    Network Working Group                                            J. Kohl
    Request for Comments: 1510                 Digital Equipment Corporation
                                                                   C. Neuman
                                                                         ISI
                                                              September 1993

                The Kerberos Network Authentication Service (V5)

    Status of this Memo

       This RFC specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" for the standardization state and status
       of this protocol.  Distribution of this memo is unlimited.

    Abstract

       This document gives an overview and specification of Version 5 of the
       protocol for the Kerberos network authentication system. Version 4,
       described elsewhere [1,2], is presently in production use at MIT's
       Project Athena, and at other Internet sites.

    Overview

       Project Athena, Athena, Athena MUSE, Discuss, Hesiod, Kerberos,
       Moira, and Zephyr are trademarks of the Massachusetts Institute of
       Technology (MIT).  No commercial use of these trademarks may be made
       without prior written permission of MIT.

       This RFC describes the concepts and model upon which the Kerberos
       network authentication system is based. It also specifies Version 5
       of the Kerberos protocol.

       The motivations, goals, assumptions, and rationale behind most design
       decisions are treated cursorily; for Version 4 they are fully
       described in the Kerberos portion of the Athena Technical Plan [1].
       The protocols are under review, and are not being submitted for
       consideration as an Internet standard at this time.  Comments are
       encouraged.  Requests for addition to an electronic mailing list for
       discussion of Kerberos, kerberos@MIT.EDU, may be addressed to
       kerberos-request@MIT.EDU.  This mailing list is gatewayed onto the
       Usenet as the group comp.protocols.kerberos.  Requests for further
       information, including documents and code availability, may be sent
       to info-kerberos@MIT.EDU.

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    RFC 1510                        Kerberos                  September 1993

    Background

       The Kerberos model is based in part on Needham and Schroeder's
       trusted third-party authentication protocol [3] and on modifications
       suggested by Denning and Sacco [4].  The original design and
       implementation of Kerberos Versions 1 through 4 was the work of two
       former Project Athena staff members, Steve Miller of Digital
       Equipment Corporation and Clifford Neuman (now at the Information
       Sciences Institute of the University of Southern California), along
       with Jerome Saltzer, Technical Director of Project Athena, and
       Jeffrey Schiller, MIT Campus Network Manager.  Many other members of
       Project Athena have also contributed to the work on Kerberos.
       Version 4 is publicly available, and has seen wide use across the
       Internet.

       Version 5 (described in this document) has evolved from Version 4
       based on new requirements and desires for features not available in
       Version 4.  Details on the differences between Kerberos Versions 4
       and 5 can be found in [5].

    Table of Contents

       1. Introduction .......................................    5
       1.1. Cross-Realm Operation ............................    7
       1.2. Environmental assumptions ........................    8
       1.3. Glossary of terms ................................    9
       2. Ticket flag uses and requests ......................   12
       2.1. Initial and pre-authenticated tickets ............   12
       2.2. Invalid tickets ..................................   12
       2.3. Renewable tickets ................................   12
       2.4. Postdated tickets ................................   13
       2.5. Proxiable and proxy tickets ......................   14
       2.6. Forwardable tickets ..............................   15
       2.7. Other KDC options ................................   15
       3. Message Exchanges ..................................   16
       3.1. The Authentication Service Exchange ..............   16
       3.1.1. Generation of KRB_AS_REQ message ...............   17
       3.1.2. Receipt of KRB_AS_REQ message ..................   17
       3.1.3. Generation of KRB_AS_REP message ...............   17
       3.1.4. Generation of KRB_ERROR message ................   19
       3.1.5. Receipt of KRB_AS_REP message ..................   19
       3.1.6. Receipt of KRB_ERROR message ...................   20
       3.2. The Client/Server Authentication Exchange ........   20
       3.2.1. The KRB_AP_REQ message .........................   20
       3.2.2. Generation of a KRB_AP_REQ message .............   20
       3.2.3. Receipt of KRB_AP_REQ message ..................   21
       3.2.4. Generation of a KRB_AP_REP message .............   23
       3.2.5. Receipt of KRB_AP_REP message ..................   23

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       3.2.6. Using the encryption key .......................   24
       3.3. The Ticket-Granting Service (TGS) Exchange .......   24
       3.3.1. Generation of KRB_TGS_REQ message ..............   25
       3.3.2. Receipt of KRB_TGS_REQ message .................   26
       3.3.3. Generation of KRB_TGS_REP message ..............   27
       3.3.3.1. Encoding the transited field .................   29
       3.3.4. Receipt of KRB_TGS_REP message .................   31
       3.4. The KRB_SAFE Exchange ............................   31
       3.4.1. Generation of a KRB_SAFE message ...............   31
       3.4.2. Receipt of KRB_SAFE message ....................   32
       3.5. The KRB_PRIV Exchange ............................   33
       3.5.1. Generation of a KRB_PRIV message ...............   33
       3.5.2. Receipt of KRB_PRIV message ....................   33
       3.6. The KRB_CRED Exchange ............................   34
       3.6.1. Generation of a KRB_CRED message ...............   34
       3.6.2. Receipt of KRB_CRED message ....................   34
       4. The Kerberos Database ..............................   35
       4.1. Database contents ................................   35
       4.2. Additional fields ................................   36
       4.3. Frequently Changing Fields .......................   37
       4.4. Site Constants ...................................   37
       5. Message Specifications .............................   38
       5.1. ASN.1 Distinguished Encoding Representation ......   38
       5.2. ASN.1 Base Definitions ...........................   38
       5.3. Tickets and Authenticators .......................   42
       5.3.1. Tickets ........................................   42
       5.3.2. Authenticators .................................   47
       5.4. Specifications for the AS and TGS exchanges ......   49
       5.4.1. KRB_KDC_REQ definition .........................   49
       5.4.2. KRB_KDC_REP definition .........................   56
       5.5. Client/Server (CS) message specifications ........   58
       5.5.1. KRB_AP_REQ definition ..........................   58
       5.5.2. KRB_AP_REP definition ..........................   60
       5.5.3. Error message reply ............................   61
       5.6. KRB_SAFE message specification ...................   61
       5.6.1. KRB_SAFE definition ............................   61
       5.7. KRB_PRIV message specification ...................   62
       5.7.1. KRB_PRIV definition ............................   62
       5.8. KRB_CRED message specification ...................   63
       5.8.1. KRB_CRED definition ............................   63
       5.9. Error message specification ......................   65
       5.9.1. KRB_ERROR definition ...........................   66
       6. Encryption and Checksum Specifications .............   67
       6.1. Encryption Specifications ........................   68
       6.2. Encryption Keys ..................................   71
       6.3. Encryption Systems ...............................   71
       6.3.1. The NULL Encryption System (null) ..............   71
       6.3.2. DES in CBC mode with a CRC-32 checksum (descbc-crc)71

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       6.3.3. DES in CBC mode with an MD4 checksum (descbc-md4)  72
       6.3.4. DES in CBC mode with an MD5 checksum (descbc-md5)  72
       6.4. Checksums ........................................   74
       6.4.1. The CRC-32 Checksum (crc32) ....................   74
       6.4.2. The RSA MD4 Checksum (rsa-md4) .................   75
       6.4.3. RSA MD4 Cryptographic Checksum Using DES
       (rsa-md4-des) .........................................   75
       6.4.4. The RSA MD5 Checksum (rsa-md5) .................   76
       6.4.5. RSA MD5 Cryptographic Checksum Using DES
       (rsa-md5-des) .........................................   76
       6.4.6. DES cipher-block chained checksum (des-mac)
       6.4.7. RSA MD4 Cryptographic Checksum Using DES
       alternative (rsa-md4-des-k) ...........................   77
       6.4.8. DES cipher-block chained checksum alternative
       (des-mac-k) ...........................................   77
       7. Naming Constraints .................................   78
       7.1. Realm Names ......................................   77
       7.2. Principal Names ..................................   79
       7.2.1. Name of server principals ......................   80
       8. Constants and other defined values .................   80
       8.1. Host address types ...............................   80
       8.2. KDC messages .....................................   81
       8.2.1. IP transport ...................................   81
       8.2.2. OSI transport ..................................   82
       8.2.3. Name of the TGS ................................   82
       8.3. Protocol constants and associated values .........   82
       9. Interoperability requirements ......................   86
       9.1. Specification 1 ..................................   86
       9.2. Recommended KDC values ...........................   88
       10. Acknowledgments ...................................   88
       11. References ........................................   89
       12. Security Considerations ...........................   90
       13. Authors' Addresses ................................   90
       A. Pseudo-code for protocol processing ................   91
       A.1. KRB_AS_REQ generation ............................   91
       A.2. KRB_AS_REQ verification and KRB_AS_REP generation    92
       A.3. KRB_AS_REP verification ..........................   95
       A.4. KRB_AS_REP and KRB_TGS_REP common checks .........   96
       A.5. KRB_TGS_REQ generation ...........................   97
       A.6. KRB_TGS_REQ verification and KRB_TGS_REP generation  98
       A.7. KRB_TGS_REP verification .........................  104
       A.8. Authenticator generation .........................  104
       A.9. KRB_AP_REQ generation ............................  105
       A.10. KRB_AP_REQ verification .........................  105
       A.11. KRB_AP_REP generation ...........................  106
       A.12. KRB_AP_REP verification .........................  107
       A.13. KRB_SAFE generation .............................  107
       A.14. KRB_SAFE verification ...........................  108

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       A.15. KRB_SAFE and KRB_PRIV common checks .............  108
       A.16. KRB_PRIV generation .............................  109
       A.17. KRB_PRIV verification ...........................  110
       A.18. KRB_CRED generation .............................  110
       A.19. KRB_CRED verification ...........................  111
       A.20. KRB_ERROR generation ............................  112

    1.  Introduction

       Kerberos provides a means of verifying the identities of principals,
       (e.g., a workstation user or a network server) on an open
       (unprotected) network.  This is accomplished without relying on
       authentication by the host operating system, without basing trust on
       host addresses, without requiring physical security of all the hosts
       on the network, and under the assumption that packets traveling along
       the network can be read, modified, and inserted at will. (Note,
       however, that many applications use Kerberos' functions only upon the
       initiation of a stream-based network connection, and assume the
       absence of any "hijackers" who might subvert such a connection.  Such
       use implicitly trusts the host addresses involved.)  Kerberos
       performs authentication under these conditions as a trusted third-
       party authentication service by using conventional cryptography,
       i.e., shared secret key.  (shared secret key - Secret and private are
       often used interchangeably in the literature.  In our usage, it takes
       two (or more) to share a secret, thus a shared DES key is a secret
       key.  Something is only private when no one but its owner knows it.
       Thus, in public key cryptosystems, one has a public and a private
       key.)

       The authentication process proceeds as follows: A client sends a
       request to the authentication server (AS) requesting "credentials"
       for a given server.  The AS responds with these credentials,
       encrypted in the client's key.  The credentials consist of 1) a
       "ticket" for the server and 2) a temporary encryption key (often
       called a "session key").  The client transmits the ticket (which
       contains the client's identity and a copy of the session key, all
       encrypted in the server's key) to the server.  The session key (now
       shared by the client and server) is used to authenticate the client,
       and may optionally be used to authenticate the server.  It may also
       be used to encrypt further communication between the two parties or
       to exchange a separate sub-session key to be used to encrypt further
       communication.

       The implementation consists of one or more authentication servers
       running on physically secure hosts.  The authentication servers
       maintain a database of principals (i.e., users and servers) and their
       secret keys. Code libraries provide encryption and implement the
       Kerberos protocol.  In order to add authentication to its

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       transactions, a typical network application adds one or two calls to
       the Kerberos library, which results in the transmission of the
       necessary messages to achieve authentication.

       The Kerberos protocol consists of several sub-protocols (or
       exchanges).  There are two methods by which a client can ask a
       Kerberos server for credentials.  In the first approach, the client
       sends a cleartext request for a ticket for the desired server to the
       AS. The reply is sent encrypted in the client's secret key. Usually
       this request is for a ticket-granting ticket (TGT) which can later be
       used with the ticket-granting server (TGS).  In the second method,
       the client sends a request to the TGS.  The client sends the TGT to
       the TGS in the same manner as if it were contacting any other
       application server which requires Kerberos credentials.  The reply is
       encrypted in the session key from the TGT.

       Once obtained, credentials may be used to verify the identity of the
       principals in a transaction, to ensure the integrity of messages
       exchanged between them, or to preserve privacy of the messages.  The
       application is free to choose whatever protection may be necessary.

       To verify the identities of the principals in a transaction, the
       client transmits the ticket to the server.  Since the ticket is sent
       "in the clear" (parts of it are encrypted, but this encryption
       doesn't thwart replay) and might be intercepted and reused by an
       attacker, additional information is sent to prove that the message
       was originated by the principal to whom the ticket was issued.  This
       information (called the authenticator) is encrypted in the session
       key, and includes a timestamp.  The timestamp proves that the message
       was recently generated and is not a replay.  Encrypting the
       authenticator in the session key proves that it was generated by a
       party possessing the session key.  Since no one except the requesting
       principal and the server know the session key (it is never sent over
       the network in the clear) this guarantees the identity of the client.

       The integrity of the messages exchanged between principals can also
       be guaranteed using the session key (passed in the ticket and
       contained in the credentials).  This approach provides detection of
       both replay attacks and message stream modification attacks.  It is
       accomplished by generating and transmitting a collision-proof
       checksum (elsewhere called a hash or digest function) of the client's
       message, keyed with the session key.  Privacy and integrity of the
       messages exchanged between principals can be secured by encrypting
       the data to be passed using the session key passed in the ticket, and
       contained in the credentials.

       The authentication exchanges mentioned above require read-only access
       to the Kerberos database.  Sometimes, however, the entries in the

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       database must be modified, such as when adding new principals or
       changing a principal's key.  This is done using a protocol between a
       client and a third Kerberos server, the Kerberos Administration
       Server (KADM).  The administration protocol is not described in this
       document. There is also a protocol for maintaining multiple copies of
       the Kerberos database, but this can be considered an implementation
       detail and may vary to support different database technologies.

    1.1.  Cross-Realm Operation

       The Kerberos protocol is designed to operate across organizational
       boundaries.  A client in one organization can be authenticated to a
       server in another.  Each organization wishing to run a Kerberos
       server establishes its own "realm".  The name of the realm in which a
       client is registered is part of the client's name, and can be used by
       the end-service to decide whether to honor a request.

       By establishing "inter-realm" keys, the administrators of two realms
       can allow a client authenticated in the local realm to use its
       authentication remotely (Of course, with appropriate permission the
       client could arrange registration of a separately-named principal in
       a remote realm, and engage in normal exchanges with that realm's
       services. However, for even small numbers of clients this becomes
       cumbersome, and more automatic methods as described here are
       necessary).  The exchange of inter-realm keys (a separate key may be
       used for each direction) registers the ticket-granting service of
       each realm as a principal in the other realm.  A client is then able
       to obtain a ticket-granting ticket for the remote realm's ticket-
       granting service from its local realm. When that ticket-granting
       ticket is used, the remote ticket-granting service uses the inter-
       realm key (which usually differs from its own normal TGS key) to
       decrypt the ticket-granting ticket, and is thus certain that it was
       issued by the client's own TGS. Tickets issued by the remote ticket-
       granting service will indicate to the end-service that the client was
       authenticated from another realm.

       A realm is said to communicate with another realm if the two realms
       share an inter-realm key, or if the local realm shares an inter-realm
       key with an intermediate realm that communicates with the remote
       realm.  An authentication path is the sequence of intermediate realms
       that are transited in communicating from one realm to another.

       Realms are typically organized hierarchically. Each realm shares a
       key with its parent and a different key with each child.  If an
       inter-realm key is not directly shared by two realms, the
       hierarchical organization allows an authentication path to be easily
       constructed.  If a hierarchical organization is not used, it may be
       necessary to consult some database in order to construct an

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       authentication path between realms.

       Although realms are typically hierarchical, intermediate realms may
       be bypassed to achieve cross-realm authentication through alternate
       authentication paths (these might be established to make
       communication between two realms more efficient).  It is important
       for the end-service to know which realms were transited when deciding
       how much faith to place in the authentication process. To facilitate
       this decision, a field in each ticket contains the names of the
       realms that were involved in authenticating the client.

    1.2.  Environmental assumptions

       Kerberos imposes a few assumptions on the environment in which it can
       properly function:

       +    "Denial of service" attacks are not solved with Kerberos.  There
            are places in these protocols where an intruder intruder can
            prevent an application from participating in the proper
            authentication steps.  Detection and solution of such attacks
            (some of which can appear to be not-uncommon "normal" failure
            modes for the system) is usually best left to the human
            administrators and users.

       +    Principals must keep their secret keys secret.  If an intruder
            somehow steals a principal's key, it will be able to masquerade
            as that principal or impersonate any server to the legitimate
            principal.

       +    "Password guessing" attacks are not solved by Kerberos.  If a
            user chooses a poor password, it is possible for an attacker to
            successfully mount an offline dictionary attack by repeatedly
            attempting to decrypt, with successive entries from a
            dictionary, messages obtained which are encrypted under a key
            derived from the user's password.

       +    Each host on the network must have a clock which is "loosely
            synchronized" to the time of the other hosts; this
            synchronization is used to reduce the bookkeeping needs of
            application servers when they do replay detection.  The degree
            of "looseness" can be configured on a per-server basis.  If the
            clocks are synchronized over the network, the clock
            synchronization protocol must itself be secured from network
            attackers.

       +    Principal identifiers are not recycled on a short-term basis.  A
            typical mode of access control will use access control lists
            (ACLs) to grant permissions to particular principals.  If a

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            stale ACL entry remains for a deleted principal and the
            principal identifier is reused, the new principal will inherit
            rights specified in the stale ACL entry. By not re-using
            principal identifiers, the danger of inadvertent access is
            removed.

    1.3.  Glossary of terms

       Below is a list of terms used throughout this document.

       Authentication      Verifying the claimed identity of a
                           principal.

       Authentication header A record containing a Ticket and an
                             Authenticator to be presented to a
                             server as part of the authentication
                             process.

       Authentication path  A sequence of intermediate realms transited
                            in the authentication process when
                            communicating from one realm to another.

       Authenticator       A record containing information that can
                           be shown to have been recently generated
                           using the session key known only by  the
                           client and server.

       Authorization       The process of determining whether a
                           client may use a service, which objects
                           the client is allowed to access, and the
                           type of access allowed for each.

       Capability          A token that grants the bearer permission
                           to access an object or service.  In
                           Kerberos, this might be a ticket whose
                           use is restricted by the contents of the
                           authorization data field, but which
                           lists no network addresses, together
                           with the session key necessary to use
                           the ticket.

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    RFC 1510                        Kerberos                  September 1993

       Ciphertext          The output of an encryption function.
                           Encryption transforms plaintext into
                           ciphertext.

       Client              A process that makes use of a network
                           service on behalf of a user.  Note that
                           in some cases a Server may itself be a
                           client of some other server (e.g., a
                           print server may be a client of a file
                           server).

       Credentials         A ticket plus the secret session key
                           necessary to successfully use that
                           ticket in an authentication exchange.

       KDC                 Key Distribution Center, a network service
                           that supplies tickets and temporary
                           session keys; or an instance of that
                           service or the host on which it runs.
                           The KDC services both initial ticket and
                           ticket-granting ticket requests.  The
                           initial ticket portion is sometimes
                           referred to as the Authentication Server
                           (or service).  The ticket-granting
                           ticket portion is sometimes referred to
                           as the ticket-granting server (or service).

       Kerberos            Aside from the 3-headed dog guarding
                           Hades, the name given to Project
                           Athena's authentication service, the
                           protocol used by that service, or the
                           code used to implement the authentication
                           service.

       Plaintext           The input to an encryption function  or
                           the output of a decryption function.
                           Decryption transforms ciphertext into
                           plaintext.

       Principal           A uniquely named client or server
                           instance that participates in a network
                           communication.

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    RFC 1510                        Kerberos                  September 1993

       Principal identifier The name used to uniquely identify each
                            different principal.

       Seal                To encipher a record containing several
                           fields in such a way that the fields
                           cannot be individually replaced without
                           either knowledge of the encryption key
                           or leaving evidence of tampering.

       Secret key          An encryption key shared by a principal
                           and the KDC, distributed outside the
                           bounds of the system, with a long lifetime.
                           In the case of a human user's
                           principal, the secret key is derived
                           from a password.

       Server              A particular Principal which provides a
                           resource to network clients.

       Service             A resource provided to network clients;
                           often provided by more than one server
                           (for example, remote file service).

       Session key         A temporary encryption key used between
                           two principals, with a lifetime limited
                           to the duration of a single login "session".

       Sub-session key     A temporary encryption key used between
                           two principals, selected and exchanged
                           by the principals using the session key,
                           and with a lifetime limited to the duration
                           of a single association.

       Ticket              A record that helps a client authenticate
                           itself to a server; it contains the
                           client's identity, a session key, a
                           timestamp, and other information, all
                           sealed using the server's secret key.
                           It only serves to authenticate a client
                           when presented along with a fresh
                           Authenticator.

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    RFC 1510                        Kerberos                  September 1993

    2.  Ticket flag uses and requests

       Each Kerberos ticket contains a set of flags which are used to
       indicate various attributes of that ticket.  Most flags may be
       requested by a client when the ticket is obtained; some are
       automatically turned on and off by a Kerberos server as required.
       The following sections explain what the various flags mean, and gives
       examples of reasons to use such a flag.

    2.1.  Initial and pre-authenticated tickets

       The INITIAL flag indicates that a ticket was issued using the AS
       protocol and not issued based on a ticket-granting ticket.
       Application servers that want to require the knowledge of a client's
       secret key (e.g., a passwordchanging program) can insist that this
       flag be set in any tickets they accept, and thus be assured that the
       client's key was recently presented to the application client.

       The PRE-AUTHENT and HW-AUTHENT flags provide addition information
       about the initial authentication, regardless of whether the current
       ticket was issued directly (in which case INITIAL will also be set)
       or issued on the basis of a ticket-granting ticket (in which case the
       INITIAL flag is clear, but the PRE-AUTHENT and HW-AUTHENT flags are
       carried forward from the ticket-granting ticket).

    2.2.  Invalid tickets

       The INVALID flag indicates that a ticket is invalid.  Application
       servers must reject tickets which have this flag set.  A postdated
       ticket will usually be issued in this form. Invalid tickets must be
       validated by the KDC before use, by presenting them to the KDC in a
       TGS request with the VALIDATE option specified.  The KDC will only
       validate tickets after their starttime has passed.  The validation is
       required so that postdated tickets which have been stolen before
       their starttime can be rendered permanently invalid (through a hot-
       list mechanism).

    2.3.  Renewable tickets

       Applications may desire to hold tickets which can be valid for long
       periods of time.  However, this can expose their credentials to
       potential theft for equally long periods, and those stolen
       credentials would be valid until the expiration time of the
       ticket(s).  Simply using shortlived tickets and obtaining new ones
       periodically would require the client to have long-term access to its
       secret key, an even greater risk.  Renewable tickets can be used to
       mitigate the consequences of theft.  Renewable tickets have two
       "expiration times": the first is when the current instance of the

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    RFC 1510                        Kerberos                  September 1993

       ticket expires, and the second is the latest permissible value for an
       individual expiration time.  An application client must periodically
       (i.e., before it expires) present a renewable ticket to the KDC, with
       the RENEW option set in the KDC request.  The KDC will issue a new
       ticket with a new session key and a later expiration time.  All other
       fields of the ticket are left unmodified by the renewal process.
       When the latest permissible expiration time arrives, the ticket
       expires permanently.  At each renewal, the KDC may consult a hot-list
       to determine if the ticket had been reported stolen since its last
       renewal; it will refuse to renew such stolen tickets, and thus the
       usable lifetime of stolen tickets is reduced.

       The RENEWABLE flag in a ticket is normally only interpreted by the
       ticket-granting service (discussed below in section 3.3).  It can
       usually be ignored by application servers.  However, some
       particularly careful application servers may wish to disallow
       renewable tickets.

       If a renewable ticket is not renewed by its  expiration time, the KDC
       will not renew the ticket.  The RENEWABLE flag is reset by default,
       but a client may request it be  set  by setting  the RENEWABLE option
       in the KRB_AS_REQ message.  If it is set, then the renew-till field
       in the ticket  contains the time after which the ticket may not be
       renewed.

    2.4.  Postdated tickets

       Applications may occasionally need to obtain tickets for use much
       later, e.g., a batch submission system would need tickets to be valid
       at the time the batch job is serviced.  However, it is dangerous to
       hold valid tickets in a batch queue, since they will be on-line
       longer and more prone to theft.  Postdated tickets provide a way to
       obtain these tickets from the KDC at job submission time, but to
       leave them "dormant" until they are activated and validated by a
       further request of the KDC.  If a ticket theft were reported in the
       interim, the KDC would refuse to validate the ticket, and the thief
       would be foiled.

       The MAY-POSTDATE flag in a ticket is normally only interpreted by the
       ticket-granting service.  It can be ignored by application servers.
       This flag must be set in a ticket-granting ticket in order to issue a
       postdated ticket based on the presented ticket. It is reset by
       default; it may be requested by a client by setting the ALLOW-
       POSTDATE option in the KRB_AS_REQ message.  This flag does not allow
       a client to obtain a postdated ticket-granting ticket; postdated
       ticket-granting tickets can only by obtained by requesting the
       postdating in the KRB_AS_REQ message.  The life (endtime-starttime)
       of a postdated ticket will be the remaining life of the ticket-

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       granting ticket at the time of the request, unless the RENEWABLE
       option is also set, in which case it can be the full life (endtime-
       starttime) of the ticket-granting ticket.  The KDC may limit how far
       in the future a ticket may be postdated.

       The POSTDATED flag indicates that a ticket has been postdated.  The
       application server can check the authtime field in the ticket to see
       when the original authentication occurred.  Some services may choose
       to reject postdated tickets, or they may only accept them within a
       certain period after the original authentication. When the KDC issues
       a POSTDATED ticket, it will also be marked as INVALID, so that the
       application client must present the ticket to the KDC to be validated
       before use.

    2.5.  Proxiable and proxy tickets

       At times it may be necessary for a principal to allow a service  to
       perform an operation on its behalf.  The service must be able to take
       on the identity of the client, but only for  a particular purpose.  A
       principal can allow a service to take on the principal's identity for
       a particular purpose by granting it a proxy.

       The PROXIABLE flag in a ticket is normally only interpreted by the
       ticket-granting service. It can be ignored by application servers.
       When set, this flag tells the ticket-granting server that it is OK to
       issue a new ticket (but not a ticket-granting ticket) with a
       different network address based on this ticket.  This flag is set by
       default.

       This flag allows a client to pass a proxy to a server to perform a
       remote request on its behalf, e.g., a print service client can give
       the print server a proxy to access the client's files on a particular
       file server in order to satisfy a print request.

       In order to complicate the use of stolen credentials, Kerberos
       tickets are usually valid from only those network addresses
       specifically included in the ticket (It is permissible to request or
       issue tickets with no network addresses specified, but we do not
       recommend it).  For this reason, a client wishing to grant a proxy
       must request a new ticket valid for the network address of the
       service to be granted the proxy.

       The PROXY flag is set in a ticket by the  TGS  when  it issues a
       proxy ticket.  Application servers may check this flag and require
       additional authentication  from  the  agent presenting the proxy in
       order to provide an audit trail.

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    2.6.  Forwardable tickets

       Authentication forwarding is an instance of the proxy case where the
       service is granted complete use of the client's identity.  An example
       where it might be used is when a user logs in to a remote system and
       wants authentication to work from that system as if the login were
       local.

       The FORWARDABLE flag in a ticket is normally only interpreted by the
       ticket-granting service.  It can be ignored by application servers.
       The FORWARDABLE flag has an interpretation similar to that of the
       PROXIABLE flag, except ticket-granting tickets may also be issued
       with different network addresses.  This flag is reset by default, but
       users may request that it be set by setting the FORWARDABLE option in
       the AS request when they request their initial ticket-granting
       ticket.

       This flag allows for authentication forwarding without requiring the
       user to enter a password again.  If the flag is not set, then
       authentication forwarding is not permitted, but the same end result
       can still be achieved if the user engages in the AS exchange with the
       requested network addresses and supplies a password.

       The FORWARDED flag is set by the TGS when a client presents a ticket
       with the FORWARDABLE flag set and requests it be set by specifying
       the FORWARDED KDC option and supplying a set of addresses for the new
       ticket.  It is also set in all tickets issued based on tickets with
       the FORWARDED flag set.  Application servers may wish to process
       FORWARDED tickets differently than non-FORWARDED tickets.

    2.7.  Other KDC options

       There are two additional options which may be set in a client's
       request of the KDC.  The RENEWABLE-OK option indicates that the
       client will accept a renewable ticket if a ticket with the requested
       life cannot otherwise be provided.  If a ticket with the requested
       life cannot be provided, then the KDC may issue a renewable ticket
       with a renew-till equal to the the requested endtime.  The value of
       the renew-till field may still be adjusted by site-determined limits
       or limits imposed by the individual principal or server.

       The ENC-TKT-IN-SKEY option is honored only by the ticket-granting
       service.  It indicates that the to-be-issued ticket for the end
       server is to be encrypted in the session key from the additional
       ticket-granting ticket provided with the request.  See section 3.3.3
       for specific details.

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    3.  Message Exchanges

       The following sections describe the interactions between network
       clients and servers and the messages involved in those exchanges.

    3.1.  The Authentication Service Exchange

                                 Summary

             Message direction       Message type    Section
             1. Client to Kerberos   KRB_AS_REQ      5.4.1
             2. Kerberos to client   KRB_AS_REP or   5.4.2
                                     KRB_ERROR       5.9.1

       The Authentication Service (AS) Exchange between the client and the
       Kerberos Authentication Server is usually initiated by a client when
       it wishes to obtain authentication credentials for a given server but
       currently holds no credentials.  The client's secret key is used for
       encryption and decryption.  This exchange is typically used at the
       initiation of a login session, to obtain credentials for a Ticket-
       Granting Server, which will subsequently be used to obtain
       credentials for other servers (see section 3.3) without requiring
       further use of the client's secret key.  This exchange is also used
       to request credentials for services which must not be mediated
       through the Ticket-Granting Service, but rather require a principal's
       secret key, such as the password-changing service.  (The password-
       changing request must not be honored unless the requester can provide
       the old password (the user's current secret key).  Otherwise, it
       would be possible for someone to walk up to an unattended session and
       change another user's password.)  This exchange does not by itself
       provide any assurance of the the identity of the user.  (To
       authenticate a user logging on to a local system, the credentials
       obtained in the AS exchange may first be used in a TGS exchange to
       obtain credentials for a local server.  Those credentials must then
       be verified by the local server through successful completion of the
       Client/Server exchange.)

       The exchange consists of two messages: KRB_AS_REQ from the client to
       Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for these
       messages are described in sections 5.4.1, 5.4.2, and 5.9.1.

       In the request, the client sends (in cleartext) its own identity and
       the identity of the server for which it is requesting credentials.
       The response, KRB_AS_REP, contains a ticket for the client to present
       to the server, and a session key that will be shared by the client
       and the server.  The session key and additional information are
       encrypted in the client's secret key.  The KRB_AS_REP message
       contains information which can be used to detect replays, and to

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       associate it with the message to which it replies.  Various errors
       can occur; these are indicated by an error response (KRB_ERROR)
       instead of the KRB_AS_REP response.  The error message is not
       encrypted.  The KRB_ERROR message also contains information which can
       be used to associate it with the message to which it replies.  The
       lack of encryption in the KRB_ERROR message precludes the ability to
       detect replays or fabrications of such messages.

       In the normal case the authentication server does not know whether
       the client is actually the principal named in the request.  It simply
       sends a reply without knowing or caring whether they are the same.
       This is acceptable because nobody but the principal whose identity
       was given in the request will be able to use the reply. Its critical
       information is encrypted in that principal's key.  The initial
       request supports an optional field that can be used to pass
       additional information that might be needed for the initial exchange.
       This field may be used for preauthentication if desired, but the
       mechanism is not currently specified.

    3.1.1. Generation of KRB_AS_REQ message

       The client may specify a number of options in the initial request.
       Among these options are whether preauthentication is to be performed;
       whether the requested ticket is to be renewable, proxiable, or
       forwardable; whether it should be postdated or allow postdating of
       derivative tickets; and whether a renewable ticket will be accepted
       in lieu of a non-renewable ticket if the requested ticket expiration
       date cannot be satisfied by a nonrenewable ticket (due to
       configuration constraints; see section 4).  See section A.1 for
       pseudocode.

       The client prepares the KRB_AS_REQ message and sends it to the KDC.

    3.1.2. Receipt of KRB_AS_REQ message

       If all goes well, processing the KRB_AS_REQ message will result in
       the creation of a ticket for the client to present to the server.
       The format for the ticket is described in section 5.3.1.  The
       contents of the ticket are determined as follows.

    3.1.3. Generation of KRB_AS_REP message

       The authentication server looks up the client and server principals
       named in the KRB_AS_REQ in its database, extracting their respective
       keys.  If required, the server pre-authenticates the request, and if
       the pre-authentication check fails, an error message with the code
       KDC_ERR_PREAUTH_FAILED is returned. If the server cannot accommodate
       the requested encryption type, an error message with code

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       KDC_ERR_ETYPE_NOSUPP is returned. Otherwise it generates a "random"
       session key ("Random" means that, among other things, it should be
       impossible to guess the next session key based on knowledge of past
       session keys.  This can only be achieved in a pseudo-random number
       generator if it is based on cryptographic principles.  It would be
       more desirable to use a truly random number generator, such as one
       based on measurements of random physical phenomena.).

       If the requested start time is absent or indicates a time in the
       past, then the start time of the ticket is set to the authentication
       server's current time. If it indicates a time in the future, but the
       POSTDATED option has not been specified, then the error
       KDC_ERR_CANNOT_POSTDATE is returned.  Otherwise the requested start
       time is checked against the policy of the local realm (the
       administrator might decide to prohibit certain types or ranges of
       postdated tickets), and if acceptable, the ticket's start time is set
       as requested and the INVALID flag is set in the new ticket. The
       postdated ticket must be validated before use by presenting it to the
       KDC after the start time has been reached.

       The expiration time of the ticket will be set to the minimum of the
       following:

       +The expiration time (endtime) requested in the KRB_AS_REQ
        message.

       +The ticket's start time plus the maximum allowable lifetime
        associated with the client principal (the authentication
        server's database includes a maximum ticket lifetime field
        in each principal's record; see section 4).

       +The ticket's start time plus the maximum allowable lifetime
        associated with the server principal.

       +The ticket's start time plus the maximum lifetime set by
        the policy of the local realm.

       If the requested expiration time minus the start time (as determined
       above) is less than a site-determined minimum lifetime, an error
       message with code KDC_ERR_NEVER_VALID is returned.  If the requested
       expiration time for the ticket exceeds what was determined as above,
       and if the "RENEWABLE-OK" option was requested, then the "RENEWABLE"
       flag is set in the new ticket, and the renew-till value is set as if
       the "RENEWABLE" option were requested (the field and option names are
       described fully in section 5.4.1).  If the RENEWABLE option has been
       requested or if the RENEWABLE-OK option has been set and a renewable
       ticket is to be issued, then the renew-till field is set to the
       minimum of:

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       +Its requested value.

       +The start time of the ticket plus the minimum of the two
        maximum renewable lifetimes associated with the principals'
        database entries.

       +The start time of the ticket plus the maximum renewable
        lifetime set by the policy of the local realm.

       The flags field of the new ticket will have the following options set
       if they have been requested and if the policy of the local realm
       allows: FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE.
       If the new ticket is postdated (the start time is in the future), its
       INVALID flag will also be set.

       If all of the above succeed, the server formats a KRB_AS_REP message
       (see section 5.4.2), copying the addresses in the request into the
       caddr of the response, placing any required pre-authentication data
       into the padata of the response, and encrypts the ciphertext part in
       the client's key using the requested encryption method, and sends it
       to the client.  See section A.2 for pseudocode.

    3.1.4. Generation of KRB_ERROR message

       Several errors can occur, and the Authentication Server responds by
       returning an error message, KRB_ERROR, to the client, with the
       error-code and e-text fields set to appropriate values.  The error
       message contents and details are described in Section 5.9.1.

    3.1.5. Receipt of KRB_AS_REP message

       If the reply message type is KRB_AS_REP, then the client verifies
       that the cname and crealm fields in the cleartext portion of the
       reply match what it requested.  If any padata fields are present,
       they may be used to derive the proper secret key to decrypt the
       message.  The client decrypts the encrypted part of the response
       using its secret key, verifies that the nonce in the encrypted part
       matches the nonce it supplied in its request (to detect replays).  It
       also verifies that the sname and srealm in the response match those
       in the request, and that the host address field is also correct.  It
       then stores the ticket, session key, start and expiration times, and
       other information for later use.  The key-expiration field from the
       encrypted part of the response may be checked to notify the user of
       impending key expiration (the client program could then suggest
       remedial action, such as a password change).  See section A.3 for
       pseudocode.

       Proper decryption of the KRB_AS_REP message is not sufficient to

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       verify the identity of the user; the user and an attacker could
       cooperate to generate a KRB_AS_REP format message which decrypts
       properly but is not from the proper KDC.  If the host wishes to
       verify the identity of the user, it must require the user to present
       application credentials which can be verified using a securely-stored
       secret key.  If those credentials can be verified, then the identity
       of the user can be assured.

    3.1.6. Receipt of KRB_ERROR message

       If the reply message type is KRB_ERROR, then the client interprets it
       as an error and performs whatever application-specific tasks are
       necessary to recover.

    3.2.  The Client/Server Authentication Exchange

                            Summary

       Message direction                         Message type    Section
       Client to Application server              KRB_AP_REQ      5.5.1
       [optional] Application server to client   KRB_AP_REP or   5.5.2
                                                 KRB_ERROR       5.9.1

       The client/server authentication (CS) exchange is used by network
       applications to authenticate the client to the server and vice versa.
       The client must have already acquired credentials for the server
       using the AS or TGS exchange.

    3.2.1. The KRB_AP_REQ message

       The KRB_AP_REQ contains authentication information which should be
       part of the first message in an authenticated transaction.  It
       contains a ticket, an authenticator, and some additional bookkeeping
       information (see section 5.5.1 for the exact format).  The ticket by
       itself is insufficient to authenticate a client, since tickets are
       passed across the network in cleartext(Tickets contain both an
       encrypted and unencrypted portion, so cleartext here refers to the
       entire unit, which can be copied from one message and replayed in
       another without any cryptographic skill.), so the authenticator is
       used to prevent invalid replay of tickets by proving to the server
       that the client knows the session key of the ticket and thus is
       entitled to use it.  The KRB_AP_REQ message is referred to elsewhere
       as the "authentication header."

    3.2.2. Generation of a KRB_AP_REQ message

       When a client wishes to initiate authentication to a server, it
       obtains (either through a credentials cache, the AS exchange, or the

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       TGS exchange) a ticket and session key for the desired service.  The
       client may re-use any tickets it holds until they expire.  The client
       then constructs a new Authenticator from the the system time, its
       name, and optionally an application specific checksum, an initial
       sequence number to be used in KRB_SAFE or KRB_PRIV messages, and/or a
       session subkey to be used in negotiations for a session key unique to
       this particular session.  Authenticators may not be re-used and will
       be rejected if replayed to a server (Note that this can make
       applications based on unreliable transports difficult to code
       correctly, if the transport might deliver duplicated messages.  In
       such cases, a new authenticator must be generated for each retry.).
       If a sequence number is to be included, it should be randomly chosen
       so that even after many messages have been exchanged it is not likely
       to collide with other sequence numbers in use.

       The client may indicate a requirement of mutual authentication or the
       use of a session-key based ticket by setting the appropriate flag(s)
       in the ap-options field of the message.

       The Authenticator is encrypted in the session key and combined with
       the ticket to form the KRB_AP_REQ message which is then sent to the
       end server along with any additional application-specific
       information.  See section A.9 for pseudocode.

    3.2.3. Receipt of KRB_AP_REQ message

       Authentication is based on the server's current time of day (clocks
       must be loosely synchronized), the authenticator, and the ticket.
       Several errors are possible.  If an error occurs, the server is
       expected to reply to the client with a KRB_ERROR message.  This
       message may be encapsulated in the application protocol if its "raw"
       form is not acceptable to the protocol. The format of error messages
       is described in section 5.9.1.

       The algorithm for verifying authentication information is as follows.
       If the message type is not KRB_AP_REQ, the server returns the
       KRB_AP_ERR_MSG_TYPE error. If the key version indicated by the Ticket
       in the KRB_AP_REQ is not one the server can use (e.g., it indicates
       an old key, and the server no longer possesses a copy of the old
       key), the KRB_AP_ERR_BADKEYVER error is returned.  If the USE-
       SESSION-KEY flag is set in the ap-options field, it indicates to the
       server that the ticket is encrypted in the session key from the
       server's ticket-granting ticket rather than its secret key (This is
       used for user-to-user authentication as described in [6]).  Since it
       is possible for the server to be registered in multiple realms, with
       different keys in each, the srealm field in the unencrypted portion
       of the ticket in the KRB_AP_REQ is used to specify which secret key
       the server should use to decrypt that ticket.  The KRB_AP_ERR_NOKEY

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       error code is returned if the server doesn't have the proper key to
       decipher the ticket.

       The ticket is decrypted using the version of the server's key
       specified by the ticket.  If the decryption routines detect a
       modification of the ticket (each encryption system must provide
       safeguards to detect modified ciphertext; see section 6), the
       KRB_AP_ERR_BAD_INTEGRITY error is returned (chances are good that
       different keys were used to encrypt and decrypt).

       The authenticator is decrypted using the session key extracted from
       the decrypted ticket.  If decryption shows it to have been modified,
       the KRB_AP_ERR_BAD_INTEGRITY error is returned.  The name and realm
       of the client from the ticket are compared against the same fields in
       the authenticator.  If they don't match, the KRB_AP_ERR_BADMATCH
       error is returned (they might not match, for example, if the wrong
       session key was used to encrypt the authenticator).  The addresses in
       the ticket (if any) are then searched for an address matching the
       operating-system reported address of the client.  If no match is
       found or the server insists on ticket addresses but none are present
       in the ticket, the KRB_AP_ERR_BADADDR error is returned.

       If the local (server) time and the client time in the authenticator
       differ by more than the allowable clock skew (e.g., 5 minutes), the
       KRB_AP_ERR_SKEW error is returned.  If the server name, along with
       the client name, time and microsecond fields from the Authenticator
       match any recently-seen such tuples, the KRB_AP_ERR_REPEAT error is
       returned (Note that the rejection here is restricted to
       authenticators from the same principal to the same server.  Other
       client principals communicating with the same server principal should
       not be have their authenticators rejected if the time and microsecond
       fields happen to match some other client's authenticator.).  The
       server must remember any authenticator presented within the allowable
       clock skew, so that a replay attempt is guaranteed to fail. If a
       server loses track of any authenticator presented within the
       allowable clock skew, it must reject all requests until the clock
       skew interval has passed.  This assures that any lost or re-played
       authenticators will fall outside the allowable clock skew and can no
       longer be successfully replayed (If this is not done, an attacker
       could conceivably record the ticket and authenticator sent over the
       network to a server, then disable the client's host, pose as the
       disabled host, and replay the ticket and authenticator to subvert the
       authentication.).  If a sequence number is provided in the
       authenticator, the server saves it for later use in processing
       KRB_SAFE and/or KRB_PRIV messages.  If a subkey is present, the
       server either saves it for later use or uses it to help generate its
       own choice for a subkey to be returned in a KRB_AP_REP message.

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       The server computes the age of the ticket: local (server) time minus
       the start time inside the Ticket.  If the start time is later than
       the current time by more than the allowable clock skew or if the
       INVALID flag is set in the ticket, the KRB_AP_ERR_TKT_NYV error is
       returned.  Otherwise, if the current time is later than end time by
       more than the allowable clock skew, the KRB_AP_ERR_TKT_EXPIRED error
       is returned.

       If all these checks succeed without an error, the server is assured
       that the client possesses the credentials of the principal named in
       the ticket and thus, the client has been authenticated to the server.
       See section A.10 for pseudocode.

    3.2.4. Generation of a KRB_AP_REP message

       Typically, a client's request will include both the authentication
       information and its initial request in the same message, and the
       server need not explicitly reply to the KRB_AP_REQ.  However, if
       mutual authentication (not only authenticating the client to the
       server, but also the server to the client) is being performed, the
       KRB_AP_REQ message will have MUTUAL-REQUIRED set in its ap-options
       field, and a KRB_AP_REP message is required in response.  As with the
       error message, this message may be encapsulated in the application
       protocol if its "raw" form is not acceptable to the application's
       protocol.  The timestamp and microsecond field used in the reply must
       be the client's timestamp and microsecond field (as provided in the
       authenticator). [Note: In the Kerberos version 4 protocol, the
       timestamp in the reply was the client's timestamp plus one.  This is
       not necessary in version 5 because version 5 messages are formatted
       in such a way that it is not possible to create the reply by
       judicious message surgery (even in encrypted form) without knowledge
       of the appropriate encryption keys.]  If a sequence number is to be
       included, it should be randomly chosen as described above for the
       authenticator.  A subkey may be included if the server desires to
       negotiate a different subkey.  The KRB_AP_REP message is encrypted in
       the session key extracted from the ticket.  See section A.11 for
       pseudocode.

    3.2.5. Receipt of KRB_AP_REP message

       If a KRB_AP_REP message is returned, the client uses the session key
       from the credentials obtained for the server (Note that for
       encrypting the KRB_AP_REP message, the sub-session key is not used,
       even if present in the Authenticator.) to decrypt the message, and
       verifies that the timestamp and microsecond fields match those in the
       Authenticator it sent to the server.  If they match, then the client
       is assured that the server is genuine. The sequence number and subkey
       (if present) are retained for later use.  See section A.12 for

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       pseudocode.

    3.2.6. Using the encryption key

       After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and
       server share an encryption key which can be used by the application.
       The "true session key" to be used for KRB_PRIV, KRB_SAFE, or other
       application-specific uses may be chosen by the application based on
       the subkeys in the KRB_AP_REP message and the authenticator
       (Implementations of the protocol may wish to provide routines to
       choose subkeys based on session keys and random numbers and to
       orchestrate a negotiated key to be returned in the KRB_AP_REP
       message.).  In some cases, the use of this session key will be
       implicit in the protocol; in others the method of use must be chosen
       from a several alternatives.  We leave the protocol negotiations of
       how to use the key (e.g., selecting an encryption or checksum type)
       to the application programmer; the Kerberos protocol does not
       constrain the implementation options.

       With both the one-way and mutual authentication exchanges, the peers
       should take care not to send sensitive information to each other
       without proper assurances.  In particular, applications that require
       privacy or integrity should use the KRB_AP_REP or KRB_ERROR responses
       from the server to client to assure both client and server of their
       peer's identity.  If an application protocol requires privacy of its
       messages, it can use the KRB_PRIV message (section 3.5). The KRB_SAFE
       message (section 3.4) can be used to assure integrity.

    3.3.  The Ticket-Granting Service (TGS) Exchange

                                 Summary

             Message direction       Message type     Section
             1. Client to Kerberos   KRB_TGS_REQ      5.4.1
             2. Kerberos to client   KRB_TGS_REP or   5.4.2
                                     KRB_ERROR        5.9.1

       The TGS exchange between a client and the Kerberos Ticket-Granting
       Server is initiated by a client when it wishes to obtain
       authentication credentials for a given server (which might be
       registered in a remote realm), when it wishes to renew or validate an
       existing ticket, or when it wishes to obtain a proxy ticket.  In the
       first case, the client must already have acquired a ticket for the
       Ticket-Granting Service using the AS exchange (the ticket-granting
       ticket is usually obtained when a client initially authenticates to
       the system, such as when a user logs in).  The message format for the
       TGS exchange is almost identical to that for the AS exchange.  The
       primary difference is that encryption and decryption in the TGS

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       exchange does not take place under the client's key.  Instead, the
       session key from the ticket-granting ticket or renewable ticket, or
       sub-session key from an Authenticator is used.  As is the case for
       all application servers, expired tickets are not accepted by the TGS,
       so once a renewable or ticket-granting ticket expires, the client
       must use a separate exchange to obtain valid tickets.

       The TGS exchange consists of two messages: A request (KRB_TGS_REQ)
       from the client to the Kerberos Ticket-Granting Server, and a reply
       (KRB_TGS_REP or KRB_ERROR).  The KRB_TGS_REQ message includes
       information authenticating the client plus a request for credentials.
       The authentication information consists of the authentication header
       (KRB_AP_REQ) which includes the client's previously obtained ticket-
       granting, renewable, or invalid ticket.  In the ticket-granting
       ticket and proxy cases, the request may include one or more of: a
       list of network addresses, a collection of typed authorization data
       to be sealed in the ticket for authorization use by the application
       server, or additional tickets (the use of which are described later).
       The TGS reply (KRB_TGS_REP) contains the requested credentials,
       encrypted in the session key from the ticket-granting ticket or
       renewable ticket, or if present, in the subsession key from the
       Authenticator (part of the authentication header). The KRB_ERROR
       message contains an error code and text explaining what went wrong.
       The KRB_ERROR message is not encrypted.  The KRB_TGS_REP message
       contains information which can be used to detect replays, and to
       associate it with the message to which it replies.  The KRB_ERROR
       message also contains information which can be used to associate it
       with the message to which it replies, but the lack of encryption in
       the KRB_ERROR message precludes the ability to detect replays or
       fabrications of such messages.

    3.3.1. Generation of KRB_TGS_REQ message

       Before sending a request to the ticket-granting service, the client
       must determine in which realm the application server is registered
       [Note: This can be accomplished in several ways.  It might be known
       beforehand (since the realm is part of the principal identifier), or
       it might be stored in a nameserver.  Presently, however, this
       information is obtained from a configuration file.  If the realm to
       be used is obtained from a nameserver, there is a danger of being
       spoofed if the nameservice providing the realm name is not
       authenticated.  This might result in the use of a realm which has
       been compromised, and would result in an attacker's ability to
       compromise the authentication of the application server to the
       client.].  If the client does not already possess a ticket-granting
       ticket for the appropriate realm, then one must be obtained.  This is
       first attempted by requesting a ticket-granting ticket for the
       destination realm from the local Kerberos server (using the

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       KRB_TGS_REQ message recursively).  The Kerberos server may return a
       TGT for the desired realm in which case one can proceed.
       Alternatively, the Kerberos server may return a TGT for a realm which
       is "closer" to the desired realm (further along the standard
       hierarchical path), in which case this step must be repeated with a
       Kerberos server in the realm specified in the returned TGT.  If
       neither are returned, then the request must be retried with a
       Kerberos server for a realm higher in the hierarchy.  This request
       will itself require a ticket-granting ticket for the higher realm
       which must be obtained by recursively applying these directions.

       Once the client obtains a ticket-granting ticket for the appropriate
       realm, it determines which Kerberos servers serve that realm, and
       contacts one. The list might be obtained through a configuration file
       or network service; as long as the secret keys exchanged by realms
       are kept secret, only denial of service results from a false Kerberos
       server.

       As in the AS exchange, the client may specify a number of options in
       the KRB_TGS_REQ message.  The client prepares the KRB_TGS_REQ
       message, providing an authentication header as an element of the
       padata field, and including the same fields as used in the KRB_AS_REQ
       message along with several optional fields: the enc-authorization-
       data field for application server use and additional tickets required
       by some options.

       In preparing the authentication header, the client can select a sub-
       session key under which the response from the Kerberos server will be
       encrypted (If the client selects a sub-session key, care must be
       taken to ensure the randomness of the selected subsession key.  One
       approach would be to generate a random number and XOR it with the
       session key from the ticket-granting ticket.). If the sub-session key
       is not specified, the session key from the ticket-granting ticket
       will be used.  If the enc-authorization-data is present, it must be
       encrypted in the sub-session key, if present, from the authenticator
       portion of the authentication header, or if not present in the
       session key from the ticket-granting ticket.

       Once prepared, the message is sent to a Kerberos server for the
       destination realm.  See section A.5 for pseudocode.

    3.3.2. Receipt of KRB_TGS_REQ message

       The KRB_TGS_REQ message is processed in a manner similar to the
       KRB_AS_REQ message, but there are many additional checks to be
       performed.  First, the Kerberos server must determine which server
       the accompanying ticket is for and it must select the appropriate key
       to decrypt it. For a normal KRB_TGS_REQ message, it will be for the

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       ticket granting service, and the TGS's key will be used.  If the TGT
       was issued by another realm, then the appropriate inter-realm key
       must be used.  If the accompanying ticket is not a ticket granting
       ticket for the current realm, but is for an application server in the
       current realm, the RENEW, VALIDATE, or PROXY options are specified in
       the request, and the server for which a ticket is requested is the
       server named in the accompanying ticket, then the KDC will decrypt
       the ticket in the authentication header using the key of the server
       for which it was issued.  If no ticket can be found in the padata
       field, the KDC_ERR_PADATA_TYPE_NOSUPP error is returned.

       Once the accompanying ticket has been decrypted, the user-supplied
       checksum in the Authenticator must be verified against the contents
       of the request, and the message rejected if the checksums do not
       match (with an error code of KRB_AP_ERR_MODIFIED) or if the checksum
       is not keyed or not collision-proof (with an error code of
       KRB_AP_ERR_INAPP_CKSUM).  If the checksum type is not supported, the
       KDC_ERR_SUMTYPE_NOSUPP error is returned.  If the authorization-data
       are present, they are decrypted using the sub-session key from the
       Authenticator.

       If any of the decryptions indicate failed integrity checks, the
       KRB_AP_ERR_BAD_INTEGRITY error is returned.

    3.3.3. Generation of KRB_TGS_REP message

       The KRB_TGS_REP message shares its format with the KRB_AS_REP
       (KRB_KDC_REP), but with its type field set to KRB_TGS_REP.  The
       detailed specification is in section 5.4.2.

       The response will include a ticket for the requested server.  The
       Kerberos database is queried to retrieve the record for the requested
       server (including the key with which the ticket will be encrypted).
       If the request is for a ticket granting ticket for a remote realm,
       and if no key is shared with the requested realm, then the Kerberos
       server will select the realm "closest" to the requested realm with
       which it does share a key, and use that realm instead. This is the
       only case where the response from the KDC will be for a different
       server than that requested by the client.

       By default, the address field, the client's name and realm, the list
       of transited realms, the time of initial authentication, the
       expiration time, and the authorization data of the newly-issued
       ticket will be copied from the ticket-granting ticket (TGT) or
       renewable ticket.  If the transited field needs to be updated, but
       the transited type is not supported, the KDC_ERR_TRTYPE_NOSUPP error
       is returned.

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       If the request specifies an endtime, then the endtime of the new
       ticket is set to the minimum of (a) that request, (b) the endtime
       from the TGT, and (c) the starttime of the TGT plus the minimum of
       the maximum life for the application server and the maximum life for
       the local realm (the maximum life for the requesting principal was
       already applied when the TGT was issued).  If the new ticket is to be
       a renewal, then the endtime above is replaced by the minimum of (a)
       the value of the renew_till field of the ticket and (b) the starttime
       for the new ticket plus the life (endtimestarttime) of the old
       ticket.

       If the FORWARDED option has been requested, then the resulting ticket
       will contain the addresses specified by the client.  This option will
       only be honored if the FORWARDABLE flag is set in the TGT.  The PROXY
       option is similar; the resulting ticket will contain the addresses
       specified by the client.  It will be honored only if the PROXIABLE
       flag in the TGT is set.  The PROXY option will not be honored on
       requests for additional ticket-granting tickets.

       If the requested start time is absent or indicates a time in the
       past, then the start time of the ticket is set to the authentication
       server's current time.  If it indicates a time in the future, but the
       POSTDATED option has not been specified or the MAY-POSTDATE flag is
       not set in the TGT, then the error KDC_ERR_CANNOT_POSTDATE is
       returned.  Otherwise, if the ticket-granting ticket has the
       MAYPOSTDATE flag set, then the resulting ticket will be postdated and
       the requested starttime is checked against the policy of the local
       realm. If acceptable, the ticket's start time is set as requested,
       and the INVALID flag is set.  The postdated ticket must be validated
       before use by presenting it to the KDC after the starttime has been
       reached. However, in no case may the starttime, endtime, or renew-
       till time of a newly-issued postdated ticket extend beyond the
       renew-till time of the ticket-granting ticket.

       If the ENC-TKT-IN-SKEY option has been specified and an additional
       ticket has been included in the request, the KDC will decrypt the
       additional ticket using the key for the server to which the
       additional ticket was issued and verify that it is a ticket-granting
       ticket.  If the name of the requested server is missing from the
       request, the name of the client in the additional ticket will be
       used.  Otherwise the name of the requested server will be compared to
       the name of the client in the additional ticket and if different, the
       request will be rejected.  If the request succeeds, the session key
       from the additional ticket will be used to encrypt the new ticket
       that is issued instead of using the key of the server for which the
       new ticket will be used (This allows easy implementation of user-to-
       user authentication [6], which uses ticket-granting ticket session
       keys in lieu of secret server keys in situations where such secret

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       keys could be easily compromised.).

       If the name of the server in the ticket that is presented to the KDC
       as part of the authentication header is not that of the ticket-
       granting server itself, and the server is registered in the realm of
       the KDC, If the RENEW option is requested, then the KDC will verify
       that the RENEWABLE flag is set in the ticket and that the renew_till
       time is still in the future.  If the VALIDATE option is rqeuested,
       the KDC will check that the starttime has passed and the INVALID flag
       is set.  If the PROXY option is requested, then the KDC will check
       that the PROXIABLE flag is set in the ticket.  If the tests succeed,
       the KDC will issue the appropriate new ticket.

       Whenever a request is made to the ticket-granting server, the
       presented ticket(s) is(are) checked against a hot-list of tickets
       which have been canceled.  This hot-list might be implemented by
       storing a range of issue dates for "suspect tickets"; if a presented
       ticket had an authtime in that range, it would be rejected.  In this
       way, a stolen ticket-granting ticket or renewable ticket cannot be
       used to gain additional tickets (renewals or otherwise) once the
       theft has been reported.  Any normal ticket obtained before it was
       reported stolen will still be valid (because they require no
       interaction with the KDC), but only until their normal expiration
       time.

       The ciphertext part of the response in the KRB_TGS_REP message is
       encrypted in the sub-session key from the Authenticator, if present,
       or the session key key from the ticket-granting ticket.  It is not
       encrypted using the client's secret key.  Furthermore, the client's
       key's expiration date and the key version number fields are left out
       since these values are stored along with the client's database
       record, and that record is not needed to satisfy a request based on a
       ticket-granting ticket.  See section A.6 for pseudocode.

    3.3.3.1.  Encoding the transited field

       If the identity of the server in the TGT that is presented to the KDC
       as part of the authentication header is that of the ticket-granting
       service, but the TGT was issued from another realm, the KDC will look
       up the inter-realm key shared with that realm and use that key to
       decrypt the ticket.  If the ticket is valid, then the KDC will honor
       the request, subject to the constraints outlined above in the section
       describing the AS exchange.  The realm part of the client's identity
       will be taken from the ticket-granting ticket.  The name of the realm
       that issued the ticket-granting ticket will be added to the transited
       field of the ticket to be issued.  This is accomplished by reading
       the transited field from the ticket-granting ticket (which is treated
       as an unordered set of realm names), adding the new realm to the set,

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       then constructing and writing out its encoded (shorthand) form (this
       may involve a rearrangement of the existing encoding).

       Note that the ticket-granting service does not add the name of its
       own realm.  Instead, its responsibility is to add the name of the
       previous realm.  This prevents a malicious Kerberos server from
       intentionally leaving out its own name (it could, however, omit other
       realms' names).

       The names of neither the local realm nor the principal's realm are to
       be included in the transited field.  They appear elsewhere in the
       ticket and both are known to have taken part in authenticating the
       principal.  Since the endpoints are not included, both local and
       single-hop inter-realm authentication result in a transited field
       that is empty.

       Because the name of each realm transited  is  added  to this field,
       it might potentially be very long.  To decrease the length of this
       field, its contents are encoded.  The initially supported encoding is
       optimized for the normal case of inter-realm communication: a
       hierarchical arrangement of realms using either domain or X.500 style
       realm names. This encoding (called DOMAIN-X500-COMPRESS) is now
       described.

       Realm names in the transited field are separated by a ",".  The ",",
       "\", trailing "."s, and leading spaces (" ") are special characters,
       and if they are part of a realm name, they must be quoted in the
       transited field by preceding them with a "\".

       A realm name ending with a "." is interpreted as  being prepended to
       the previous realm.  For example, we can encode traversal of EDU,
       MIT.EDU,  ATHENA.MIT.EDU,  WASHINGTON.EDU, and CS.WASHINGTON.EDU as:

                  "EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".

       Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were endpoints,
       that they would not be included in this field, and we would have:

                  "EDU,MIT.,WASHINGTON.EDU"

       A realm name beginning with a "/" is interpreted as being appended to
       the previous realm (For the purpose of appending, the realm preceding
       the first listed realm is considered to be the null realm ("")).  If
       it is to stand by itself, then it should be preceded by a space ("
       ").  For example, we can encode traversal of /COM/HP/APOLLO, /COM/HP,
       /COM, and /COM/DEC as:

                  "/COM,/HP,/APOLLO, /COM/DEC".

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       Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints,
       they they would not be included in this field, and we would have:

                  "/COM,/HP"

       A null subfield preceding or following a "," indicates that all
       realms between the previous realm and the next realm have been
       traversed (For the purpose of interpreting null subfields, the
       client's realm is considered to precede those in the transited field,
       and the server's realm is considered to follow them.). Thus, ","
       means that all realms along the path between the client and the
       server have been traversed.  ",EDU, /COM," means that that all realms
       from the client's realm up to EDU (in a domain style hierarchy) have
       been traversed, and that everything from /COM down to the server's
       realm in an X.500 style has also been traversed.  This could occur if
       the EDU realm in one hierarchy shares an inter-realm key directly
       with the /COM realm in another hierarchy.

    3.3.4. Receipt of KRB_TGS_REP message

       When the KRB_TGS_REP is received by the client, it is processed in
       the same manner as the KRB_AS_REP processing described above.  The
       primary difference is that the ciphertext part of the response must
       be decrypted using the session key from the ticket-granting ticket
       rather than the client's secret key.  See section A.7 for pseudocode.

    3.4.  The KRB_SAFE Exchange

       The KRB_SAFE message may be used by clients requiring the ability to
       detect modifications of messages they exchange.  It achieves this by
       including a keyed collisionproof checksum of the user data and some
       control information.  The checksum is keyed with an encryption key
       (usually the last key negotiated via subkeys, or the session key if
       no negotiation has occured).

    3.4.1. Generation of a KRB_SAFE message

       When an application wishes to send a KRB_SAFE message, it collects
       its data and the appropriate control information and computes a
       checksum over them.  The checksum algorithm should be some sort of
       keyed one-way hash function (such as the RSA-MD5-DES checksum
       algorithm specified in section 6.4.5, or the DES MAC), generated
       using the sub-session key if present, or the session key.  Different
       algorithms may be selected by changing the checksum type in the
       message.  Unkeyed or non-collision-proof checksums are not suitable
       for this use.

       The control information for the KRB_SAFE message includes both a

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       timestamp and a sequence number.  The designer of an application
       using the KRB_SAFE message must choose at least one of the two
       mechanisms.  This choice should be based on the needs of the
       application protocol.

       Sequence numbers are useful when all messages sent will be received
       by one's peer.  Connection state is presently required to maintain
       the session key, so maintaining the next sequence number should not
       present an additional problem.

       If the application protocol is expected to tolerate lost messages
       without them being resent, the use of the timestamp is the
       appropriate replay detection mechanism.  Using timestamps is also the
       appropriate mechanism for multi-cast protocols where all of one's
       peers share a common sub-session key, but some messages will be sent
       to a subset of one's peers.

       After computing the checksum, the client then transmits the
       information and checksum to the recipient in the message format
       specified in section 5.6.1.

    3.4.2. Receipt of KRB_SAFE message

       When an application receives a KRB_SAFE message, it verifies it as
       follows.  If any error occurs, an error code is reported for use by
       the application.

       The message is first checked by verifying that the protocol version
       and type fields match the current version and KRB_SAFE, respectively.
       A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
       error.  The application verifies that the checksum used is a
       collisionproof keyed checksum, and if it is not, a
       KRB_AP_ERR_INAPP_CKSUM error is generated.  The recipient verifies
       that the operating system's report of the sender's address matches
       the sender's address in the message, and (if a recipient address is
       specified or the recipient requires an address) that one of the
       recipient's addresses appears as the recipient's address in the
       message.  A failed match for either case generates a
       KRB_AP_ERR_BADADDR error.  Then the timestamp and usec and/or the
       sequence number fields are checked.  If timestamp and usec are
       expected and not present, or they are present but not current, the
       KRB_AP_ERR_SKEW error is generated.  If the server name, along with
       the client name, time and microsecond fields from the Authenticator
       match any recently-seen such tuples, the KRB_AP_ERR_REPEAT error is
       generated.  If an incorrect sequence number is included, or a
       sequence number is expected but not present, the KRB_AP_ERR_BADORDER
       error is generated.  If neither a timestamp and usec or a sequence
       number is present, a KRB_AP_ERR_MODIFIED error is generated.

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       Finally, the checksum is computed over the data and control
       information, and if it doesn't match the received checksum, a
       KRB_AP_ERR_MODIFIED error is generated.

       If all the checks succeed, the application is assured that the
       message was generated by its peer and was not modified in transit.

    3.5.  The KRB_PRIV Exchange

       The KRB_PRIV message may be used by clients requiring confidentiality
       and the ability to detect modifications of exchanged messages.  It
       achieves this by encrypting the messages and adding control
       information.

    3.5.1. Generation of a KRB_PRIV message

       When an application wishes to send a KRB_PRIV message, it collects
       its data and the appropriate control information (specified in
       section 5.7.1) and encrypts them under an encryption key (usually the
       last key negotiated via subkeys, or the session key if no negotiation
       has occured).  As part of the control information, the client must
       choose to use either a timestamp or a sequence number (or both); see
       the discussion in section 3.4.1 for guidelines on which to use.
       After the user data and control information are encrypted, the client
       transmits the ciphertext and some "envelope" information to the
       recipient.

    3.5.2. Receipt of KRB_PRIV message

       When an application receives a KRB_PRIV message, it verifies it as
       follows.  If any error occurs, an error code is reported for use by
       the application.

       The message is first checked by verifying that the protocol version
       and type fields match the current version and KRB_PRIV, respectively.
       A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
       error.  The application then decrypts the ciphertext and processes
       the resultant plaintext. If decryption shows the data to have been
       modified, a KRB_AP_ERR_BAD_INTEGRITY error is generated.  The
       recipient verifies that the operating system's report of the sender's
       address matches the sender's address in the message, and (if a
       recipient address is specified or the recipient requires an address)
       that one of the recipient's addresses appears as the recipient's
       address in the message.  A failed match for either case generates a
       KRB_AP_ERR_BADADDR error.  Then the timestamp and usec and/or the
       sequence number fields are checked. If timestamp and usec are
       expected and not present, or they are present but not current, the
       KRB_AP_ERR_SKEW error is generated.  If the server name, along with

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       the client name, time and microsecond fields from the Authenticator
       match any recently-seen such tuples, the KRB_AP_ERR_REPEAT error is
       generated.  If an incorrect sequence number is included, or a
       sequence number is expected but not present, the KRB_AP_ERR_BADORDER
       error is generated.  If neither a timestamp and usec or a sequence
       number is present, a KRB_AP_ERR_MODIFIED error is generated.

       If all the checks succeed, the application can assume the message was
       generated by its peer, and was securely transmitted (without
       intruders able to see the unencrypted contents).

    3.6.  The KRB_CRED Exchange

       The KRB_CRED message may be used by clients requiring the ability to
       send Kerberos credentials from one host to another.  It achieves this
       by sending the tickets together with encrypted data containing the
       session keys and other information associated with the tickets.

    3.6.1. Generation of a KRB_CRED message

       When an application wishes to send a KRB_CRED message it first (using
       the KRB_TGS exchange) obtains credentials to be sent to the remote
       host.  It then constructs a KRB_CRED message using the ticket or
       tickets so obtained, placing the session key needed to use each
       ticket in the key field of the corresponding KrbCredInfo sequence of
       the encrypted part of the the KRB_CRED message.

       Other information associated with each ticket and obtained during the
       KRB_TGS exchange is also placed in the corresponding KrbCredInfo
       sequence in the encrypted part of the KRB_CRED message.  The current
       time and, if specifically required by the application the nonce, s-
       address, and raddress fields, are placed in the encrypted part of the
       KRB_CRED message which is then encrypted under an encryption key
       previosuly exchanged in the KRB_AP exchange (usually the last key
       negotiated via subkeys, or the session key if no negotiation has
       occured).

    3.6.2. Receipt of KRB_CRED message

       When an application receives a KRB_CRED message, it verifies it.  If
       any error occurs, an error code is reported for use by the
       application.  The message is verified by checking that the protocol
       version and type fields match the current version and KRB_CRED,
       respectively.  A mismatch generates a KRB_AP_ERR_BADVERSION or
       KRB_AP_ERR_MSG_TYPE error.  The application then decrypts the
       ciphertext and processes the resultant plaintext. If decryption shows
       the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY error is
       generated.

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       If present or required, the recipient verifies that the operating
       system's report of the sender's address matches the sender's address
       in the message, and that one of the recipient's addresses appears as
       the recipient's address in the message.  A failed match for either
       case generates a KRB_AP_ERR_BADADDR error.  The timestamp and usec
       fields (and the nonce field if required) are checked next.  If the
       timestamp and usec are not present, or they are present but not
       current, the KRB_AP_ERR_SKEW error is generated.

       If all the checks succeed, the application stores each of the new
       tickets in its ticket cache together with the session key and other
       information in the corresponding KrbCredInfo sequence from the
       encrypted part of the KRB_CRED message.

    4.  The Kerberos Database

       The Kerberos server must have access to a database containing the
       principal identifiers and secret keys of principals to be
       authenticated (The implementation of the Kerberos server need not
       combine the database and the server on the same machine; it is
       feasible to store the principal database in, say, a network name
       service, as long as the entries stored therein are protected from
       disclosure to and modification by unauthorized parties.  However, we
       recommend against such strategies, as they can make system management
       and threat analysis quite complex.).

    4.1.  Database contents

       A database entry should contain at least the following fields:

       Field                Value

       name                 Principal's identifier
       key                  Principal's secret key
       p_kvno               Principal's key version
       max_life             Maximum lifetime for Tickets
       max_renewable_life   Maximum total lifetime for renewable
                            Tickets

       The name field is an encoding of the principal's identifier.  The key
       field contains an encryption key.  This key is the principal's secret
       key.  (The key can be encrypted before storage under a Kerberos
       "master key" to protect it in case the database is compromised but
       the master key is not.  In that case, an extra field must be added to
       indicate the master key version used, see below.) The p_kvno field is
       the key version number of the principal's secret key.  The max_life
       field contains the maximum allowable lifetime (endtime - starttime)
       for any Ticket issued for this principal.  The max_renewable_life

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       field contains the maximum allowable total lifetime for any renewable
       Ticket issued for this principal.  (See section 3.1 for a description
       of how these lifetimes are used in determining the lifetime of a
       given Ticket.)

       A server may provide KDC service to several realms, as long as the
       database representation provides a mechanism to distinguish between
       principal records with identifiers which differ only in the realm
       name.

       When an application server's key changes, if the change is routine
       (i.e.,  not the result of disclosure of the old key), the old key
       should be retained by the server until all tickets that had been
       issued using that key have expired.  Because of this, it is possible
       for several keys to be active for a single principal.  Ciphertext
       encrypted in a principal's key is always tagged with the version of
       the key that was used for encryption, to help the recipient find the
       proper key for decryption.

       When more than one key is active for a particular principal, the
       principal will have more than one record in the Kerberos database.
       The keys and key version numbers will differ between the records (the
       rest of the fields may or may not be the same). Whenever Kerberos
       issues a ticket, or responds to a request for initial authentication,
       the most recent key (known by the Kerberos server) will be used for
       encryption.  This is the key with the highest key version number.

    4.2.  Additional fields

       Project Athena's KDC implementation uses additional fields in its
       database:

       Field        Value

       K_kvno       Kerberos' key version
       expiration   Expiration date for entry
       attributes   Bit field of attributes
       mod_date     Timestamp of last modification
       mod_name     Modifying principal's identifier

       The K_kvno field indicates the key version of the Kerberos master key
       under which the principal's secret key is encrypted.

       After an entry's expiration date has passed, the KDC will return an
       error to any client attempting to gain tickets as or for the
       principal.  (A database may want to maintain two expiration dates:
       one for the principal, and one for the principal's current key.  This
       allows password aging to work independently of the principal's

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       expiration date.  However, due to the limited space in the responses,
       the KDC must combine the key expiration and principal expiration date
       into a single value called "key_exp", which is used as a hint to the
       user to take administrative action.)

       The attributes field is a bitfield used to govern the operations
       involving the principal.  This field might be useful in conjunction
       with user registration procedures, for site-specific policy
       implementations (Project Athena currently uses it for their user
       registration process controlled by the system-wide database service,
       Moira [7]), or to identify the "string to key" conversion algorithm
       used for a principal's key.  (See the discussion of the padata field
       in section 5.4.2 for details on why this can be useful.)  Other bits
       are used to indicate that certain ticket options should not be
       allowed in tickets encrypted under a principal's key (one bit each):
       Disallow issuing postdated tickets, disallow issuing forwardable
       tickets, disallow issuing tickets based on TGT authentication,
       disallow issuing renewable tickets, disallow issuing proxiable
       tickets, and disallow issuing tickets for which the principal is the
       server.

       The mod_date field contains the time of last modification of the
       entry, and the mod_name field contains the name of the principal
       which last modified the entry.

    4.3.  Frequently Changing Fields

       Some KDC implementations may wish to maintain the last time that a
       request was made by a particular principal.  Information that might
       be maintained includes the time of the last request, the time of the
       last request for a ticket-granting ticket, the time of the last use
       of a ticket-granting ticket, or other times.  This information can
       then be returned to the user in the last-req field (see section 5.2).

       Other frequently changing information that can be maintained is the
       latest expiration time for any tickets that have been issued using
       each key.  This field would be used to indicate how long old keys
       must remain valid to allow the continued use of outstanding tickets.

    4.4.  Site Constants

       The KDC implementation should have the following configurable
       constants or options, to allow an administrator to make and enforce
       policy decisions:

       + The minimum supported lifetime (used to determine whether the
          KDC_ERR_NEVER_VALID error should be returned). This constant
          should reflect reasonable expectations of round-trip time to the

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          KDC, encryption/decryption time, and processing time by the client
          and target server, and it should allow for a minimum "useful"
          lifetime.

       + The maximum allowable total (renewable) lifetime of a ticket
          (renew_till - starttime).

       + The maximum allowable lifetime of a ticket (endtime - starttime).

       + Whether to allow the issue of tickets with empty address fields
          (including the ability to specify that such tickets may only be
          issued if the request specifies some authorization_data).

       + Whether proxiable, forwardable, renewable or post-datable tickets
          are to be issued.

    5.  Message Specifications

       The following sections describe the exact contents and encoding of
       protocol messages and objects.  The ASN.1 base definitions are
       presented in the first subsection.  The remaining subsections specify
       the protocol objects (tickets and authenticators) and messages.
       Specification of encryption and checksum techniques, and the fields
       related to them, appear in section 6.

    5.1.  ASN.1 Distinguished Encoding Representation

       All uses of ASN.1 in Kerberos shall use the Distinguished Encoding
       Representation of the data elements as described in the X.509
       specification, section 8.7 [8].

    5.2.  ASN.1 Base Definitions

       The following ASN.1 base definitions are used in the rest of this
       section. Note that since the underscore character (_) is not
       permitted in ASN.1 names, the hyphen (-) is used in its place for the
       purposes of ASN.1 names.

       Realm ::=           GeneralString
       PrincipalName ::=   SEQUENCE {
                           name-type[0]     INTEGER,
                           name-string[1]   SEQUENCE OF GeneralString
       }

       Kerberos realms are encoded as GeneralStrings. Realms shall not
       contain a character with the code 0 (the ASCII NUL).  Most realms
       will usually consist of several components separated by periods (.),
       in the style of Internet Domain Names, or separated by slashes (/) in

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       the style of X.500 names.  Acceptable forms for realm names are
       specified in section 7.  A PrincipalName is a typed sequence of
       components consisting of the following sub-fields:

       name-type This field specifies the type of name that follows.
                 Pre-defined values for this field are
                 specified in section 7.2.  The name-type should be
                 treated as a hint.  Ignoring the name type, no two
                 names can be the same (i.e., at least one of the
                 components, or the realm, must be different).
                 This constraint may be eliminated in the future.

       name-string This field encodes a sequence of components that
                   form a name, each component encoded as a General
                   String.  Taken together, a PrincipalName and a Realm
                   form a principal identifier.  Most PrincipalNames
                   will have only a few components (typically one or two).

               KerberosTime ::=   GeneralizedTime
                                  -- Specifying UTC time zone (Z)

       The timestamps used in Kerberos are encoded as GeneralizedTimes.  An
       encoding shall specify the UTC time zone (Z) and shall not include
       any fractional portions of the seconds.  It further shall not include
       any separators.  Example: The only valid format for UTC time 6
       minutes, 27 seconds after 9 pm on 6 November 1985 is 19851106210627Z.

        HostAddress ::=     SEQUENCE  {
                            addr-type[0]             INTEGER,
                            address[1]               OCTET STRING
        }

        HostAddresses ::=   SEQUENCE OF SEQUENCE {
                            addr-type[0]             INTEGER,
                            address[1]               OCTET STRING
        }

       The host adddress encodings consists of two fields:

       addr-type  This field specifies the type of  address that
                  follows. Pre-defined values for this field are
                  specified in section 8.1.

       address   This field encodes a single address of type addr-type.

       The two forms differ slightly. HostAddress contains exactly one

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       address; HostAddresses contains a sequence of possibly many
       addresses.

       AuthorizationData ::=   SEQUENCE OF SEQUENCE {
                               ad-type[0]               INTEGER,
                               ad-data[1]               OCTET STRING
       }

       ad-data   This field contains authorization data to be
                 interpreted according to the value of the
                 corresponding ad-type field.

       ad-type   This field specifies the format for the ad-data
                 subfield.  All negative values are reserved for
                 local use.  Non-negative values are reserved for
                 registered use.

                       APOptions ::=   BIT STRING {
                                       reserved(0),
                                       use-session-key(1),
                                       mutual-required(2)
                       }

                       TicketFlags ::=   BIT STRING {
                                         reserved(0),
                                         forwardable(1),
                                         forwarded(2),
                                         proxiable(3),
                                         proxy(4),
                                         may-postdate(5),
                                         postdated(6),
                                         invalid(7),
                                         renewable(8),
                                         initial(9),
                                         pre-authent(10),
                                         hw-authent(11)
                       }

                      KDCOptions ::=   BIT STRING {
                                       reserved(0),
                                       forwardable(1),
                                       forwarded(2),
                                       proxiable(3),
                                       proxy(4),
                                       allow-postdate(5),
                                       postdated(6),

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                                       unused7(7),
                                       renewable(8),
                                       unused9(9),
                                       unused10(10),
                                       unused11(11),
                                       renewable-ok(27),
                                       enc-tkt-in-skey(28),
                                       renew(30),
                                       validate(31)
                      }

                LastReq ::=   SEQUENCE OF SEQUENCE {
                              lr-type[0]               INTEGER,
                              lr-value[1]              KerberosTime
                }

       lr-type   This field indicates how the following lr-value
                 field is to be interpreted.  Negative values indicate
                 that the information pertains only to the
                 responding server.  Non-negative values pertain to
                 all servers for the realm.

                 If the lr-type field is zero (0), then no information
                 is conveyed by the lr-value subfield.  If the
                 absolute value of the lr-type field is one (1),
                 then the lr-value subfield is the time of last
                 initial request for a TGT.  If it is two (2), then
                 the lr-value subfield is the time of last initial
                 request.  If it is three (3), then the lr-value
                 subfield is the time of issue for the newest
                 ticket-granting ticket used. If it is four (4),
                 then the lr-value subfield is the time of the last
                 renewal.  If it is five (5), then the lr-value
                 subfield is the time of last request (of any
                 type).

       lr-value  This field contains the time of the last request.
                 The time must be interpreted according to the contents
                 of the accompanying lr-type subfield.

       See section 6 for the definitions of Checksum, ChecksumType,
       EncryptedData, EncryptionKey, EncryptionType, and KeyType.

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    5.3.  Tickets and Authenticators

       This section describes the format and encryption parameters for
       tickets and authenticators.  When a ticket or authenticator is
       included in a protocol message it is treated as an opaque object.

    5.3.1. Tickets

       A ticket is a record that helps a client authenticate to a service.
       A Ticket contains the following information:

    Ticket ::=                    [APPLICATION 1] SEQUENCE {
                                  tkt-vno[0]                   INTEGER,
                                  realm[1]                     Realm,
                                  sname[2]                     PrincipalName,
                                  enc-part[3]                  EncryptedData
    }
    -- Encrypted part of ticket
    EncTicketPart ::=     [APPLICATION 3] SEQUENCE {
                          flags[0]             TicketFlags,
                          key[1]               EncryptionKey,
                          crealm[2]            Realm,
                          cname[3]             PrincipalName,
                          transited[4]         TransitedEncoding,
                          authtime[5]          KerberosTime,
                          starttime[6]         KerberosTime OPTIONAL,
                          endtime[7]           KerberosTime,
                          renew-till[8]        KerberosTime OPTIONAL,
                          caddr[9]             HostAddresses OPTIONAL,
                          authorization-data[10]   AuthorizationData OPTIONAL
    }
    -- encoded Transited field
    TransitedEncoding ::=         SEQUENCE {
                                  tr-type[0]  INTEGER, -- must be registered
                                  contents[1]          OCTET STRING
    }

       The encoding of EncTicketPart is encrypted in the key shared by
       Kerberos and the end server (the server's secret key).  See section 6
       for the format of the ciphertext.

       tkt-vno   This field specifies the version number for the ticket
                 format.  This document describes version number 5.

       realm     This field specifies the realm that issued a ticket.  It
                 also serves to identify the realm part of the server's
                 principal identifier.  Since a Kerberos server can only
                 issue tickets for servers within its realm, the two will

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                 always be identical.

       sname     This field specifies the name part of the server's
                 identity.

       enc-part  This field holds the encrypted encoding of the
                 EncTicketPart sequence.

       flags     This field indicates which of various options were used or
                 requested when the ticket was issued.  It is a bit-field,
                 where the selected options are indicated by the bit being
                 set (1), and the unselected options and reserved fields
                 being reset (0).  Bit 0 is the most significant bit.  The
                 encoding of the bits is specified in section 5.2.  The
                 flags are described in more detail above in section 2.  The
                 meanings of the flags are:

                 Bit(s)    Name        Description

                 0         RESERVED    Reserved for future expansion of this
                                       field.

                 1         FORWARDABLE The FORWARDABLE flag is normally only
                                       interpreted by the TGS, and can be
                                       ignored by end servers.  When set,
                                       this flag tells the ticket-granting
                                       server that it is OK to issue a new
                                       ticket- granting ticket with a
                                       different network address based on
                                       the presented ticket.

                 2         FORWARDED   When set, this flag indicates that
                                       the ticket has either been forwarded
                                       or was issued based on authentication
                                       involving a forwarded ticket-granting
                                       ticket.

                 3         PROXIABLE   The PROXIABLE flag is normally only
                                       interpreted by the TGS, and can be
                                       ignored by end servers. The PROXIABLE
                                       flag has an interpretation identical
                                       to that of the FORWARDABLE flag,
                                       except that the PROXIABLE flag tells
                                       the ticket-granting server that only
                                       non- ticket-granting tickets may be
                                       issued with different network
                                       addresses.

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                 4         PROXY      When set, this flag indicates that a
                                       ticket is a proxy.

                 5         MAY-POSTDATE The MAY-POSTDATE flag is normally
                                       only interpreted by the TGS, and can
                                       be ignored by end servers.  This flag
                                       tells the ticket-granting server that
                                       a post- dated ticket may be issued
                                       based on this ticket-granting ticket.

                 6         POSTDATED   This flag indicates that this ticket
                                       has been postdated.  The end-service
                                       can check the authtime field to see
                                       when the original authentication
                                       occurred.

                 7         INVALID     This flag indicates that a ticket is
                                       invalid, and it must be validated by
                                       the KDC before use.  Application
                                       servers must reject tickets which
                                       have this flag set.

                 8         RENEWABLE   The RENEWABLE flag is normally only
                                       interpreted by the TGS, and can
                                       usually be ignored by end servers
                                       (some particularly careful servers
                                       may wish to disallow renewable
                                       tickets).  A renewable ticket can be
                                       used to obtain a replacement ticket
                                       that expires at a later date.

                 9         INITIAL     This flag indicates that this ticket
                                       was issued using the AS protocol, and
                                       not issued based on a ticket-granting
                                       ticket.

                 10        PRE-AUTHENT This flag indicates that during
                                       initial authentication, the client
                                       was authenticated by the KDC before a
                                       ticket was issued.  The strength of
                                       the preauthentication method is not
                                       indicated, but is acceptable to the
                                       KDC.

                 11        HW-AUTHENT  This flag indicates that the protocol
                                       employed for initial authentication
                                       required the use of hardware expected
                                       to be possessed solely by the named

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                                       client.  The hardware authentication
                                       method is selected by the KDC and the
                                       strength of the method is not
                                       indicated.

                 12-31     RESERVED    Reserved for future use.

       key       This field exists in the ticket and the KDC response and is
                 used to pass the session key from Kerberos to the
                 application server and the client.  The field's encoding is
                 described in section 6.2.

       crealm    This field contains the name of the realm in which the
                 client is registered and in which initial authentication
                 took place.

       cname     This field contains the name part of the client's principal
                 identifier.

       transited This field lists the names of the Kerberos realms that took
                 part in authenticating the user to whom this ticket was
                 issued.  It does not specify the order in which the realms
                 were transited.  See section 3.3.3.1 for details on how
                 this field encodes the traversed realms.

       authtime  This field indicates the time of initial authentication for
                 the named principal.  It is the time of issue for the
                 original ticket on which this ticket is based.  It is
                 included in the ticket to provide additional information to
                 the end service, and  to provide  the necessary information
                 for implementation of a `hot list' service at the KDC.   An
                 end service that is particularly paranoid could refuse to
                 accept tickets for which the initial authentication
                 occurred "too far" in the past.

                 This field is also returned as part of the response from
                 the KDC.  When returned as part of the response to initial
                 authentication (KRB_AS_REP), this is the current time on
                 the Kerberos server (It is NOT recommended that this time
                 value be used to adjust the workstation's clock since the
                 workstation cannot reliably determine that such a
                 KRB_AS_REP actually came from the proper KDC in a timely
                 manner.).

       starttime This field in the ticket specifies the time after which the
                 ticket is valid.  Together with endtime, this field
                 specifies the life of the ticket.   If it is absent from
                 the ticket, its value should be treated as that of the

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                 authtime field.

       endtime   This field contains the time after which the ticket will
                 not be honored (its expiration time).  Note that individual
                 services may place their own limits on the life of a ticket
                 and may reject tickets which have not yet expired.  As
                 such, this is really an upper bound on the expiration time
                 for the ticket.

       renew-till This field is only present in tickets that have the
                 RENEWABLE flag set in the flags field.  It indicates the
                 maximum endtime that may be included in a renewal.  It can
                 be thought of as the absolute expiration time for the
                 ticket, including all renewals.

       caddr     This field in a ticket contains zero (if omitted) or more
                 (if present) host addresses.  These are the addresses from
                 which the ticket can be used.  If there are no addresses,
                 the ticket can be used from any location.  The decision
                 by the KDC to issue or by the end server to accept zero-
                 address tickets is a policy decision and is left to the
                 Kerberos and end-service administrators; they may refuse to
                 issue or accept such tickets.  The suggested and default
                 policy, however, is that such tickets will only be issued
                 or accepted when additional information that can be used to
                 restrict the use of the ticket is included in the
                 authorization_data field.  Such a ticket is a capability.

                 Network addresses are included in the ticket to make it
                 harder for an attacker to use stolen credentials. Because
                 the session key is not sent over the network in cleartext,
                 credentials can't be stolen simply by listening to the
                 network; an attacker has to gain access to the session key
                 (perhaps through operating system security breaches or a
                 careless user's unattended session) to make use of stolen
                 tickets.

                 It is important to note that the network address from which
                 a connection is received cannot be reliably determined.
                 Even if it could be, an attacker who has compromised the
                 client's workstation could use the credentials from there.
                 Including the network addresses only makes it more
                 difficult, not impossible, for an attacker to walk off with
                 stolen credentials and then use them from a "safe"
                 location.

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       authorization-data The authorization-data field is used to pass
                 authorization data from the principal on whose behalf a
                 ticket was issued to the application service.  If no
                 authorization data is included, this field will be left
                 out.  The data in this field are specific to the end
                 service.  It is expected that the field will contain the
                 names of service specific objects, and the rights to those
                 objects.  The format for this field is described in section
                 5.2.  Although Kerberos is not concerned with the format of
                 the contents of the subfields, it does carry type
                 information (ad-type).

                 By using the authorization_data field, a principal is able
                 to issue a proxy that is valid for a specific purpose.  For
                 example, a client wishing to print a file can obtain a file
                 server proxy to be passed to the print server.  By
                 specifying the name of the file in the authorization_data
                 field, the file server knows that the print server can only
                 use the client's rights when accessing the particular file
                 to be printed.

                 It is interesting to note that if one specifies the
                 authorization-data field of a proxy and leaves the host
                 addresses blank, the resulting ticket and session key can
                 be treated as a capability.  See [9] for some suggested
                 uses of this field.

                 The authorization-data field is optional and does not have
                 to be included in a ticket.

    5.3.2. Authenticators

       An authenticator is a record sent with a ticket to a server to
       certify the client's knowledge of the encryption key in the ticket,
       to help the server detect replays, and to help choose a "true session
       key" to use with the particular session.  The encoding is encrypted
       in the ticket's session key shared by the client and the server:

    -- Unencrypted authenticator
    Authenticator ::=    [APPLICATION 2] SEQUENCE    {
                   authenticator-vno[0]          INTEGER,
                   crealm[1]                     Realm,
                   cname[2]                      PrincipalName,
                   cksum[3]                      Checksum OPTIONAL,
                   cusec[4]                      INTEGER,
                   ctime[5]                      KerberosTime,
                   subkey[6]                     EncryptionKey OPTIONAL,
                   seq-number[7]                 INTEGER OPTIONAL,

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                   authorization-data[8]         AuthorizationData OPTIONAL
                         }

       authenticator-vno This field specifies the version number for the
                 format of the authenticator. This document specifies
                 version 5.

       crealm and cname These fields are the same as those described for the
                 ticket in section 5.3.1.

       cksum     This field contains a checksum of the the application data
                 that accompanies the KRB_AP_REQ.

       cusec     This field contains the microsecond part of the client's
                 timestamp.  Its value (before encryption) ranges from 0 to
                 999999.  It often appears along with ctime.  The two fields
                 are used together to specify a reasonably accurate
                 timestamp.

       ctime     This field contains the current time on the client's host.

       subkey    This field contains the client's choice for an encryption
                 key which is to be used to protect this specific
                 application session. Unless an application specifies
                 otherwise, if this field is left out the session key from
                 the ticket will be used.

       seq-number This optional field includes the initial sequence number
                 to be used by the KRB_PRIV or KRB_SAFE messages when
                 sequence numbers are used to detect replays (It may also be
                 used by application specific messages).  When included in
                 the authenticator this field specifies the initial sequence
                 number for messages from the client to the server.  When
                 included in the AP-REP message, the initial sequence number
                 is that for messages from the server to the client.  When
                 used in KRB_PRIV or KRB_SAFE messages, it is incremented by
                 one after each message is sent.

                 For sequence numbers to adequately support the detection of
                 replays they should be non-repeating, even across
                 connection boundaries. The initial sequence number should
                 be random and uniformly distributed across the full space
                 of possible sequence numbers, so that it cannot be guessed
                 by an attacker and so that it and the successive sequence
                 numbers do not repeat other sequences.

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       authorization-data This field is the same as described for the ticket
                 in section 5.3.1.  It is optional and will only appear when
                 additional restrictions are to be placed on the use of a
                 ticket, beyond those carried in the ticket itself.

    5.4.  Specifications for the AS and TGS exchanges

       This section specifies the format of the messages used in exchange
       between the client and the Kerberos server.  The format of possible
       error messages appears in section 5.9.1.

    5.4.1. KRB_KDC_REQ definition

       The KRB_KDC_REQ message has no type of its own.  Instead, its type is
       one of KRB_AS_REQ or KRB_TGS_REQ depending on whether the request is
       for an initial ticket or an additional ticket.  In either case, the
       message is sent from the client to the Authentication Server to
       request credentials for a service.

    The message fields are:

    AS-REQ ::=         [APPLICATION 10] KDC-REQ
    TGS-REQ ::=        [APPLICATION 12] KDC-REQ

    KDC-REQ ::=        SEQUENCE {
               pvno[1]               INTEGER,
               msg-type[2]           INTEGER,
               padata[3]             SEQUENCE OF PA-DATA OPTIONAL,
               req-body[4]           KDC-REQ-BODY
    }

    PA-DATA ::=        SEQUENCE {
               padata-type[1]        INTEGER,
               padata-value[2]       OCTET STRING,
                             -- might be encoded AP-REQ
    }

    KDC-REQ-BODY ::=   SEQUENCE {
                kdc-options[0]       KDCOptions,
                cname[1]             PrincipalName OPTIONAL,
                             -- Used only in AS-REQ
                realm[2]             Realm, -- Server's realm
                             -- Also client's in AS-REQ
                sname[3]             PrincipalName OPTIONAL,
                from[4]              KerberosTime OPTIONAL,
                till[5]              KerberosTime,
                rtime[6]             KerberosTime OPTIONAL,
                nonce[7]             INTEGER,

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                etype[8]             SEQUENCE OF INTEGER, -- EncryptionType,
                             -- in preference order
                addresses[9]         HostAddresses OPTIONAL,
                enc-authorization-data[10]   EncryptedData OPTIONAL,
                             -- Encrypted AuthorizationData encoding
                additional-tickets[11]       SEQUENCE OF Ticket OPTIONAL
    }

       The fields in this message are:

       pvno      This field is included in each message, and specifies the
                 protocol version number.  This document specifies protocol
                 version 5.

       msg-type  This field indicates the type of a protocol message.  It
                 will almost always be the same as the application
                 identifier associated with a message.  It is included to
                 make the identifier more readily accessible to the
                 application.  For the KDC-REQ message, this type will be
                 KRB_AS_REQ or KRB_TGS_REQ.

       padata    The padata (pre-authentication data) field contains a of
                 authentication information which may be needed before
                 credentials can be issued or decrypted.  In the case of
                 requests for additional tickets (KRB_TGS_REQ), this field
                 will include an element with padata-type of PA-TGS-REQ and
                 data of an authentication header (ticket-granting ticket
                 and authenticator). The checksum in the authenticator
                 (which must be collisionproof) is to be computed over the
                 KDC-REQ-BODY encoding.  In most requests for initial
                 authentication (KRB_AS_REQ) and most replies (KDC-REP), the
                 padata field will be left out.

                 This field may also contain information needed by certain
                 extensions to the Kerberos protocol.  For example, it might
                 be used to initially verify the identity of a client before
                 any response is returned.  This is accomplished with a
                 padata field with padata-type equal to PA-ENC-TIMESTAMP and
                 padata-value defined as follows:

       padata-type     ::= PA-ENC-TIMESTAMP
       padata-value    ::= EncryptedData -- PA-ENC-TS-ENC

       PA-ENC-TS-ENC   ::= SEQUENCE {
               patimestamp[0]               KerberosTime, -- client's time
               pausec[1]                    INTEGER OPTIONAL
       }

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                 with patimestamp containing the client's time and pausec
                 containing the microseconds which may be omitted if a
                 client will not generate more than one request per second.
                 The ciphertext (padata-value) consists of the PA-ENC-TS-ENC
                 sequence, encrypted using the client's secret key.

                 The padata field can also contain information needed to
                 help the KDC or the client select the key needed for
                 generating or decrypting the response.  This form of the
                 padata is useful for supporting the use of certain
                 "smartcards" with Kerberos.  The details of such extensions
                 are beyond the scope of this specification.  See [10] for
                 additional uses of this field.

       padata-type The padata-type element of the padata field indicates the
                 way that the padata-value element is to be interpreted.
                 Negative values of padata-type are reserved for
                 unregistered use; non-negative values are used for a
                 registered interpretation of the element type.

       req-body  This field is a placeholder delimiting the extent of the
                 remaining fields.  If a checksum is to be calculated over
                 the request, it is calculated over an encoding of the KDC-
                 REQ-BODY sequence which is enclosed within the req-body
                 field.

       kdc-options This field appears in the KRB_AS_REQ and KRB_TGS_REQ
                 requests to the KDC and indicates the flags that the client
                 wants set on the tickets as well as other information that
                 is to modify the behavior of the KDC. Where appropriate,
                 the name of an option may be the same as the flag that is
                 set by that option.  Although in most case, the bit in the
                 options field will be the same as that in the flags field,
                 this is not guaranteed, so it is not acceptable to simply
                 copy the options field to the flags field.  There are
                 various checks that must be made before honoring an option
                 anyway.

                 The kdc_options field is a bit-field, where the selected
                 options are indicated by the bit being set (1), and the
                 unselected options and reserved fields being reset (0).
                 The encoding of the bits is specified in section 5.2.  The
                 options are described in more detail above in section 2.
                 The meanings of the options are:

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                 Bit(s)  Name         Description

                 0       RESERVED     Reserved for future expansion of this
                                      field.

                 1       FORWARDABLE  The FORWARDABLE option indicates that
                                      the ticket to be issued is to have its
                                      forwardable flag set.  It may only be
                                      set on the initial request, or in a
                                      subsequent request if the ticket-
                                      granting ticket on which it is based
                                      is also forwardable.

                 2       FORWARDED    The FORWARDED option is only specified
                                      in a request to the ticket-granting
                                      server and will only be honored if the
                                      ticket-granting ticket in the request
                                      has its FORWARDABLE bit set.  This
                                      option indicates that this is a
                                      request for forwarding. The
                                      address(es) of the host from which the
                                      resulting ticket is to be valid are
                                      included in the addresses field of the
                                      request.

                 3       PROXIABLE    The PROXIABLE option indicates that
                                      the ticket to be issued is to have its
                                      proxiable flag set. It may only be set
                                      on the initial request, or in a
                                      subsequent request if the ticket-
                                      granting ticket on which it is based
                                      is also proxiable.

                 4       PROXY        The PROXY option indicates that this
                                      is a request for a proxy.  This option
                                      will only be honored if the ticket-
                                      granting ticket in the request has its
                                      PROXIABLE bit set.  The address(es) of
                                      the host from which the resulting
                                      ticket is to be valid are included in
                                      the addresses field of the request.

                 5       ALLOW-POSTDATE The ALLOW-POSTDATE option indicates
                                      that the ticket to be issued is to
                                      have its MAY-POSTDATE flag set.  It
                                      may only be set on the initial
                                      request, or in a subsequent request if

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                                      the ticket-granting ticket on which it
                                      is based also has its MAY-POSTDATE
                                      flag set.

                 6       POSTDATED    The POSTDATED option indicates that
                                      this is a request for a postdated
                                      ticket.  This option will only be
                                      honored if the ticket-granting ticket
                                      on which it is based has its MAY-
                                      POSTDATE flag set.  The resulting
                                      ticket will also have its INVALID flag
                                      set, and that flag may be reset by a
                                      subsequent request to the KDC after
                                      the starttime in the ticket has been
                                      reached.

                 7       UNUSED       This option is presently unused.

                 8       RENEWABLE    The RENEWABLE option indicates that
                                      the ticket to be issued is to have its
                                      RENEWABLE flag set.  It may only be
                                      set on the initial request, or when
                                      the ticket-granting ticket on which
                                      the request is based is also
                                      renewable.  If this option is
                                      requested, then the rtime field in the
                                      request contains the desired absolute
                                      expiration time for the ticket.

                 9-26    RESERVED     Reserved for future use.

                 27      RENEWABLE-OK The RENEWABLE-OK option indicates that
                                      a renewable ticket will be acceptable
                                      if a ticket with the requested life
                                      cannot otherwise be provided.  If a
                                      ticket with the requested life cannot
                                      be provided, then a renewable ticket
                                      may be issued with a renew-till equal
                                      to the the requested endtime.  The
                                      value of the renew-till field may
                                      still be limited by local limits, or
                                      limits selected by the individual
                                      principal or server.

                 28      ENC-TKT-IN-SKEY This option is used only by the
                                      ticket-granting service.  The ENC-
                                      TKT-IN-SKEY option indicates that the
                                      ticket for the end server is to be

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                                      encrypted in the session key from the
                                      additional ticket-granting ticket
                                      provided.

                 29      RESERVED     Reserved for future use.

                 30      RENEW        This option is used only by the
                                      ticket-granting service.  The RENEW
                                      option indicates that the present
                                      request is for a renewal.  The ticket
                                      provided is encrypted in the secret
                                      key for the server on which it is
                                      valid.  This option will only be
                                      honored if the ticket to be renewed
                                      has its RENEWABLE flag set and if the
                                      time in its renew till field has not
                                      passed.  The ticket to be renewed is
                                      passed in the padata field as part of
                                      the authentication header.

                 31      VALIDATE     This option is used only by the
                                      ticket-granting service.  The VALIDATE
                                      option indicates that the request is
                                      to validate a postdated ticket.  It
                                      will only be honored if the ticket
                                      presented is postdated, presently has
                                      its INVALID flag set, and would be
                                      otherwise usable at this time.  A
                                      ticket cannot be validated before its
                                      starttime.  The ticket presented for
                                      validation is encrypted in the key of
                                      the server for which it is valid and
                                      is passed in the padata field as part
                                      of the authentication header.

       cname and sname These fields are the same as those described for the
                 ticket in section 5.3.1.  sname may only be absent when the
                 ENC-TKT-IN-SKEY option is specified.  If absent, the name
                 of the server is taken from the name of the client in the
                 ticket passed as additional-tickets.

       enc-authorization-data The enc-authorization-data, if present (and it
                 can only be present in the TGS_REQ form), is an encoding of
                 the desired authorization-data encrypted under the sub-
                 session key if present in the Authenticator, or
                 alternatively from the session key in the ticket-granting
                 ticket, both from the padata field in the KRB_AP_REQ.

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       realm     This field specifies the realm part of the server's
                 principal identifier. In the AS exchange, this is also the
                 realm part of the client's principal identifier.

       from      This field is included in the KRB_AS_REQ and KRB_TGS_REQ
                 ticket requests when the requested ticket is to be
                 postdated.  It specifies the desired start time for the
                 requested ticket.

       till      This field contains the expiration date requested by the
                 client in a ticket request.

       rtime     This field is the requested renew-till time sent from a
                 client to the KDC in a ticket request.  It is optional.

       nonce     This field is part of the KDC request and response.  It it
                 intended to hold a random number generated by the client.
                 If the same number is included in the encrypted response
                 from the KDC, it provides evidence that the response is
                 fresh and has not been replayed by an attacker.  Nonces
                 must never be re-used.  Ideally, it should be gen erated
                 randomly, but if the correct time is known, it may suffice
                 (Note, however, that if the time is used as the nonce, one
                 must make sure that the workstation time is monotonically
                 increasing.  If the time is ever reset backwards, there is
                 a small, but finite, probability that a nonce will be
                 reused.).

       etype     This field specifies the desired encryption algorithm to be
                 used in the response.

       addresses This field is included in the initial request for tickets,
                 and optionally included in requests for additional tickets
                 from the ticket-granting server.  It specifies the
                 addresses from which the requested ticket is to be valid.
                 Normally it includes the addresses for the client's host.
                 If a proxy is requested, this field will contain other
                 addresses.  The contents of this field are usually copied
                 by the KDC into the caddr field of the resulting ticket.

       additional-tickets Additional tickets may be optionally included in a
                 request to the ticket-granting server.  If the ENC-TKT-IN-
                 SKEY option has been specified, then the session key from
                 the additional ticket will be used in place of the server's
                 key to encrypt the new ticket.  If more than one option
                 which requires additional tickets has been specified, then
                 the additional tickets are used in the order specified by
                 the ordering of the options bits (see kdc-options, above).

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       The application code will be either ten (10) or twelve (12) depending
       on whether the request is for an initial ticket (AS-REQ) or for an
       additional ticket (TGS-REQ).

       The optional fields (addresses, authorization-data and additional-
       tickets) are only included if necessary to perform the operation
       specified in the kdc-options field.

       It should be noted that in KRB_TGS_REQ, the protocol version number
       appears twice and two different message types appear: the KRB_TGS_REQ
       message contains these fields as does the authentication header
       (KRB_AP_REQ) that is passed in the padata field.

    5.4.2. KRB_KDC_REP definition

       The KRB_KDC_REP message format is used for the reply from the KDC for
       either an initial (AS) request or a subsequent (TGS) request.  There
       is no message type for KRB_KDC_REP.  Instead, the type will be either
       KRB_AS_REP or KRB_TGS_REP.  The key used to encrypt the ciphertext
       part of the reply depends on the message type.  For KRB_AS_REP, the
       ciphertext is encrypted in the client's secret key, and the client's
       key version number is included in the key version number for the
       encrypted data.  For KRB_TGS_REP, the ciphertext is encrypted in the
       sub-session key from the Authenticator, or if absent, the session key
       from the ticket-granting ticket used in the request.  In that case,
       no version number will be present in the EncryptedData sequence.

       The KRB_KDC_REP message contains the following fields:

       AS-REP ::=    [APPLICATION 11] KDC-REP
       TGS-REP ::=   [APPLICATION 13] KDC-REP

       KDC-REP ::=   SEQUENCE {
                     pvno[0]                    INTEGER,
                     msg-type[1]                INTEGER,
                     padata[2]                  SEQUENCE OF PA-DATA OPTIONAL,
                     crealm[3]                  Realm,
                     cname[4]                   PrincipalName,
                     ticket[5]                  Ticket,
                     enc-part[6]                EncryptedData
       }

       EncASRepPart ::=    [APPLICATION 25[25]] EncKDCRepPart
       EncTGSRepPart ::=   [APPLICATION 26] EncKDCRepPart

       EncKDCRepPart ::=   SEQUENCE {
                   key[0]                       EncryptionKey,
                   last-req[1]                  LastReq,

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                   nonce[2]                     INTEGER,
                   key-expiration[3]            KerberosTime OPTIONAL,
                   flags[4]                     TicketFlags,
                   authtime[5]                  KerberosTime,
                   starttime[6]                 KerberosTime OPTIONAL,
                   endtime[7]                   KerberosTime,
                   renew-till[8]                KerberosTime OPTIONAL,
                   srealm[9]                    Realm,
                   sname[10]                    PrincipalName,
                   caddr[11]                    HostAddresses OPTIONAL
       }

       NOTE: In EncASRepPart, the application code in the encrypted
             part of a message provides an additional check that
             the message was decrypted properly.

       pvno and msg-type These fields are described above in section 5.4.1.
                 msg-type is either KRB_AS_REP or KRB_TGS_REP.

       padata    This field is described in detail in section 5.4.1.  One
                 possible use for this field is to encode an alternate
                 "mix-in" string to be used with a string-to-key algorithm
                 (such as is described in section 6.3.2). This ability is
                 useful to ease transitions if a realm name needs to change
                 (e.g., when a company is acquired); in such a case all
                 existing password-derived entries in the KDC database would
                 be flagged as needing a special mix-in string until the
                 next password change.

       crealm, cname, srealm and sname These fields are the same as those
                 described for the ticket in section 5.3.1.

       ticket    The newly-issued ticket, from section 5.3.1.

       enc-part  This field is a place holder for the ciphertext and related
                 information that forms the encrypted part of a message.
                 The description of the encrypted part of the message
                 follows each appearance of this field.  The encrypted part
                 is encoded as described in section 6.1.

       key       This field is the same as described for the ticket in
                 section 5.3.1.

       last-req  This field is returned by the KDC and specifies the time(s)
                 of the last request by a principal.  Depending on what
                 information is available, this might be the last time that
                 a request for a ticket-granting ticket was made, or the
                 last time that a request based on a ticket-granting ticket

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                 was successful.  It also might cover all servers for a
                 realm, or just the particular server. Some implementations
                 may display this information to the user to aid in
                 discovering unauthorized use of one's identity.  It is
                 similar in spirit to the last login time displayed when
                 logging into timesharing systems.

       nonce     This field is described above in section 5.4.1.

       key-expiration The key-expiration field is part of the response from
                 the KDC and specifies the time that the client's secret key
                 is due to expire.  The expiration might be the result of
                 password aging or an account expiration.  This field will
                 usually be left out of the TGS reply since the response to
                 the TGS request is encrypted in a session key and no client
                 information need be retrieved from the KDC database.  It is
                 up to the application client (usually the login program) to
                 take appropriate action (such as notifying the user) if the
                 expira    tion time is imminent.

       flags, authtime, starttime, endtime, renew-till and caddr These
                 fields are duplicates of those found in the encrypted
                 portion of the attached ticket (see section 5.3.1),
                 provided so the client may verify they match the intended
                 request and to assist in proper ticket caching.  If the
                 message is of type KRB_TGS_REP, the caddr field will only
                 be filled in if the request was for a proxy or forwarded
                 ticket, or if the user is substituting a subset of the
                 addresses from the ticket granting ticket.  If the client-
                 requested addresses are not present or not used, then the
                 addresses contained in the ticket will be the same as those
                 included in the ticket-granting ticket.

    5.5.  Client/Server (CS) message specifications

       This section specifies the format of the messages used for the
       authentication of the client to the application server.

    5.5.1. KRB_AP_REQ definition

       The KRB_AP_REQ message contains the Kerberos protocol version number,
       the message type KRB_AP_REQ, an options field to indicate any options
       in use, and the ticket and authenticator themselves.  The KRB_AP_REQ
       message is often referred to as the "authentication header".

       AP-REQ ::=      [APPLICATION 14] SEQUENCE {
                       pvno[0]                       INTEGER,
                       msg-type[1]                   INTEGER,

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                       ap-options[2]                 APOptions,
                       ticket[3]                     Ticket,
                       authenticator[4]              EncryptedData
       }

       APOptions ::=   BIT STRING {
                       reserved(0),
                       use-session-key(1),
                       mutual-required(2)
       }

       pvno and msg-type These fields are described above in section 5.4.1.
                 msg-type is KRB_AP_REQ.

       ap-options This field appears in the application request (KRB_AP_REQ)
                 and affects the way the request is processed.  It is a
                 bit-field, where the selected options are indicated by the
                 bit being set (1), and the unselected options and reserved
                 fields being reset (0).  The encoding of the bits is
                 specified in section 5.2.  The meanings of the options are:

                 Bit(s)  Name           Description

                 0       RESERVED       Reserved for future expansion of
                                      this field.

                 1       USE-SESSION-KEYThe USE-SESSION-KEY option indicates
                                      that the ticket the client is
                                      presenting to a server is encrypted in
                                      the session key from the server's
                                      ticket-granting ticket. When this
                                      option is not specified, the ticket is
                                      encrypted in the server's secret key.

                 2       MUTUAL-REQUIREDThe MUTUAL-REQUIRED option tells the
                                      server that the client requires mutual
                                      authentication, and that it must
                                      respond with a KRB_AP_REP message.

                 3-31    RESERVED       Reserved for future use.

       ticket    This field is a ticket authenticating the client to the
                 server.

       authenticator This contains the authenticator, which includes the
                 client's choice of a subkey.  Its encoding is described in
                 section 5.3.2.

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    5.5.2.  KRB_AP_REP definition

       The KRB_AP_REP message contains the Kerberos protocol version number,
       the message type, and an encrypted timestamp. The message is sent in
       in response to an application request (KRB_AP_REQ) where the mutual
       authentication option has been selected in the ap-options field.

       AP-REP ::=         [APPLICATION 15] SEQUENCE {
                  pvno[0]                   INTEGER,
                  msg-type[1]               INTEGER,
                  enc-part[2]               EncryptedData
       }

       EncAPRepPart ::=   [APPLICATION 27]     SEQUENCE {
                  ctime[0]                  KerberosTime,
                  cusec[1]                  INTEGER,
                  subkey[2]                 EncryptionKey OPTIONAL,
                  seq-number[3]             INTEGER OPTIONAL
       }

       NOTE: in EncAPRepPart, the application code in the encrypted part of
       a message provides an additional check that the message was decrypted
       properly.

       The encoded EncAPRepPart is encrypted in the shared session key of
       the ticket.  The optional subkey field can be used in an
       application-arranged negotiation to choose a per association session
       key.

       pvno and msg-type These fields are described above in section 5.4.1.
                 msg-type is KRB_AP_REP.

       enc-part  This field is described above in section 5.4.2.

       ctime     This field contains the current time on the client's host.

       cusec     This field contains the microsecond part of the client's
                 timestamp.

       subkey    This field contains an encryption key which is to be used
                 to protect this specific application session.  See section
                 3.2.6 for specifics on how this field is used to negotiate
                 a key.  Unless an application specifies otherwise, if this
                 field is left out, the sub-session key from the
                 authenticator, or if also left out, the session key from
                 the ticket will be used.

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    5.5.3. Error message reply

       If an error occurs while processing the application request, the
       KRB_ERROR message will be sent in response.  See section 5.9.1 for
       the format of the error message.  The cname and crealm fields may be
       left out if the server cannot determine their appropriate values from
       the corresponding KRB_AP_REQ message.  If the authenticator was
       decipherable, the ctime and cusec fields will contain the values from
       it.

    5.6.  KRB_SAFE message specification

       This section specifies the format of a message that can be used by
       either side (client or server) of an application to send a tamper-
       proof message to its peer. It presumes that a session key has
       previously been exchanged (for example, by using the
       KRB_AP_REQ/KRB_AP_REP messages).

    5.6.1. KRB_SAFE definition

       The KRB_SAFE message contains user data along with a collision-proof
       checksum keyed with the session key.  The message fields are:

       KRB-SAFE ::=        [APPLICATION 20] SEQUENCE {
                   pvno[0]               INTEGER,
                   msg-type[1]           INTEGER,
                   safe-body[2]          KRB-SAFE-BODY,
                   cksum[3]              Checksum
       }

       KRB-SAFE-BODY ::=   SEQUENCE {
                   user-data[0]          OCTET STRING,
                   timestamp[1]          KerberosTime OPTIONAL,
                   usec[2]               INTEGER OPTIONAL,
                   seq-number[3]         INTEGER OPTIONAL,
                   s-address[4]          HostAddress,
                   r-address[5]          HostAddress OPTIONAL
       }

       pvno and msg-type These fields are described above in section 5.4.1.
                 msg-type is KRB_SAFE.

       safe-body This field is a placeholder for the body of the KRB-SAFE
                 message.  It is to be encoded separately and then have the
                 checksum computed over it, for use in the cksum field.

       cksum     This field contains the checksum of the application data.
                 Checksum details are described in section 6.4.  The

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                 checksum is computed over the encoding of the KRB-SAFE-BODY
                 sequence.

       user-data This field is part of the KRB_SAFE and KRB_PRIV messages
                 and contain the application specific data that is being
                 passed from the sender to the recipient.

       timestamp This field is part of the KRB_SAFE and KRB_PRIV messages.
                 Its contents are the current time as known by the sender of
                 the message. By checking the timestamp, the recipient of
                 the message is able to make sure that it was recently
                 generated, and is not a replay.

       usec      This field is part of the KRB_SAFE and KRB_PRIV headers.
                 It contains the microsecond part of the timestamp.

       seq-number This field is described above in section 5.3.2.

       s-address This field specifies the address in use by the sender of
                 the message.

       r-address This field specifies the address in use by the recipient of
                 the message.  It may be omitted for some uses (such as
                 broadcast protocols), but the recipient may arbitrarily
                 reject such messages.  This field along with s-address can
                 be used to help detect messages which have been incorrectly
                 or maliciously delivered to the wrong recipient.

    5.7.  KRB_PRIV message specification

       This section specifies the format of a message that can be used by
       either side (client or server) of an application to securely and
       privately send a message to its peer.  It presumes that a session key
       has previously been exchanged (for example, by using the
       KRB_AP_REQ/KRB_AP_REP messages).

    5.7.1. KRB_PRIV definition

       The KRB_PRIV message contains user data encrypted in the Session Key.
       The message fields are:

       KRB-PRIV ::=         [APPLICATION 21] SEQUENCE {
                    pvno[0]                   INTEGER,
                    msg-type[1]               INTEGER,
                    enc-part[3]               EncryptedData
       }

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       EncKrbPrivPart ::=   [APPLICATION 28] SEQUENCE {
                    user-data[0]              OCTET STRING,
                    timestamp[1]              KerberosTime OPTIONAL,
                    usec[2]                   INTEGER OPTIONAL,
                    seq-number[3]             INTEGER OPTIONAL,
                    s-address[4]              HostAddress, -- sender's addr
                    r-address[5]              HostAddress OPTIONAL
                                                          -- recip's addr
       }

       NOTE: In EncKrbPrivPart, the application code in the encrypted part
       of a message provides an additional check that the message was
       decrypted properly.

       pvno and msg-type These fields are described above in section 5.4.1.
                 msg-type is KRB_PRIV.

       enc-part  This field holds an encoding of the EncKrbPrivPart sequence
                 encrypted under the session key (If supported by the
                 encryption method in use, an initialization vector may be
                 passed to the encryption procedure, in order to achieve
                 proper cipher chaining.  The initialization vector might
                 come from the last block of the ciphertext from the
                 previous KRB_PRIV message, but it is the application's
                 choice whether or not to use such an initialization vector.
                 If left out, the default initialization vector for the
                 encryption algorithm will be used.).  This encrypted
                 encoding is used for the enc-part field of the KRB-PRIV
                 message.  See section 6 for the format of the ciphertext.

       user-data, timestamp, usec, s-address and r-address These fields are
                 described above in section 5.6.1.

       seq-number This field is described above in section 5.3.2.

    5.8.  KRB_CRED message specification

       This section specifies the format of a message that can be used to
       send Kerberos credentials from one principal to another.  It is
       presented here to encourage a common mechanism to be used by
       applications when forwarding tickets or providing proxies to
       subordinate servers.  It presumes that a session key has already been
       exchanged perhaps by using the KRB_AP_REQ/KRB_AP_REP messages.

    5.8.1. KRB_CRED definition

       The KRB_CRED message contains a sequence of tickets to be sent and
       information needed to use the tickets, including the session key from

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       each.  The information needed to use the tickets is encryped under an
       encryption key previously exchanged.  The message fields are:

       KRB-CRED         ::= [APPLICATION 22]   SEQUENCE {
                        pvno[0]                INTEGER,
                        msg-type[1]            INTEGER, -- KRB_CRED
                        tickets[2]             SEQUENCE OF Ticket,
                        enc-part[3]            EncryptedData
       }

       EncKrbCredPart   ::= [APPLICATION 29]   SEQUENCE {
                        ticket-info[0]         SEQUENCE OF KrbCredInfo,
                        nonce[1]               INTEGER OPTIONAL,
                        timestamp[2]           KerberosTime OPTIONAL,
                        usec[3]                INTEGER OPTIONAL,
                        s-address[4]           HostAddress OPTIONAL,
                        r-address[5]           HostAddress OPTIONAL
       }

       KrbCredInfo      ::=                    SEQUENCE {
                        key[0]                 EncryptionKey,
                        prealm[1]              Realm OPTIONAL,
                        pname[2]               PrincipalName OPTIONAL,
                        flags[3]               TicketFlags OPTIONAL,
                        authtime[4]            KerberosTime OPTIONAL,
                        starttime[5]           KerberosTime OPTIONAL,
                        endtime[6]             KerberosTime OPTIONAL
                        renew-till[7]          KerberosTime OPTIONAL,
                        srealm[8]              Realm OPTIONAL,
                        sname[9]               PrincipalName OPTIONAL,
                        caddr[10]              HostAddresses OPTIONAL
       }

       pvno and msg-type These fields are described above in section 5.4.1.
                 msg-type is KRB_CRED.

       tickets
                   These are the tickets obtained from the KDC specifically
                 for use by the intended recipient.  Successive tickets are
                 paired with the corresponding KrbCredInfo sequence from the
                 enc-part of the KRB-CRED message.

       enc-part  This field holds an encoding of the EncKrbCredPart sequence
                 encrypted under the session key shared between the sender
                 and the intended recipient.  This encrypted encoding is
                 used for the enc-part field of the KRB-CRED message.  See
                 section 6 for the format of the ciphertext.

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       nonce     If practical, an application may require the inclusion of a
                 nonce generated by the recipient of the message. If the
                 same value is included as the nonce in the message, it
                 provides evidence that the message is fresh and has not
                 been replayed by an attacker.  A nonce must never be re-
                 used; it should be generated randomly by the recipient of
                 the message and provided to the sender of the mes  sage in
                 an application specific manner.

       timestamp and usec These fields specify the time that the KRB-CRED
                 message was generated.  The time is used to provide
                 assurance that the message is fresh.

       s-address and r-address These fields are described above in section
                 5.6.1.  They are used optionally to provide additional
                 assurance of the integrity of the KRB-CRED message.

       key       This field exists in the corresponding ticket passed by the
                 KRB-CRED message and is used to pass the session key from
                 the sender to the intended recipient.  The field's encoding
                 is described in section 6.2.

       The following fields are optional.   If present, they can be
       associated with the credentials in the remote ticket file.  If left
       out, then it is assumed that the recipient of the credentials already
       knows their value.

       prealm and pname The name and realm of the delegated principal
                 identity.

       flags, authtime,  starttime,  endtime, renew-till,  srealm, sname,
                 and caddr These fields contain the values of the
                 corresponding fields from the ticket found in the ticket
                 field.  Descriptions of the fields are identical to the
                 descriptions in the KDC-REP message.

    5.9.  Error message specification

       This section specifies the format for the KRB_ERROR message.  The
       fields included in the message are intended to return as much
       information as possible about an error.  It is not expected that all
       the information required by the fields will be available for all
       types of errors.  If the appropriate information is not available
       when the message is composed, the corresponding field will be left
       out of the message.

       Note that since the KRB_ERROR message is not protected by any
       encryption, it is quite possible for an intruder to synthesize or

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       modify such a message.  In particular, this means that the client
       should not use any fields in this message for security-critical
       purposes, such as setting a system clock or generating a fresh
       authenticator.  The message can be useful, however, for advising a
       user on the reason for some failure.

    5.9.1. KRB_ERROR definition

       The KRB_ERROR message consists of the following fields:

       KRB-ERROR ::=   [APPLICATION 30] SEQUENCE {
                       pvno[0]               INTEGER,
                       msg-type[1]           INTEGER,
                       ctime[2]              KerberosTime OPTIONAL,
                       cusec[3]              INTEGER OPTIONAL,
                       stime[4]              KerberosTime,
                       susec[5]              INTEGER,
                       error-code[6]         INTEGER,
                       crealm[7]             Realm OPTIONAL,
                       cname[8]              PrincipalName OPTIONAL,
                       realm[9]              Realm, -- Correct realm
                       sname[10]             PrincipalName, -- Correct name
                       e-text[11]            GeneralString OPTIONAL,
                       e-data[12]            OCTET STRING OPTIONAL
       }

       pvno and msg-type These fields are described above in section 5.4.1.
                 msg-type is KRB_ERROR.

       ctime     This field is described above in section 5.4.1.

       cusec     This field is described above in section 5.5.2.

       stime     This field contains the current time on the server.  It is
                 of type KerberosTime.

       susec     This field contains the microsecond part of the server's
                 timestamp.  Its value ranges from 0 to 999. It appears
                 along with stime. The two fields are used in conjunction to
                 specify a reasonably accurate timestamp.

       error-code This field contains the error code returned by Kerberos or
                 the server when a request fails.  To interpret the value of
                 this field see the list of error codes in section 8.
                 Implementations are encouraged to provide for national
                 language support in the display of error messages.

       crealm, cname, srealm and sname These fields are described above in

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                 section 5.3.1.

       e-text    This field contains additional text to help explain the
                 error code associated with the failed request (for example,
                 it might include a principal name which was unknown).

       e-data    This field contains additional data about the error for use
                 by the application to help it recover from or handle the
                 error.  If the errorcode is KDC_ERR_PREAUTH_REQUIRED, then
                 the e-data field will contain an encoding of a sequence of
                 padata fields, each corresponding to an acceptable pre-
                 authentication method and optionally containing data for
                 the method:

          METHOD-DATA ::=    SEQUENCE of PA-DATA

       If the error-code is KRB_AP_ERR_METHOD, then the e-data field will
       contain an encoding of the following sequence:

          METHOD-DATA ::=    SEQUENCE {
                             method-type[0]   INTEGER,
                             method-data[1]   OCTET STRING OPTIONAL
           }

       method-type will indicate the required alternate method; method-data
       will contain any required additional information.

    6.  Encryption and Checksum Specifications

       The Kerberos protocols described in this document are designed to use
       stream encryption ciphers, which can be simulated using commonly
       available block encryption ciphers, such as the Data Encryption
       Standard [11], in conjunction with block chaining and checksum
       methods [12].  Encryption is used to prove the identities of the
       network entities participating in message exchanges.  The Key
       Distribution Center for each realm is trusted by all principals
       registered in that realm to store a secret key in confidence.  Proof
       of knowledge of this secret key is used to verify the authenticity of
       a principal.

       The KDC uses the principal's secret key (in the AS exchange) or a
       shared session key (in the TGS exchange) to encrypt responses to
       ticket requests; the ability to obtain the secret key or session key
       implies the knowledge of the appropriate keys and the identity of the
       KDC. The ability of a principal to decrypt the KDC response and
       present a Ticket and a properly formed Authenticator (generated with
       the session key from the KDC response) to a service verifies the
       identity of the principal; likewise the ability of the service to

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       extract the session key from the Ticket and prove its knowledge
       thereof in a response verifies the identity of the service.

       The Kerberos protocols generally assume that the encryption used is
       secure from cryptanalysis; however, in some cases, the order of
       fields in the encrypted portions of messages are arranged to minimize
       the effects of poorly chosen keys.  It is still important to choose
       good keys.  If keys are derived from user-typed passwords, those
       passwords need to be well chosen to make brute force attacks more
       difficult.  Poorly chosen keys still make easy targets for intruders.

       The following sections specify the encryption and checksum mechanisms
       currently defined for Kerberos.  The encodings, chaining, and padding
       requirements for each are described.  For encryption methods, it is
       often desirable to place random information (often referred to as a
       confounder) at the start of the message.  The requirements for a
       confounder are specified with each encryption mechanism.

       Some encryption systems use a block-chaining method to improve the
       the security characteristics of the ciphertext.  However, these
       chaining methods often don't provide an integrity check upon
       decryption.  Such systems (such as DES in CBC mode) must be augmented
       with a checksum of the plaintext which can be verified at decryption
       and used to detect any tampering or damage.  Such checksums should be
       good at detecting burst errors in the input.  If any damage is
       detected, the decryption routine is expected to return an error
       indicating the failure of an integrity check. Each encryption type is
       expected to provide and verify an appropriate checksum. The
       specification of each encryption method sets out its checksum
       requirements.

       Finally, where a key is to be derived from a user's password, an
       algorithm for converting the password to a key of the appropriate
       type is included.  It is desirable for the string to key function to
       be one-way, and for the mapping to be different in different realms.
       This is important because users who are registered in more than one
       realm will often use the same password in each, and it is desirable
       that an attacker compromising the Kerberos server in one realm not
       obtain or derive the user's key in another.

       For a discussion of the integrity characteristics of the candidate
       encryption and checksum methods considered for Kerberos, the the
       reader is referred to [13].

    6.1.  Encryption Specifications

       The following ASN.1 definition describes all encrypted messages.  The
       enc-part field which appears in the unencrypted part of messages in

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       section 5 is a sequence consisting of an encryption type, an optional
       key version number, and the ciphertext.

       EncryptedData ::=   SEQUENCE {
                           etype[0]     INTEGER, -- EncryptionType
                           kvno[1]      INTEGER OPTIONAL,
                           cipher[2]    OCTET STRING -- ciphertext
       }

       etype     This field identifies which encryption algorithm was used
                 to encipher the cipher.  Detailed specifications for
                 selected encryption types appear later in this section.

       kvno      This field contains the version number of the key under
                 which data is encrypted.  It is only present in messages
                 encrypted under long lasting keys, such as principals'
                 secret keys.

       cipher    This field contains the enciphered text, encoded as an
                 OCTET STRING.

       The cipher field is generated by applying the specified encryption
       algorithm to data composed of the message and algorithm-specific
       inputs.  Encryption mechanisms defined for use with Kerberos must
       take sufficient measures to guarantee the integrity of the plaintext,
       and we recommend they also take measures to protect against
       precomputed dictionary attacks.  If the encryption algorithm is not
       itself capable of doing so, the protections can often be enhanced by
       adding a checksum and a confounder.

       The suggested format for the data to be encrypted includes a
       confounder, a checksum, the encoded plaintext, and any necessary
       padding.  The msg-seq field contains the part of the protocol message
       described in section 5 which is to be encrypted.  The confounder,
       checksum, and padding are all untagged and untyped, and their length
       is exactly sufficient to hold the appropriate item.  The type and
       length is implicit and specified by the particular encryption type
       being used (etype).  The format for the data to be encrypted is
       described in the following diagram:

             +-----------+----------+-------------+-----+
             |confounder |   check  |   msg-seq   | pad |
             +-----------+----------+-------------+-----+

       The format cannot be described in ASN.1, but for those who prefer an
       ASN.1-like notation:

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    CipherText ::=   ENCRYPTED       SEQUENCE {
             confounder[0]   UNTAGGED OCTET STRING(conf_length)     OPTIONAL,
             check[1]        UNTAGGED OCTET STRING(checksum_length) OPTIONAL,
             msg-seq[2]      MsgSequence,
             pad             UNTAGGED OCTET STRING(pad_length) OPTIONAL
    }

       In the above specification, UNTAGGED OCTET STRING(length) is the
       notation for an octet string with its tag and length removed.  It is
       not a valid ASN.1 type.  The tag bits and length must be removed from
       the confounder since the purpose of the confounder is so that the
       message starts with random data, but the tag and its length are
       fixed.  For other fields, the length and tag would be redundant if
       they were included because they are specified by the encryption type.

       One generates a random confounder of the appropriate length, placing
       it in confounder; zeroes out check; calculates the appropriate
       checksum over confounder, check, and msg-seq, placing the result in
       check; adds the necessary padding; then encrypts using the specified
       encryption type and the appropriate key.

       Unless otherwise specified, a definition of an encryption algorithm
       that specifies a checksum, a length for the confounder field, or an
       octet boundary for padding uses this ciphertext format (The ordering
       of the fields in the CipherText is important.  Additionally, messages
       encoded in this format must include a length as part of the msg-seq
       field.  This allows the recipient to verify that the message has not
       been truncated.  Without a length, an attacker could use a chosen
       plaintext attack to generate a message which could be truncated,
       while leaving the checksum intact.  Note that if the msg-seq is an
       encoding of an ASN.1 SEQUENCE or OCTET STRING, then the length is
       part of that encoding.). Those fields which are not specified will be
       omitted.

       In the interest of allowing all implementations using a particular
       encryption type to communicate with all others using that type, the
       specification of an encryption type defines any checksum that is
       needed as part of the encryption process.  If an alternative checksum
       is to be used, a new encryption type must be defined.

       Some cryptosystems require additional information beyond the key and
       the data to be encrypted. For example, DES, when used in cipher-
       block-chaining mode, requires an initialization vector.  If required,
       the description for each encryption type must specify the source of
       such additional information.

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    6.2.  Encryption Keys

       The sequence below shows the encoding of an encryption key:

              EncryptionKey ::=   SEQUENCE {
                                  keytype[0]    INTEGER,
                                  keyvalue[1]   OCTET STRING
              }

       keytype   This field specifies the type of encryption key that
                 follows in the keyvalue field.  It will almost always
                 correspond to the encryption algorithm used to generate the
                 EncryptedData, though more than one algorithm may use the
                 same type of key (the mapping is many to one).  This might
                 happen, for example, if the encryption algorithm uses an
                 alternate checksum algorithm for an integrity check, or a
                 different chaining mechanism.

       keyvalue  This field contains the key itself, encoded as an octet
                 string.

       All negative values for the  encryption key type are reserved for
       local use.  All non-negative values are reserved for officially
       assigned type fields and interpretations.

    6.3.  Encryption Systems

    6.3.1. The NULL Encryption System (null)

       If no encryption is in use, the encryption system is said to be the
       NULL encryption system.  In the NULL encryption system there is no
       checksum, confounder or padding.  The ciphertext is simply the
       plaintext.  The NULL Key is used by the null encryption system and is
       zero octets in length, with keytype zero (0).

    6.3.2. DES in CBC mode with a CRC-32 checksum (des-cbc-crc)

       The des-cbc-crc encryption mode encrypts information under the Data
       Encryption Standard [11] using the cipher block chaining mode [12].
       A CRC-32 checksum (described in ISO 3309 [14]) is applied to the
       confounder and message sequence (msg-seq) and placed in the cksum
       field.  DES blocks are 8 bytes.  As a result, the data to be
       encrypted (the concatenation of confounder, checksum, and message)
       must be padded to an 8 byte boundary before encryption.  The details
       of the encryption of this data are identical to those for the des-
       cbc-md5 encryption mode.

       Note that, since the CRC-32 checksum is not collisionproof, an

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       attacker could use a probabilistic chosenplaintext attack to generate
       a valid message even if a confounder is used [13]. The use of
       collision-proof checksums is recommended for environments where such
       attacks represent a significant threat.  The use of the CRC-32 as the
       checksum for ticket or authenticator is no longer mandated as an
       interoperability requirement for Kerberos Version 5 Specification 1
       (See section 9.1 for specific details).

    6.3.3. DES in CBC mode with an MD4 checksum (des-cbc-md4)

       The des-cbc-md4 encryption mode encrypts information under the Data
       Encryption Standard [11] using the cipher block chaining mode [12].
       An MD4 checksum (described in [15]) is applied to the confounder and
       message sequence (msg-seq) and placed in the cksum field.  DES blocks
       are 8 bytes.  As a result, the data to be encrypted (the
       concatenation of confounder, checksum, and message) must be padded to
       an 8 byte boundary before encryption.  The details of the encryption
       of this data are identical to those for the descbc-md5 encryption
       mode.

    6.3.4. DES in CBC mode with an MD5 checksum (des-cbc-md5)

       The des-cbc-md5 encryption mode encrypts information under the Data
       Encryption Standard [11] using the cipher block chaining mode [12].
       An MD5 checksum (described in [16]) is applied to the confounder and
       message sequence (msg-seq) and placed in the cksum field.  DES blocks
       are 8 bytes.  As a result, the data to be encrypted (the
       concatenation of confounder, checksum, and message) must be padded to
       an 8 byte boundary before encryption.

       Plaintext and DES ciphtertext are encoded as 8-octet blocks which are
       concatenated to make the 64-bit inputs for the DES algorithms.  The
       first octet supplies the 8 most significant bits (with the octet's
       MSbit used as the DES input block's MSbit, etc.), the second octet
       the next 8 bits, ..., and the eighth octet supplies the 8 least
       significant bits.

       Encryption under DES using cipher block chaining requires an
       additional input in the form of an initialization vector.  Unless
       otherwise specified, zero should be used as the initialization
       vector.  Kerberos' use of DES requires an 8-octet confounder.

       The DES specifications identify some "weak" and "semiweak" keys;
       those keys shall not be used for encrypting messages for use in
       Kerberos.  Additionally, because of the way that keys are derived for
       the encryption of checksums, keys shall not be used that yield "weak"
       or "semi-weak" keys when eXclusive-ORed with the constant
       F0F0F0F0F0F0F0F0.

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       A DES key is 8 octets of data, with keytype one (1).  This consists
       of 56 bits of key, and 8 parity bits (one per octet).  The key is
       encoded as a series of 8 octets written in MSB-first order. The bits
       within the key are also encoded in MSB order.  For example, if the
       encryption key is:
       (B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8) where
       B1,B2,...,B56 are the key bits in MSB order, and P1,P2,...,P8 are the
       parity bits, the first octet of the key would be B1,B2,...,B7,P1
       (with B1 as the MSbit).  [See the FIPS 81 introduction for
       reference.]

       To generate a DES key from a text string (password), the text string
       normally must have the realm and each component of the principal's
       name appended(In some cases, it may be necessary to use a different
       "mix-in" string for compatibility reasons; see the discussion of
       padata in section 5.4.2.), then padded with ASCII nulls to an 8 byte
       boundary.  This string is then fan-folded and eXclusive-ORed with
       itself to form an 8 byte DES key.  The parity is corrected on the
       key, and it is used to generate a DES CBC checksum on the initial
       string (with the realm and name appended).  Next, parity is corrected
       on the CBC checksum.  If the result matches a "weak" or "semiweak"
       key as described in the DES specification, it is eXclusive-ORed with
       the constant 00000000000000F0.  Finally, the result is returned as
       the key.  Pseudocode follows:

            string_to_key(string,realm,name) {
                 odd = 1;
                 s = string + realm;
                 for(each component in name) {
                      s = s + component;
                 }
                 tempkey = NULL;
                 pad(s); /* with nulls to 8 byte boundary */
                 for(8byteblock in s) {
                      if(odd == 0)  {
                          odd = 1;
                          reverse(8byteblock)
                      }
                      else odd = 0;
                      tempkey = tempkey XOR 8byteblock;
                 }
                 fixparity(tempkey);
                 key = DES-CBC-check(s,tempkey);
                 fixparity(key);
                 if(is_weak_key_key(key))
                      key = key XOR 0xF0;
                 return(key);
            }

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    6.4.  Checksums

       The following is the ASN.1 definition used for a checksum:

                Checksum ::=   SEQUENCE {
                               cksumtype[0]   INTEGER,
                               checksum[1]    OCTET STRING
                }

       cksumtype This field indicates the algorithm used to generate the
                 accompanying checksum.

       checksum  This field contains the checksum itself, encoded
                 as an octet string.

       Detailed specification of selected checksum types appear later in
       this section.  Negative values for the checksum type are reserved for
       local use.  All non-negative values are reserved for officially
       assigned type fields and interpretations.

       Checksums used by Kerberos can be classified by two properties:
       whether they are collision-proof, and whether they are keyed.  It is
       infeasible to find two plaintexts which generate the same checksum
       value for a collision-proof checksum.  A key is required to perturb
       or initialize the algorithm in a keyed checksum.  To prevent
       message-stream modification by an active attacker, unkeyed checksums
       should only be used when the checksum and message will be
       subsequently encrypted (e.g., the checksums defined as part of the
       encryption algorithms covered earlier in this section).  Collision-
       proof checksums can be made tamper-proof as well if the checksum
       value is encrypted before inclusion in a message.  In such cases, the
       composition of the checksum and the encryption algorithm must be
       considered a separate checksum algorithm (e.g., RSA-MD5 encrypted
       using DES is a new checksum algorithm of type RSA-MD5-DES).  For most
       keyed checksums, as well as for the encrypted forms of collisionproof
       checksums, Kerberos prepends a confounder before the checksum is
       calculated.

    6.4.1. The CRC-32 Checksum (crc32)

       The CRC-32 checksum calculates a checksum based on a cyclic
       redundancy check as described in ISO 3309 [14].  The resulting
       checksum is four (4) octets in length.  The CRC-32 is neither keyed
       nor collision-proof.  The use of this checksum is not recommended.
       An attacker using a probabilistic chosen-plaintext attack as
       described in [13] might be able to generate an alternative message
       that satisfies the checksum.  The use of collision-proof checksums is
       recommended for environments where such attacks represent a

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       significant threat.

    6.4.2. The RSA MD4 Checksum (rsa-md4)

       The RSA-MD4 checksum calculates a checksum using the RSA MD4
       algorithm [15].  The algorithm takes as input an input message of
       arbitrary length and produces as output a 128-bit (16 octet)
       checksum.  RSA-MD4 is believed to be collision-proof.

    6.4.3. RSA MD4 Cryptographic Checksum Using DES (rsa-md4des)

       The RSA-MD4-DES checksum calculates a keyed collisionproof checksum
       by prepending an 8 octet confounder before the text, applying the RSA
       MD4 checksum algorithm, and encrypting the confounder and the
       checksum using DES in cipher-block-chaining (CBC) mode using a
       variant of the key, where the variant is computed by eXclusive-ORing
       the key with the constant F0F0F0F0F0F0F0F0 (A variant of the key is
       used to limit the use of a key to a particular function, separating
       the functions of generating a checksum from other encryption
       performed using the session key.  The constant F0F0F0F0F0F0F0F0 was
       chosen because it maintains key parity.  The properties of DES
       precluded the use of the complement.  The same constant is used for
       similar purpose in the Message Integrity Check in the Privacy
       Enhanced Mail standard.).  The initialization vector should be zero.
       The resulting checksum is 24 octets long (8 octets of which are
       redundant).  This checksum is tamper-proof and believed to be
       collision-proof.

       The DES specifications identify some "weak keys"; those keys shall
       not be used for generating RSA-MD4 checksums for use in Kerberos.

       The format for the checksum is described in the following diagram:

          +--+--+--+--+--+--+--+--
          |  des-cbc(confounder
          +--+--+--+--+--+--+--+--

                        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                            rsa-md4(confounder+msg),key=var(key),iv=0)  |
                        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

       The format cannot be described in ASN.1, but for those who prefer an
       ASN.1-like notation:

       rsa-md4-des-checksum ::=   ENCRYPTED       UNTAGGED SEQUENCE {
                                  confounder[0]   UNTAGGED OCTET STRING(8),
                                  check[1]        UNTAGGED OCTET STRING(16)
       }

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    6.4.4. The RSA MD5 Checksum (rsa-md5)

       The RSA-MD5 checksum calculates a checksum using the RSA MD5
       algorithm [16].  The algorithm takes as input an input message of
       arbitrary length and produces as output a 128-bit (16 octet)
       checksum.  RSA-MD5 is believed to be collision-proof.

    6.4.5. RSA MD5 Cryptographic Checksum Using DES (rsa-md5des)

       The RSA-MD5-DES checksum calculates a keyed collisionproof checksum
       by prepending an 8 octet confounder before the text, applying the RSA
       MD5 checksum algorithm, and encrypting the confounder and the
       checksum using DES in cipher-block-chaining (CBC) mode using a
       variant of the key, where the variant is computed by eXclusive-ORing
       the key with the constant F0F0F0F0F0F0F0F0.  The initialization
       vector should be zero.  The resulting checksum is 24 octets long (8
       octets of which are redundant).  This checksum is tamper-proof and
       believed to be collision-proof.

       The DES specifications identify some "weak keys"; those keys shall
       not be used for encrypting RSA-MD5 checksums for use in Kerberos.

       The format for the checksum is described in the following diagram:

          +--+--+--+--+--+--+--+--
          |  des-cbc(confounder
          +--+--+--+--+--+--+--+--

                         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                             rsa-md5(confounder+msg),key=var(key),iv=0)  |
                         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

       The format cannot be described in ASN.1, but for those who prefer an
       ASN.1-like notation:

       rsa-md5-des-checksum ::=   ENCRYPTED       UNTAGGED SEQUENCE {
                                  confounder[0]   UNTAGGED OCTET STRING(8),
                                  check[1]        UNTAGGED OCTET STRING(16)
       }

    6.4.6. DES cipher-block chained checksum (des-mac)

       The DES-MAC checksum is computed by prepending an 8 octet confounder
       to the plaintext, performing a DES CBC-mode encryption on the result
       using the key and an initialization vector of zero, taking the last
       block of the ciphertext, prepending the same confounder and
       encrypting the pair using DES in cipher-block-chaining (CBC) mode
       using a a variant of the key, where the variant is computed by

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       eXclusive-ORing the key with the constant F0F0F0F0F0F0F0F0.  The
       initialization vector should be zero.  The resulting checksum is 128
       bits (16 octets) long, 64 bits of which are redundant. This checksum
       is tamper-proof and collision-proof.

       The format for the checksum is described in the following diagram:

          +--+--+--+--+--+--+--+--
          |   des-cbc(confounder
          +--+--+--+--+--+--+--+--

                         +-----+-----+-----+-----+-----+-----+-----+-----+
                           des-mac(conf+msg,iv=0,key),key=var(key),iv=0) |
                         +-----+-----+-----+-----+-----+-----+-----+-----+

       The format cannot be described in ASN.1, but for those who prefer an
       ASN.1-like notation:

       des-mac-checksum ::=    ENCRYPTED       UNTAGGED SEQUENCE {
                               confounder[0]   UNTAGGED OCTET STRING(8),
                               check[1]        UNTAGGED OCTET STRING(8)
       }

       The DES specifications identify some "weak" and "semiweak" keys;
       those keys shall not be used for generating DES-MAC checksums for use
       in Kerberos, nor shall a key be used whose veriant is "weak" or
       "semi-weak".

    6.4.7. RSA MD4 Cryptographic Checksum Using DES alternative
           (rsa-md4-des-k)

       The RSA-MD4-DES-K checksum calculates a keyed collision-proof
       checksum by applying the RSA MD4 checksum algorithm and encrypting
       the results using DES in cipherblock-chaining (CBC) mode using a DES
       key as both key and initialization vector. The resulting checksum is
       16 octets long. This checksum is tamper-proof and believed to be
       collision-proof.  Note that this checksum type is the old method for
       encoding the RSA-MD4-DES checksum and it is no longer recommended.

    6.4.8. DES cipher-block chained checksum alternative (desmac-k)

       The DES-MAC-K checksum is computed by performing a DES CBC-mode
       encryption of the plaintext, and using the last block of the
       ciphertext as the checksum value. It is keyed with an encryption key
       and an initialization vector; any uses which do not specify an
       additional initialization vector will use the key as both key and
       initialization vector.  The resulting checksum is 64 bits (8 octets)
       long. This checksum is tamper-proof and collision-proof.  Note that

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       this checksum type is the old method for encoding the DESMAC checksum
       and it is no longer recommended.

       The DES specifications identify some "weak keys"; those keys shall
       not be used for generating DES-MAC checksums for use in Kerberos.

    7.  Naming Constraints

    7.1.  Realm Names

       Although realm names are encoded as GeneralStrings and although a
       realm can technically select any name it chooses, interoperability
       across realm boundaries requires agreement on how realm names are to
       be assigned, and what information they imply.

       To enforce these conventions, each realm must conform to the
       conventions itself, and it must require that any realms with which
       inter-realm keys are shared also conform to the conventions and
       require the same from its neighbors.

       There are presently four styles of realm names: domain, X500, other,
       and reserved.  Examples of each style follow:

            domain:   host.subdomain.domain (example)
              X500:   C=US/O=OSF (example)
             other:   NAMETYPE:rest/of.name=without-restrictions (example)
          reserved:   reserved, but will not conflict with above

       Domain names must look like domain names: they consist of components
       separated by periods (.) and they contain neither colons (:) nor
       slashes (/).

       X.500 names contain an equal (=) and cannot contain a colon (:)
       before the equal.  The realm names for X.500 names will be string
       representations of the names with components separated by slashes.
       Leading and trailing slashes will not be included.

       Names that fall into the other category must begin with a prefix that
       contains no equal (=) or period (.) and the prefix must be followed
       by a colon (:) and the rest of the name. All prefixes must be
       assigned before they may be used.  Presently none are assigned.

       The reserved category includes strings which do not fall into the
       first three categories.  All names in this category are reserved. It
       is unlikely that names will be assigned to this category unless there
       is a very strong argument for not using the "other" category.

       These rules guarantee that there will be no conflicts between the

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       various name styles.  The following additional constraints apply to
       the assignment of realm names in the domain and X.500 categories: the
       name of a realm for the domain or X.500 formats must either be used
       by the organization owning (to whom it was assigned) an Internet
       domain name or X.500 name, or in the case that no such names are
       registered, authority to use a realm name may be derived from the
       authority of the parent realm.  For example, if there is no domain
       name for E40.MIT.EDU, then the administrator of the MIT.EDU realm can
       authorize the creation of a realm with that name.

       This is acceptable because the organization to which the parent is
       assigned is presumably the organization authorized to assign names to
       its children in the X.500 and domain name systems as well.  If the
       parent assigns a realm name without also registering it in the domain
       name or X.500 hierarchy, it is the parent's responsibility to make
       sure that there will not in the future exists a name identical to the
       realm name of the child unless it is assigned to the same entity as
       the realm name.

    7.2.  Principal Names

       As was the case for realm names, conventions are needed to ensure
       that all agree on what information is implied by a principal name.
       The name-type field that is part of the principal name indicates the
       kind of information implied by the name.  The name-type should be
       treated as a hint.  Ignoring the name type, no two names can be the
       same (i.e., at least one of the components, or the realm, must be
       different).  This constraint may be eliminated in the future.  The
       following name types are defined:

          name-type      value   meaning
          NT-UNKNOWN       0     Name type not known
          NT-PRINCIPAL     1     Just the name of the principal as in
                                 DCE, or for users
          NT-SRV-INST      2     Service and other unique instance (krbtgt)
          NT-SRV-HST       3     Service with host name as instance
                                 (telnet, rcommands)
          NT-SRV-XHST      4     Service with host as remaining components
          NT-UID           5     Unique ID

       When a name implies no information other than its uniqueness at a
       particular time the name type PRINCIPAL should be used.  The
       principal name type should be used for users, and it might also be
       used for a unique server.  If the name is a unique machine generated
       ID that is guaranteed never to be reassigned then the name type of
       UID should be used (note that it is generally a bad idea to reassign
       names of any type since stale entries might remain in access control
       lists).

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       If the first component of a name identifies a service and the
       remaining components identify an instance of the service in a server
       specified manner, then the name type of SRV-INST should be used.  An
       example of this name type is the Kerberos ticket-granting ticket
       which has a first component of krbtgt and a second component
       identifying the realm for which the ticket is valid.

       If instance is a single component following the service name and the
       instance identifies the host on which the server is running, then the
       name type SRV-HST should be used. This type is typically used for
       Internet services such as telnet and the Berkeley R commands.  If the
       separate components of the host name appear as successive components
       following the name of the service, then the name type SRVXHST should
       be used.  This type might be used to identify servers on hosts with
       X.500 names where the slash (/) might otherwise be ambiguous.

       A name type of UNKNOWN should be used when the form of the name is
       not known. When comparing names, a name of type UNKNOWN will match
       principals authenticated with names of any type.  A principal
       authenticated with a name of type UNKNOWN, however, will only match
       other names of type UNKNOWN.

       Names of any type with an initial component of "krbtgt" are reserved
       for the Kerberos ticket granting service.  See section 8.2.3 for the
       form of such names.

    7.2.1. Name of server principals

       The principal identifier for a server on a host will generally be
       composed of two parts: (1) the realm of the KDC with which the server
       is registered, and (2) a two-component name of type NT-SRV-HST if the
       host name is an Internet domain name or a multi-component name of
       type NT-SRV-XHST if the name of the host is of a form such as X.500
       that allows slash (/) separators.  The first component of the two- or
       multi-component name will identify the service and the latter
       components will identify the host.  Where the name of the host is not
       case sensitive (for example, with Internet domain names) the name of
       the host must be lower case.  For services such as telnet and the
       Berkeley R commands which run with system privileges, the first
       component will be the string "host" instead of a service specific
       identifier.

    8.  Constants and other defined values

    8.1.  Host address types

       All negative values for the host address type are reserved for local
       use.  All non-negative values are reserved for officially assigned

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       type fields and interpretations.

       The values of the types for the following addresses are chosen to
       match the defined address family constants in the Berkeley Standard
       Distributions of Unix.  They can be found in <sys/socket.h> with
       symbolic names AF_xxx (where xxx is an abbreviation of the address
       family name).

       Internet addresses

          Internet addresses are 32-bit (4-octet) quantities, encoded in MSB
          order.  The type of internet addresses is two (2).

       CHAOSnet addresses

          CHAOSnet addresses are 16-bit (2-octet) quantities, encoded in MSB
          order.  The type of CHAOSnet addresses is five (5).

       ISO addresses

          ISO addresses are variable-length.  The type of ISO addresses is
          seven (7).

       Xerox Network Services (XNS) addresses

          XNS addresses are 48-bit (6-octet) quantities, encoded in MSB
          order.  The type of XNS addresses is six (6).

       AppleTalk Datagram Delivery Protocol (DDP) addresses

          AppleTalk DDP addresses consist of an 8-bit node number and a 16-
          bit network number.  The first octet of the address is the node
          number; the remaining two octets encode the network number in MSB
          order. The type of AppleTalk DDP addresses is sixteen (16).

       DECnet Phase IV addresses

          DECnet Phase IV addresses are 16-bit addresses, encoded in LSB
          order.  The type of DECnet Phase IV addresses is twelve (12).

    8.2.  KDC messages

    8.2.1. IP transport

       When contacting a Kerberos server (KDC) for a KRB_KDC_REQ request
       using IP transport, the client shall send a UDP datagram containing
       only an encoding of the request to port 88 (decimal) at the KDC's IP

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       address; the KDC will respond with a reply datagram containing only
       an encoding of the reply message (either a KRB_ERROR or a
       KRB_KDC_REP) to the sending port at the sender's IP address.

    8.2.2. OSI transport

       During authentication of an OSI client to and OSI server, the mutual
       authentication of an OSI server to an OSI client, the transfer of
       credentials from an OSI client to an OSI server, or during exchange
       of private or integrity checked messages, Kerberos protocol messages
       may be treated as opaque objects and the type of the authentication
       mechanism will be:

       OBJECT IDENTIFIER ::= {iso (1), org(3), dod(5),internet(1),
                              security(5), kerberosv5(2)}

       Depending on the situation, the opaque object will be an
       authentication header (KRB_AP_REQ), an authentication reply
       (KRB_AP_REP), a safe message (KRB_SAFE), a private message
       (KRB_PRIV), or a credentials message (KRB_CRED).  The opaque data
       contains an application code as specified in the ASN.1 description
       for each message.  The application code may be used by Kerberos to
       determine the message type.

    8.2.3. Name of the TGS

       The principal identifier of the ticket-granting service shall be
       composed of three parts: (1) the realm of the KDC issuing the TGS
       ticket (2) a two-part name of type NT-SRVINST, with the first part
       "krbtgt" and the second part the name of the realm which will accept
       the ticket-granting ticket.  For example, a ticket-granting ticket
       issued by the ATHENA.MIT.EDU realm to be used to get tickets from the
       ATHENA.MIT.EDU KDC has a principal identifier of "ATHENA.MIT.EDU"
       (realm), ("krbtgt", "ATHENA.MIT.EDU") (name).  A ticket-granting
       ticket issued by the ATHENA.MIT.EDU realm to be used to get tickets
       from the MIT.EDU realm has a principal identifier of "ATHENA.MIT.EDU"
       (realm), ("krbtgt", "MIT.EDU") (name).

    8.3.  Protocol constants and associated values

       The following tables list constants used in the protocol and defines
       their meanings.

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    ---------------+-----------+----------+----------------+---------------
    Encryption type|etype value|block size|minimum pad size|confounder size
    ---------------+-----------+----------+----------------+---------------
    NULL                0            1              0              0
    des-cbc-crc         1            8              4              8
    des-cbc-md4         2            8              0              8
    des-cbc-md5         3            8              0              8

    -------------------------------+-------------------+-------------
    Checksum type                  |sumtype value      |checksum size
    -------------------------------+-------------------+-------------
    CRC32                           1                   4
    rsa-md4                         2                   16
    rsa-md4-des                     3                   24
    des-mac                         4                   16
    des-mac-k                       5                   8
    rsa-md4-des-k                   6                   16
    rsa-md5                         7                   16
    rsa-md5-des                     8                   24

    -------------------------------+-----------------
    padata type                    |padata-type value
    -------------------------------+-----------------
    PA-TGS-REQ                      1
    PA-ENC-TIMESTAMP                2
    PA-PW-SALT                      3

    -------------------------------+-------------
    authorization data type        |ad-type value
    -------------------------------+-------------
    reserved values                 0-63
    OSF-DCE                         64
    SESAME                          65

    -------------------------------+-----------------
    alternate authentication type  |method-type value
    -------------------------------+-----------------
    reserved values                 0-63
    ATT-CHALLENGE-RESPONSE          64

    -------------------------------+-------------
    transited encoding type        |tr-type value
    -------------------------------+-------------
    DOMAIN-X500-COMPRESS            1
    reserved values                 all others

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    --------------+-------+-----------------------------------------
    Label         |Value  |Meaning or MIT code
    --------------+-------+-----------------------------------------

    pvno             5     current Kerberos protocol version number

    message types

    KRB_AS_REQ      10     Request for initial authentication
    KRB_AS_REP      11     Response to KRB_AS_REQ request
    KRB_TGS_REQ     12     Request for authentication based on TGT
    KRB_TGS_REP     13     Response to KRB_TGS_REQ request
    KRB_AP_REQ      14     application request to server
    KRB_AP_REP      15     Response to KRB_AP_REQ_MUTUAL
    KRB_SAFE        20     Safe (checksummed) application message
    KRB_PRIV        21     Private (encrypted) application message
    KRB_CRED        22     Private (encrypted) message to forward
                           credentials
    KRB_ERROR       30     Error response

    name types

    KRB_NT_UNKNOWN   0   Name type not known
    KRB_NT_PRINCIPAL 1   Just the name of the principal as in DCE, or
                         for users
    KRB_NT_SRV_INST  2   Service and other unique instance (krbtgt)
    KRB_NT_SRV_HST   3   Service with host name as instance (telnet,
                         rcommands)
    KRB_NT_SRV_XHST  4   Service with host as remaining components
    KRB_NT_UID       5   Unique ID

    error codes

    KDC_ERR_NONE                   0   No error
    KDC_ERR_NAME_EXP               1   Client's entry in database has
                                       expired
    KDC_ERR_SERVICE_EXP            2   Server's entry in database has
                                       expired
    KDC_ERR_BAD_PVNO               3   Requested protocol version number
                                       not supported
    KDC_ERR_C_OLD_MAST_KVNO        4   Client's key encrypted in old
                                       master key
    KDC_ERR_S_OLD_MAST_KVNO        5   Server's key encrypted in old
                                       master key
    KDC_ERR_C_PRINCIPAL_UNKNOWN    6   Client not found in Kerberos database
    KDC_ERR_S_PRINCIPAL_UNKNOWN    7   Server not found in Kerberos database
    KDC_ERR_PRINCIPAL_NOT_UNIQUE   8   Multiple principal entries in
                                       database

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    KDC_ERR_NULL_KEY               9   The client or server has a null key
    KDC_ERR_CANNOT_POSTDATE       10   Ticket not eligible for postdating
    KDC_ERR_NEVER_VALID           11   Requested start time is later than
                                       end time
    KDC_ERR_POLICY                12   KDC policy rejects request
    KDC_ERR_BADOPTION             13   KDC cannot accommodate requested
                                       option
    KDC_ERR_ETYPE_NOSUPP          14   KDC has no support for encryption
                                       type
    KDC_ERR_SUMTYPE_NOSUPP        15   KDC has no support for checksum type
    KDC_ERR_PADATA_TYPE_NOSUPP    16   KDC has no support for padata type
    KDC_ERR_TRTYPE_NOSUPP         17   KDC has no support for transited type
    KDC_ERR_CLIENT_REVOKED        18   Clients credentials have been revoked
    KDC_ERR_SERVICE_REVOKED       19   Credentials for server have been
                                       revoked
    KDC_ERR_TGT_REVOKED           20   TGT has been revoked
    KDC_ERR_CLIENT_NOTYET         21   Client not yet valid - try again
                                       later
    KDC_ERR_SERVICE_NOTYET        22   Server not yet valid - try again
                                       later
    KDC_ERR_KEY_EXPIRED           23   Password has expired - change
                                       password to reset
    KDC_ERR_PREAUTH_FAILED        24   Pre-authentication information
                                       was invalid
    KDC_ERR_PREAUTH_REQUIRED      25   Additional pre-authentication
                                       required*
    KRB_AP_ERR_BAD_INTEGRITY      31   Integrity check on decrypted field
                                       failed
    KRB_AP_ERR_TKT_EXPIRED        32   Ticket expired
    KRB_AP_ERR_TKT_NYV            33   Ticket not yet valid
    KRB_AP_ERR_REPEAT             34   Request is a replay
    KRB_AP_ERR_NOT_US             35   The ticket isn't for us
    KRB_AP_ERR_BADMATCH           36   Ticket and authenticator don't match
    KRB_AP_ERR_SKEW               37   Clock skew too great
    KRB_AP_ERR_BADADDR            38   Incorrect net address
    KRB_AP_ERR_BADVERSION         39   Protocol version mismatch
    KRB_AP_ERR_MSG_TYPE           40   Invalid msg type
    KRB_AP_ERR_MODIFIED           41   Message stream modified
    KRB_AP_ERR_BADORDER           42   Message out of order
    KRB_AP_ERR_BADKEYVER          44   Specified version of key is not
                                       available
    KRB_AP_ERR_NOKEY              45   Service key not available
    KRB_AP_ERR_MUT_FAIL           46   Mutual authentication failed
    KRB_AP_ERR_BADDIRECTION       47   Incorrect message direction
    KRB_AP_ERR_METHOD             48   Alternative authentication method
                                       required*
    KRB_AP_ERR_BADSEQ             49   Incorrect sequence number in message
    KRB_AP_ERR_INAPP_CKSUM        50   Inappropriate type of checksum in

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                                       message
    KRB_ERR_GENERIC               60   Generic error (description in e-text)
    KRB_ERR_FIELD_TOOLONG         61   Field is too long for this
                                       implementation

       *This error carries additional information in the e-data field.  The
       contents of the e-data field for this message is described in section
       5.9.1.

    9.  Interoperability requirements

       Version 5 of the Kerberos protocol supports a myriad of options.
       Among these are multiple encryption and checksum types, alternative
       encoding schemes for the transited field, optional mechanisms for
       pre-authentication, the handling of tickets with no addresses,
       options for mutual authentication, user to user authentication,
       support for proxies, forwarding, postdating, and renewing tickets,
       the format of realm names, and the handling of authorization data.

       In order to ensure the interoperability of realms, it is necessary to
       define a minimal configuration which must be supported by all
       implementations.  This minimal configuration is subject to change as
       technology does. For example, if at some later date it is discovered
       that one of the required encryption or checksum algorithms is not
       secure, it will be replaced.

    9.1.  Specification 1

       This section defines the first specification of these options.
       Implementations which are configured in this way can be said to
       support Kerberos Version 5 Specification 1 (5.1).

       Encryption and checksum methods

       The following encryption and checksum mechanisms must be supported.
       Implementations may support other mechanisms as well, but the
       additional mechanisms may only be used when communicating with
       principals known to also support them: Encryption: DES-CBC-MD5
       Checksums: CRC-32, DES-MAC, DES-MAC-K, and DES-MD5

       Realm Names

       All implementations must understand hierarchical realms in both the
       Internet Domain and the X.500 style.  When a ticket granting ticket
       for an unknown realm is requested, the KDC must be able to determine
       the names of the intermediate realms between the KDCs realm and the
       requested realm.

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       Transited field encoding

       DOMAIN-X500-COMPRESS (described in section 3.3.3.1) must be
       supported.  Alternative encodings may be supported, but they may be
       used only when that encoding is supported by ALL intermediate realms.

       Pre-authentication methods

       The TGS-REQ method must be supported.  The TGS-REQ method is not used
       on the initial request. The PA-ENC-TIMESTAMP method must be supported
       by clients but whether it is enabled by default may be determined on
       a realm by realm basis. If not used in the initial request and the
       error KDC_ERR_PREAUTH_REQUIRED is returned specifying PA-ENCTIMESTAMP
       as an acceptable method, the client should retry the initial request
       using the PA-ENC-TIMESTAMP preauthentication method. Servers need not
       support the PAENC-TIMESTAMP method, but if not supported the server
       should ignore the presence of PA-ENC-TIMESTAMP pre-authentication in
       a request.

       Mutual authentication

       Mutual authentication (via the KRB_AP_REP message) must be supported.

       Ticket addresses and flags

       All KDC's must pass on tickets that carry no addresses (i.e.,  if a
       TGT contains no addresses, the KDC will return derivative tickets),
       but each realm may set its own policy for issuing such tickets, and
       each application server will set its own policy with respect to
       accepting them. By default, servers should not accept them.

       Proxies and forwarded tickets must be supported.  Individual realms
       and application servers can set their own policy on when such tickets
       will be accepted.

       All implementations must recognize renewable and postdated tickets,
       but need not actually implement them.  If these options are not
       supported, the starttime and endtime in the ticket shall specify a
       ticket's entire useful life.  When a postdated ticket is decoded by a
       server, all implementations shall make the presence of the postdated
       flag visible to the calling server.

       User-to-user authentication

       Support for user to user authentication (via the ENC-TKTIN-SKEY KDC
       option) must be provided by implementations, but individual realms
       may decide as a matter of policy to reject such requests on a per-
       principal or realm-wide basis.

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       Authorization data

       Implementations must pass all authorization data subfields from
       ticket-granting tickets to any derivative tickets unless directed to
       suppress a subfield as part of the definition of that registered
       subfield type (it is never incorrect to pass on a subfield, and no
       registered subfield types presently specify suppression at the KDC).

       Implementations must make the contents of any authorization data
       subfields available to the server when a ticket is used.
       Implementations are not required to allow clients to specify the
       contents of the authorization data fields.

    9.2.  Recommended KDC values

       Following is a list of recommended values for a KDC implementation,
       based on the list of suggested configuration constants (see section
       4.4).

       minimum lifetime                5 minutes

       maximum renewable lifetime      1 week

       maximum ticket lifetime         1 day

       empty addresses                 only when suitable restrictions appear
                                       in authorization data

       proxiable, etc.                 Allowed.

    10.  Acknowledgments

       Early versions of this document, describing version 4 of the
       protocol, were written by Jennifer Steiner (formerly at Project
       Athena); these drafts provided an excellent starting point for this
       current version 5 specification.  Many people in the Internet
       community have contributed ideas and suggested protocol changes for
       version 5. Notable contributions came from Ted Anderson, Steve
       Bellovin and Michael Merritt [17], Daniel Bernstein, Mike Burrows,
       Donald Davis, Ravi Ganesan, Morrie Gasser, Virgil Gligor, Bill
       Griffeth, Mark Lillibridge, Mark Lomas, Steve Lunt, Piers McMahon,
       Joe Pato, William Sommerfeld, Stuart Stubblebine, Ralph Swick, Ted
       T'so, and Stanley Zanarotti.  Many others commented and helped shape
       this specification into its current form.

    Kohl & Neuman                                                  [Page 88]

    RFC 1510                        Kerberos                  September 1993

    11.  References

       [1]  Miller, S., Neuman, C., Schiller, J., and  J. Saltzer, "Section
            E.2.1: Kerberos  Authentication and Authorization System",
            M.I.T. Project Athena, Cambridge, Massachusetts, December 21,
            1987.

       [2]  Steiner, J., Neuman, C., and J. Schiller, "Kerberos: An
            Authentication Service for Open Network Systems", pp. 191-202 in
            Usenix Conference Proceedings, Dallas, Texas, February, 1988.

       [3]  Needham, R., and M. Schroeder, "Using Encryption for
            Authentication in Large Networks of Computers", Communications
            of the ACM, Vol. 21 (12), pp. 993-999, December 1978.

       [4]  Denning, D., and G. Sacco, "Time stamps in Key Distribution
            Protocols", Communications of the ACM, Vol. 24 (8), pp. 533-536,
            August 1981.

       [5]  Kohl, J., Neuman, C., and T. Ts'o, "The Evolution of the
            Kerberos Authentication Service", in an IEEE Computer Society
            Text soon to be published, June 1992.

       [6]  Davis, D., and R. Swick, "Workstation Services and Kerberos
            Authentication at Project Athena", Technical Memorandum TM-424,
            MIT Laboratory for Computer Science, February 1990.

       [7]  Levine, P., Gretzinger, M, Diaz, J., Sommerfeld, W., and K.
            Raeburn, "Section E.1: Service Management System, M.I.T.
            Project Athena, Cambridge, Mas sachusetts (1987).

       [8]  CCITT, Recommendation X.509: The Directory Authentication
            Framework, December 1988.

       [9]  Neuman, C., "Proxy-Based Authorization and Accounting for
            Distributed Systems," in Proceedings of the 13th International
            Conference on Distributed Computing Systems", Pittsburgh, PA,
            May 1993.

       [10] Pato, J., "Using Pre-Authentication to Avoid Password Guessing
            Attacks", Open Software Foundation DCE Request for Comments 26,
            December 1992.

       [11] National Bureau of Standards, U.S. Department of Commerce, "Data
            Encryption Standard", Federal Information Processing Standards
            Publication 46, Washington, DC (1977).

    Kohl & Neuman                                                  [Page 89]

    RFC 1510                        Kerberos                  September 1993

       [12] National Bureau of Standards, U.S. Department of Commerce, "DES
            Modes of Operation", Federal Information Processing Standards
            Publication 81, Springfield, VA, December 1980.

       [13] Stubblebine S., and V. Gligor, "On Message Integrity in
            Cryptographic Protocols", in Proceedings of the IEEE Symposium
            on Research in Security and Privacy, Oakland, California, May
            1992.

       [14] International Organization for Standardization, "ISO Information
            Processing Systems - Data Communication High-Level Data Link
            Control Procedure - Frame Structure", IS 3309, October 1984, 3rd
            Edition.

       [15] Rivest, R., "The MD4 Message Digest Algorithm", RFC 1320, MIT
            Laboratory for Computer Science, April 1992.

       [16] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, MIT
            Laboratory for Computer Science, April 1992.

       [17] Bellovin S., and M. Merritt, "Limitations of the Kerberos
            Authentication System", Computer Communications Review, Vol.
            20(5), pp. 119-132, October 1990.

    12.  Security Considerations

       Security issues are discussed throughout this memo.

    13.  Authors' Addresses

       John Kohl
       Digital Equipment Corporation
       110 Spit Brook Road, M/S ZKO3-3/U14
       Nashua, NH  03062

       Phone: 603-881-2481
       EMail: jtkohl@zk3.dec.com

       B. Clifford Neuman
       USC/Information Sciences Institute
       4676 Admiralty Way #1001
       Marina del Rey, CA 90292-6695

       Phone: 310-822-1511
       EMail: bcn@isi.edu

    Kohl & Neuman                                                  [Page 90]

    RFC 1510                        Kerberos                  September 1993

    A.  Pseudo-code for protocol processing

       This appendix provides pseudo-code describing how the messages are to
       be constructed and interpreted by clients and servers.

    A.1.  KRB_AS_REQ generation
            request.pvno := protocol version; /* pvno = 5 */
            request.msg-type := message type; /* type = KRB_AS_REQ */

            if(pa_enc_timestamp_required) then
                    request.padata.padata-type = PA-ENC-TIMESTAMP;
                    get system_time;
                    padata-body.patimestamp,pausec = system_time;
                    encrypt padata-body into request.padata.padata-value
                            using client.key; /* derived from password */
            endif

            body.kdc-options := users's preferences;
            body.cname := user's name;
            body.realm := user's realm;
            body.sname := service's name; /* usually "krbtgt",
                                             "localrealm" */
            if (body.kdc-options.POSTDATED is set) then
                    body.from := requested starting time;
            else
                    omit body.from;
            endif
            body.till := requested end time;
            if (body.kdc-options.RENEWABLE is set) then
                    body.rtime := requested final renewal time;
            endif
            body.nonce := random_nonce();
            body.etype := requested etypes;
            if (user supplied addresses) then
                    body.addresses := user's addresses;
            else
                    omit body.addresses;
            endif
            omit body.enc-authorization-data;
            request.req-body := body;

            kerberos := lookup(name of local kerberos server (or servers));
            send(packet,kerberos);

            wait(for response);
            if (timed_out) then
                    retry or use alternate server;
            endif

    Kohl & Neuman                                                  [Page 91]

    RFC 1510                        Kerberos                  September 1993

    A.2.  KRB_AS_REQ verification and KRB_AS_REP generation
            decode message into req;

            client := lookup(req.cname,req.realm);
            server := lookup(req.sname,req.realm);
            get system_time;
            kdc_time := system_time.seconds;

            if (!client) then
                    /* no client in Database */
                    error_out(KDC_ERR_C_PRINCIPAL_UNKNOWN);
            endif
            if (!server) then
                    /* no server in Database */
                    error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
            endif

            if(client.pa_enc_timestamp_required and
               pa_enc_timestamp not present) then
                    error_out(KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP));
            endif

            if(pa_enc_timestamp present) then
                    decrypt req.padata-value into decrypted_enc_timestamp
                            using client.key;
                            using auth_hdr.authenticator.subkey;
                    if (decrypt_error()) then
                            error_out(KRB_AP_ERR_BAD_INTEGRITY);
                    if(decrypted_enc_timestamp is not within allowable
                            skew) then error_out(KDC_ERR_PREAUTH_FAILED);
                    endif
                    if(decrypted_enc_timestamp and usec is replay)
                            error_out(KDC_ERR_PREAUTH_FAILED);
                    endif
                    add decrypted_enc_timestamp and usec to replay cache;
            endif

            use_etype := first supported etype in req.etypes;

            if (no support for req.etypes) then
                    error_out(KDC_ERR_ETYPE_NOSUPP);
            endif

            new_tkt.vno := ticket version; /* = 5 */
            new_tkt.sname := req.sname;
            new_tkt.srealm := req.srealm;
            reset all flags in new_tkt.flags;

    Kohl & Neuman                                                  [Page 92]

    RFC 1510                        Kerberos                  September 1993

            /* It should be noted that local policy may affect the  */
            /* processing of any of these flags.  For example, some */
            /* realms may refuse to issue renewable tickets         */

            if (req.kdc-options.FORWARDABLE is set) then
                    set new_tkt.flags.FORWARDABLE;
            endif
            if (req.kdc-options.PROXIABLE is set) then
                    set new_tkt.flags.PROXIABLE;
            endif
            if (req.kdc-options.ALLOW-POSTDATE is set) then
                    set new_tkt.flags.ALLOW-POSTDATE;
            endif
            if ((req.kdc-options.RENEW is set) or
                (req.kdc-options.VALIDATE is set) or
                (req.kdc-options.PROXY is set) or
                (req.kdc-options.FORWARDED is set) or
                (req.kdc-options.ENC-TKT-IN-SKEY is set)) then
                    error_out(KDC_ERR_BADOPTION);
            endif

            new_tkt.session := random_session_key();
            new_tkt.cname := req.cname;
            new_tkt.crealm := req.crealm;
            new_tkt.transited := empty_transited_field();

            new_tkt.authtime := kdc_time;

            if (req.kdc-options.POSTDATED is set) then
               if (against_postdate_policy(req.from)) then
                    error_out(KDC_ERR_POLICY);
               endif
               set new_tkt.flags.INVALID;
               new_tkt.starttime := req.from;
            else
               omit new_tkt.starttime; /* treated as authtime when
                                          omitted */
            endif
            if (req.till = 0) then
                    till := infinity;
            else
                    till := req.till;
            endif

            new_tkt.endtime := min(till,
                                  new_tkt.starttime+client.max_life,
                                  new_tkt.starttime+server.max_life,
                                  new_tkt.starttime+max_life_for_realm);

    Kohl & Neuman                                                  [Page 93]

    RFC 1510                        Kerberos                  September 1993

            if ((req.kdc-options.RENEWABLE-OK is set) and
                (new_tkt.endtime < req.till)) then
                    /* we set the RENEWABLE option for later processing */
                    set req.kdc-options.RENEWABLE;
                    req.rtime := req.till;
            endif

            if (req.rtime = 0) then
                    rtime := infinity;
            else
                    rtime := req.rtime;
            endif

            if (req.kdc-options.RENEWABLE is set) then
                    set new_tkt.flags.RENEWABLE;
                    new_tkt.renew-till := min(rtime,
                    new_tkt.starttime+client.max_rlife,
                    new_tkt.starttime+server.max_rlife,
                    new_tkt.starttime+max_rlife_for_realm);
            else
                    omit new_tkt.renew-till; /* only present if RENEWABLE */
            endif

            if (req.addresses) then
                    new_tkt.caddr := req.addresses;
            else
                    omit new_tkt.caddr;
            endif

            new_tkt.authorization_data := empty_authorization_data();

            encode to-be-encrypted part of ticket into OCTET STRING;
            new_tkt.enc-part := encrypt OCTET STRING
                using etype_for_key(server.key), server.key, server.p_kvno;

            /* Start processing the response */

            resp.pvno := 5;
            resp.msg-type := KRB_AS_REP;
            resp.cname := req.cname;
            resp.crealm := req.realm;
            resp.ticket := new_tkt;

            resp.key := new_tkt.session;
            resp.last-req := fetch_last_request_info(client);
            resp.nonce := req.nonce;
            resp.key-expiration := client.expiration;

    Kohl & Neuman                                                  [Page 94]

    RFC 1510                        Kerberos                  September 1993

            resp.flags := new_tkt.flags;

            resp.authtime := new_tkt.authtime;
            resp.starttime := new_tkt.starttime;
            resp.endtime := new_tkt.endtime;

            if (new_tkt.flags.RENEWABLE) then
                    resp.renew-till := new_tkt.renew-till;
            endif

            resp.realm := new_tkt.realm;
            resp.sname := new_tkt.sname;

            resp.caddr := new_tkt.caddr;

            encode body of reply into OCTET STRING;

            resp.enc-part := encrypt OCTET STRING
                             using use_etype, client.key, client.p_kvno;
            send(resp);

    A.3.  KRB_AS_REP verification
            decode response into resp;

            if (resp.msg-type = KRB_ERROR) then
                    if(error = KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP))
                            then set pa_enc_timestamp_required;
                            goto KRB_AS_REQ;
                    endif
                    process_error(resp);
                    return;
            endif

            /* On error, discard the response, and zero the session key */
            /* from the response immediately */

            key = get_decryption_key(resp.enc-part.kvno, resp.enc-part.etype,
                                     resp.padata);
            unencrypted part of resp := decode of decrypt of resp.enc-part
                                    using resp.enc-part.etype and key;
            zero(key);

            if (common_as_rep_tgs_rep_checks fail) then
                    destroy resp.key;
                    return error;
            endif

            if near(resp.princ_exp) then

    Kohl & Neuman                                                  [Page 95]

    RFC 1510                        Kerberos                  September 1993

                    print(warning message);
            endif
            save_for_later(ticket,session,client,server,times,flags);

    A.4.  KRB_AS_REP and KRB_TGS_REP common checks
            if (decryption_error() or
                (req.cname != resp.cname) or
                (req.realm != resp.crealm) or
                (req.sname != resp.sname) or
                (req.realm != resp.realm) or
                (req.nonce != resp.nonce) or
                (req.addresses != resp.caddr)) then
                    destroy resp.key;
                    return KRB_AP_ERR_MODIFIED;
            endif

            /* make sure no flags are set that shouldn't be, and that  */
            /* all that should be are set                              */
            if (!check_flags_for_compatability(req.kdc-options,resp.flags))
                    then destroy resp.key;
                    return KRB_AP_ERR_MODIFIED;
            endif

            if ((req.from = 0) and
                (resp.starttime is not within allowable skew)) then
                    destroy resp.key;
                    return KRB_AP_ERR_SKEW;
            endif
            if ((req.from != 0) and (req.from != resp.starttime)) then
                    destroy resp.key;
                    return KRB_AP_ERR_MODIFIED;
            endif
            if ((req.till != 0) and (resp.endtime > req.till)) then
                    destroy resp.key;
                    return KRB_AP_ERR_MODIFIED;
            endif

            if ((req.kdc-options.RENEWABLE is set) and
                (req.rtime != 0) and (resp.renew-till > req.rtime)) then
                    destroy resp.key;
                    return KRB_AP_ERR_MODIFIED;
            endif
            if ((req.kdc-options.RENEWABLE-OK is set) and
                (resp.flags.RENEWABLE) and
                (req.till != 0) and
                (resp.renew-till > req.till)) then
                    destroy resp.key;
                    return KRB_AP_ERR_MODIFIED;

    Kohl & Neuman                                                  [Page 96]

    RFC 1510                        Kerberos                  September 1993

            endif

    A.5.  KRB_TGS_REQ generation
            /* Note that make_application_request might have to     */
            /* recursivly call this routine to get the appropriate  */
            /* ticket-granting ticket                               */

            request.pvno := protocol version; /* pvno = 5 */
            request.msg-type := message type; /* type = KRB_TGS_REQ */

            body.kdc-options := users's preferences;
            /* If the TGT is not for the realm of the end-server  */
            /* then the sname will be for a TGT for the end-realm */
            /* and the realm of the requested ticket (body.realm) */
            /* will be that of the TGS to which the TGT we are    */
            /* sending applies                                    */
            body.sname := service's name;
            body.realm := service's realm;

            if (body.kdc-options.POSTDATED is set) then
                    body.from := requested starting time;
            else
                    omit body.from;
            endif
            body.till := requested end time;
            if (body.kdc-options.RENEWABLE is set) then
                    body.rtime := requested final renewal time;
            endif
            body.nonce := random_nonce();
            body.etype := requested etypes;
            if (user supplied addresses) then
                    body.addresses := user's addresses;
            else
                    omit body.addresses;
            endif

            body.enc-authorization-data := user-supplied data;
            if (body.kdc-options.ENC-TKT-IN-SKEY) then
                    body.additional-tickets_ticket := second TGT;
            endif

            request.req-body := body;
            check := generate_checksum (req.body,checksumtype);

            request.padata[0].padata-type := PA-TGS-REQ;
            request.padata[0].padata-value := create a KRB_AP_REQ using
                                          the TGT and checksum

    Kohl & Neuman                                                  [Page 97]

    RFC 1510                        Kerberos                  September 1993

            /* add in any other padata as required/supplied */

            kerberos := lookup(name of local kerberose server (or servers));
            send(packet,kerberos);

            wait(for response);
            if (timed_out) then
                    retry or use alternate server;
            endif

    A.6.  KRB_TGS_REQ verification and KRB_TGS_REP generation
            /* note that reading the application request requires first
            determining the server for which a ticket was issued, and
            choosing the correct key for decryption.  The name of the
            server appears in the plaintext part of the ticket. */

            if (no KRB_AP_REQ in req.padata) then
                    error_out(KDC_ERR_PADATA_TYPE_NOSUPP);
            endif
            verify KRB_AP_REQ in req.padata;

            /* Note that the realm in which the Kerberos server is
            operating is determined by the instance from the
            ticket-granting ticket.  The realm in the ticket-granting
            ticket is the realm under which the ticket granting ticket was
            issued.  It is possible for a single Kerberos server to
            support more than one realm. */

            auth_hdr := KRB_AP_REQ;
            tgt := auth_hdr.ticket;

            if (tgt.sname is not a TGT for local realm and is not
                    req.sname) then error_out(KRB_AP_ERR_NOT_US);

            realm := realm_tgt_is_for(tgt);

            decode remainder of request;

            if (auth_hdr.authenticator.cksum is missing) then
                    error_out(KRB_AP_ERR_INAPP_CKSUM);
            endif
            if (auth_hdr.authenticator.cksum type is not supported) then
                    error_out(KDC_ERR_SUMTYPE_NOSUPP);
            endif
            if (auth_hdr.authenticator.cksum is not both collision-proof
                and keyed)  then
                    error_out(KRB_AP_ERR_INAPP_CKSUM);
            endif

    Kohl & Neuman                                                  [Page 98]

    RFC 1510                        Kerberos                  September 1993

            set computed_checksum := checksum(req);
            if (computed_checksum != auth_hdr.authenticatory.cksum) then
                    error_out(KRB_AP_ERR_MODIFIED);
            endif

            server := lookup(req.sname,realm);

            if (!server) then
                    if (is_foreign_tgt_name(server)) then
                            server := best_intermediate_tgs(server);
                    else
                            /* no server in Database */
                            error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
                    endif
            endif

            session := generate_random_session_key();

            use_etype := first supported etype in req.etypes;

            if (no support for req.etypes) then
                    error_out(KDC_ERR_ETYPE_NOSUPP);
            endif

            new_tkt.vno := ticket version; /* = 5 */
            new_tkt.sname := req.sname;
            new_tkt.srealm := realm;
            reset all flags in new_tkt.flags;

            /* It should be noted that local policy may affect the  */
            /* processing of any of these flags.  For example, some */
            /* realms may refuse to issue renewable tickets         */

            new_tkt.caddr := tgt.caddr;
            resp.caddr := NULL; /* We only include this if they change */
            if (req.kdc-options.FORWARDABLE is set) then
                    if (tgt.flags.FORWARDABLE is reset) then
                            error_out(KDC_ERR_BADOPTION);
                    endif
                    set new_tkt.flags.FORWARDABLE;
            endif
            if (req.kdc-options.FORWARDED is set) then
                    if (tgt.flags.FORWARDABLE is reset) then
                            error_out(KDC_ERR_BADOPTION);
                    endif
                    set new_tkt.flags.FORWARDED;
                    new_tkt.caddr := req.addresses;

    Kohl & Neuman                                                  [Page 99]

    RFC 1510                        Kerberos                  September 1993

                    resp.caddr := req.addresses;
            endif
            if (tgt.flags.FORWARDED is set) then
                    set new_tkt.flags.FORWARDED;
            endif

            if (req.kdc-options.PROXIABLE is set) then
                    if (tgt.flags.PROXIABLE is reset)
                            error_out(KDC_ERR_BADOPTION);
                    endif
                    set new_tkt.flags.PROXIABLE;
            endif
            if (req.kdc-options.PROXY is set) then
                    if (tgt.flags.PROXIABLE is reset) then
                            error_out(KDC_ERR_BADOPTION);
                    endif
                    set new_tkt.flags.PROXY;
                    new_tkt.caddr := req.addresses;
                    resp.caddr := req.addresses;
            endif

            if (req.kdc-options.POSTDATE is set) then
                    if (tgt.flags.POSTDATE is reset)
                            error_out(KDC_ERR_BADOPTION);
                    endif
                    set new_tkt.flags.POSTDATE;
            endif
            if (req.kdc-options.POSTDATED is set) then
                    if (tgt.flags.POSTDATE is reset) then
                            error_out(KDC_ERR_BADOPTION);
                    endif
                    set new_tkt.flags.POSTDATED;
                    set new_tkt.flags.INVALID;
                    if (against_postdate_policy(req.from)) then
                            error_out(KDC_ERR_POLICY);
                    endif
                    new_tkt.starttime := req.from;
            endif

            if (req.kdc-options.VALIDATE is set) then
                    if (tgt.flags.INVALID is reset) then
                            error_out(KDC_ERR_POLICY);
                    endif
                    if (tgt.starttime > kdc_time) then
                            error_out(KRB_AP_ERR_NYV);
                    endif
                    if (check_hot_list(tgt)) then

    Kohl & Neuman                                                 [Page 100]

    RFC 1510                        Kerberos                  September 1993

                            error_out(KRB_AP_ERR_REPEAT);
                    endif
                    tkt := tgt;
                    reset new_tkt.flags.INVALID;
            endif

            if (req.kdc-options.(any flag except ENC-TKT-IN-SKEY, RENEW,
                                 and those already processed) is set) then
                    error_out(KDC_ERR_BADOPTION);
            endif

            new_tkt.authtime := tgt.authtime;

            if (req.kdc-options.RENEW is set) then
              /* Note that if the endtime has already passed, the ticket */
              /* would have been rejected in the initial authentication  */
              /* stage, so there is no need to check again here          */
                    if (tgt.flags.RENEWABLE is reset) then
                            error_out(KDC_ERR_BADOPTION);
                    endif
                    if (tgt.renew-till >= kdc_time) then
                            error_out(KRB_AP_ERR_TKT_EXPIRED);
                    endif
                    tkt := tgt;
                    new_tkt.starttime := kdc_time;
                    old_life := tgt.endttime - tgt.starttime;
                    new_tkt.endtime := min(tgt.renew-till,
                                           new_tkt.starttime + old_life);
            else
                    new_tkt.starttime := kdc_time;
                    if (req.till = 0) then
                            till := infinity;
                    else
                            till := req.till;
                    endif
                    new_tkt.endtime := min(till,
                                       new_tkt.starttime+client.max_life,
                                       new_tkt.starttime+server.max_life,
                                       new_tkt.starttime+max_life_for_realm,
                                       tgt.endtime);

                    if ((req.kdc-options.RENEWABLE-OK is set) and
                        (new_tkt.endtime < req.till) and
                        (tgt.flags.RENEWABLE is set) then
                            /* we set the RENEWABLE option for later  */
                            /* processing                             */
                            set req.kdc-options.RENEWABLE;
                            req.rtime := min(req.till, tgt.renew-till);

    Kohl & Neuman                                                 [Page 101]

    RFC 1510                        Kerberos                  September 1993

                    endif
            endif

            if (req.rtime = 0) then
                    rtime := infinity;
            else
                    rtime := req.rtime;
            endif

            if ((req.kdc-options.RENEWABLE is set) and
                (tgt.flags.RENEWABLE is set)) then
                    set new_tkt.flags.RENEWABLE;
                    new_tkt.renew-till := min(rtime,
                    new_tkt.starttime+client.max_rlife,
                    new_tkt.starttime+server.max_rlife,
                    new_tkt.starttime+max_rlife_for_realm,
                    tgt.renew-till);
            else
                    new_tkt.renew-till := OMIT;
                                  /* leave the renew-till field out */
            endif
            if (req.enc-authorization-data is present) then
                    decrypt req.enc-authorization-data
                            into    decrypted_authorization_data
                            using auth_hdr.authenticator.subkey;
                    if (decrypt_error()) then
                            error_out(KRB_AP_ERR_BAD_INTEGRITY);
                    endif
            endif
            new_tkt.authorization_data :=
            req.auth_hdr.ticket.authorization_data +
                                     decrypted_authorization_data;

            new_tkt.key := session;
            new_tkt.crealm := tgt.crealm;
            new_tkt.cname := req.auth_hdr.ticket.cname;

            if (realm_tgt_is_for(tgt) := tgt.realm) then
                    /* tgt issued by local realm */
                    new_tkt.transited := tgt.transited;
            else
                    /* was issued for this realm by some other realm */
                    if (tgt.transited.tr-type not supported) then
                            error_out(KDC_ERR_TRTYPE_NOSUPP);
                    endif
                    new_tkt.transited
                       := compress_transited(tgt.transited + tgt.realm)
            endif

    Kohl & Neuman                                                 [Page 102]

    RFC 1510                        Kerberos                  September 1993

            encode encrypted part of new_tkt into OCTET STRING;
            if (req.kdc-options.ENC-TKT-IN-SKEY is set) then
                    if (server not specified) then
                            server = req.second_ticket.client;
                    endif
                    if ((req.second_ticket is not a TGT) or
                        (req.second_ticket.client != server)) then
                            error_out(KDC_ERR_POLICY);
                    endif

                    new_tkt.enc-part := encrypt OCTET STRING using
                            using etype_for_key(second-ticket.key),
                                                          second-ticket.key;
            else
                    new_tkt.enc-part := encrypt OCTET STRING
                            using etype_for_key(server.key), server.key,
                                                          server.p_kvno;
            endif

            resp.pvno := 5;
            resp.msg-type := KRB_TGS_REP;
            resp.crealm := tgt.crealm;
            resp.cname := tgt.cname;
            resp.ticket := new_tkt;

            resp.key := session;
            resp.nonce := req.nonce;
            resp.last-req := fetch_last_request_info(client);
            resp.flags := new_tkt.flags;

            resp.authtime := new_tkt.authtime;
            resp.starttime := new_tkt.starttime;
            resp.endtime := new_tkt.endtime;

            omit resp.key-expiration;

            resp.sname := new_tkt.sname;
            resp.realm := new_tkt.realm;

            if (new_tkt.flags.RENEWABLE) then
                    resp.renew-till := new_tkt.renew-till;
            endif

            encode body of reply into OCTET STRING;

            if (req.padata.authenticator.subkey)
                    resp.enc-part := encrypt OCTET STRING using use_etype,

    Kohl & Neuman                                                 [Page 103]

    RFC 1510                        Kerberos                  September 1993

                            req.padata.authenticator.subkey;
            else resp.enc-part := encrypt OCTET STRING
                                  using use_etype, tgt.key;

            send(resp);

    A.7.  KRB_TGS_REP verification
            decode response into resp;

            if (resp.msg-type = KRB_ERROR) then
                    process_error(resp);
                    return;
            endif

            /* On error, discard the response, and zero the session key from
            the response immediately */

            if (req.padata.authenticator.subkey)
                    unencrypted part of resp :=
                            decode of decrypt of resp.enc-part
                            using resp.enc-part.etype and subkey;
            else unencrypted part of resp :=
                            decode of decrypt of resp.enc-part
                            using resp.enc-part.etype and tgt's session key;
            if (common_as_rep_tgs_rep_checks fail) then
                    destroy resp.key;
                    return error;
            endif

            check authorization_data as necessary;
            save_for_later(ticket,session,client,server,times,flags);

    A.8.  Authenticator generation
            body.authenticator-vno := authenticator vno; /* = 5 */
            body.cname, body.crealm := client name;
            if (supplying checksum) then
                    body.cksum := checksum;
            endif
            get system_time;
            body.ctime, body.cusec := system_time;
            if (selecting sub-session key) then
                    select sub-session key;
                    body.subkey := sub-session key;
            endif
            if (using sequence numbers) then
                    select initial sequence number;
                    body.seq-number := initial sequence;
            endif

    Kohl & Neuman                                                 [Page 104]

    RFC 1510                        Kerberos                  September 1993

    A.9.  KRB_AP_REQ generation
            obtain ticket and session_key from cache;

            packet.pvno := protocol version; /* 5 */
            packet.msg-type := message type; /* KRB_AP_REQ */

            if (desired(MUTUAL_AUTHENTICATION)) then
                    set packet.ap-options.MUTUAL-REQUIRED;
            else
                    reset packet.ap-options.MUTUAL-REQUIRED;
            endif
            if (using session key for ticket) then
                    set packet.ap-options.USE-SESSION-KEY;
            else
                    reset packet.ap-options.USE-SESSION-KEY;
            endif
            packet.ticket := ticket; /* ticket */
            generate authenticator;
            encode authenticator into OCTET STRING;
            encrypt OCTET STRING into packet.authenticator
                                 using session_key;

    A.10.  KRB_AP_REQ verification
            receive packet;
            if (packet.pvno != 5) then
                    either process using other protocol spec
                    or error_out(KRB_AP_ERR_BADVERSION);
            endif
            if (packet.msg-type != KRB_AP_REQ) then
                    error_out(KRB_AP_ERR_MSG_TYPE);
            endif
            if (packet.ticket.tkt_vno != 5) then
                    either process using other protocol spec
                    or error_out(KRB_AP_ERR_BADVERSION);
            endif
            if (packet.ap_options.USE-SESSION-KEY is set) then
                    retrieve session key from ticket-granting ticket for
                     packet.ticket.{sname,srealm,enc-part.etype};
            else
               retrieve service key for
               packet.ticket.{sname,srealm,enc-part.etype,enc-part.skvno};
            endif
            if (no_key_available) then
                    if (cannot_find_specified_skvno) then
                            error_out(KRB_AP_ERR_BADKEYVER);
                    else
                            error_out(KRB_AP_ERR_NOKEY);
                    endif

    Kohl & Neuman                                                 [Page 105]

    RFC 1510                        Kerberos                  September 1993

            endif
            decrypt packet.ticket.enc-part into decr_ticket
                                           using retrieved key;
            if (decryption_error()) then
                    error_out(KRB_AP_ERR_BAD_INTEGRITY);
            endif
            decrypt packet.authenticator into decr_authenticator
                    using decr_ticket.key;
            if (decryption_error()) then
                    error_out(KRB_AP_ERR_BAD_INTEGRITY);
            endif
            if (decr_authenticator.{cname,crealm} !=
                decr_ticket.{cname,crealm}) then
                    error_out(KRB_AP_ERR_BADMATCH);
            endif
            if (decr_ticket.caddr is present) then
                    if (sender_address(packet) is not in decr_ticket.caddr)
                            then error_out(KRB_AP_ERR_BADADDR);
                    endif
            elseif (application requires addresses) then
                    error_out(KRB_AP_ERR_BADADDR);
            endif
            if (not in_clock_skew(decr_authenticator.ctime,
                                  decr_authenticator.cusec)) then
                    error_out(KRB_AP_ERR_SKEW);
            endif
            if (repeated(decr_authenticator.{ctime,cusec,cname,crealm}))
                    then error_out(KRB_AP_ERR_REPEAT);
            endif
            save_identifier(decr_authenticator.{ctime,cusec,cname,crealm});
            get system_time;
            if ((decr_ticket.starttime-system_time > CLOCK_SKEW) or
                (decr_ticket.flags.INVALID is set)) then
                    /* it hasn't yet become valid */
                    error_out(KRB_AP_ERR_TKT_NYV);
            endif
            if (system_time-decr_ticket.endtime > CLOCK_SKEW) then
                    error_out(KRB_AP_ERR_TKT_EXPIRED);
            endif
            /* caller must check decr_ticket.flags for any pertinent */
            /* details */
            return(OK, decr_ticket, packet.ap_options.MUTUAL-REQUIRED);

    A.11.  KRB_AP_REP generation
            packet.pvno := protocol version; /* 5 */
            packet.msg-type := message type; /* KRB_AP_REP */
            body.ctime := packet.ctime;
            body.cusec := packet.cusec;

    Kohl & Neuman                                                 [Page 106]

    RFC 1510                        Kerberos                  September 1993

            if (selecting sub-session key) then
                    select sub-session key;
                    body.subkey := sub-session key;
            endif
            if (using sequence numbers) then
                    select initial sequence number;
                    body.seq-number := initial sequence;
            endif

            encode body into OCTET STRING;

            select encryption type;
            encrypt OCTET STRING into packet.enc-part;

    A.12.  KRB_AP_REP verification
            receive packet;
            if (packet.pvno != 5) then
                    either process using other protocol spec
                    or error_out(KRB_AP_ERR_BADVERSION);
            endif
            if (packet.msg-type != KRB_AP_REP) then
                    error_out(KRB_AP_ERR_MSG_TYPE);
            endif
            cleartext := decrypt(packet.enc-part)
                         using ticket's session key;
            if (decryption_error()) then
                    error_out(KRB_AP_ERR_BAD_INTEGRITY);
            endif
            if (cleartext.ctime != authenticator.ctime) then
                    error_out(KRB_AP_ERR_MUT_FAIL);
            endif
            if (cleartext.cusec != authenticator.cusec) then
                    error_out(KRB_AP_ERR_MUT_FAIL);
            endif
            if (cleartext.subkey is present) then
                    save cleartext.subkey for future use;
            endif
            if (cleartext.seq-number is present) then
                    save cleartext.seq-number for future verifications;
            endif
            return(AUTHENTICATION_SUCCEEDED);

    A.13.  KRB_SAFE generation
            collect user data in buffer;

            /* assemble packet: */
            packet.pvno := protocol version; /* 5 */
            packet.msg-type := message type; /* KRB_SAFE */

    Kohl & Neuman                                                 [Page 107]

    RFC 1510                        Kerberos                  September 1993

            body.user-data := buffer; /* DATA */
            if (using timestamp) then
                    get system_time;
                    body.timestamp, body.usec := system_time;
            endif
            if (using sequence numbers) then
                    body.seq-number := sequence number;
            endif
            body.s-address := sender host addresses;
            if (only one recipient) then
                    body.r-address := recipient host address;
            endif
            checksum.cksumtype := checksum type;
            compute checksum over body;
            checksum.checksum := checksum value; /* checksum.checksum */
            packet.cksum := checksum;
            packet.safe-body := body;

    A.14.  KRB_SAFE verification
            receive packet;
            if (packet.pvno != 5) then
                    either process using other protocol spec
                    or error_out(KRB_AP_ERR_BADVERSION);
            endif
            if (packet.msg-type != KRB_SAFE) then
                    error_out(KRB_AP_ERR_MSG_TYPE);
            endif
            if (packet.checksum.cksumtype is not both collision-proof
                                                 and keyed) then
                    error_out(KRB_AP_ERR_INAPP_CKSUM);
            endif
            if (safe_priv_common_checks_ok(packet)) then
                    set computed_checksum := checksum(packet.body);
                    if (computed_checksum != packet.checksum) then
                            error_out(KRB_AP_ERR_MODIFIED);
                    endif
                    return (packet, PACKET_IS_GENUINE);
            else
                    return common_checks_error;
            endif

    A.15.  KRB_SAFE and KRB_PRIV common checks
            if (packet.s-address != O/S_sender(packet)) then
                /* O/S report of sender not who claims to have sent it */
                error_out(KRB_AP_ERR_BADADDR);
            endif
            if ((packet.r-address is present) and
                (packet.r-address != local_host_address)) then

    Kohl & Neuman                                                 [Page 108]

    RFC 1510                        Kerberos                  September 1993

                    /* was not sent to proper place */
                    error_out(KRB_AP_ERR_BADADDR);
            endif
            if (((packet.timestamp is present) and
                 (not in_clock_skew(packet.timestamp,packet.usec))) or
                (packet.timestamp is not present and timestamp expected))
                    then error_out(KRB_AP_ERR_SKEW);
            endif
            if (repeated(packet.timestamp,packet.usec,packet.s-address))
                    then error_out(KRB_AP_ERR_REPEAT);
            endif
            if (((packet.seq-number is present) and
                 ((not in_sequence(packet.seq-number)))) or
                (packet.seq-number is not present and sequence expected))
                    then error_out(KRB_AP_ERR_BADORDER);
            endif
            if (packet.timestamp not present and
                packet.seq-number not present) then
                    error_out(KRB_AP_ERR_MODIFIED);
            endif

            save_identifier(packet.{timestamp,usec,s-address},
                            sender_principal(packet));

            return PACKET_IS_OK;

    A.16.  KRB_PRIV generation
            collect user data in buffer;

            /* assemble packet: */
            packet.pvno := protocol version; /* 5 */
            packet.msg-type := message type; /* KRB_PRIV */

            packet.enc-part.etype := encryption type;

            body.user-data := buffer;
            if (using timestamp) then
                    get system_time;
                    body.timestamp, body.usec := system_time;
            endif
            if (using sequence numbers) then
                    body.seq-number := sequence number;
            endif
            body.s-address := sender host addresses;
            if (only one recipient) then
                    body.r-address := recipient host address;
            endif

    Kohl & Neuman                                                 [Page 109]

    RFC 1510                        Kerberos                  September 1993

            encode body into OCTET STRING;

            select encryption type;
            encrypt OCTET STRING into packet.enc-part.cipher;

    A.17.  KRB_PRIV verification
            receive packet;
            if (packet.pvno != 5) then
                    either process using other protocol spec
                    or error_out(KRB_AP_ERR_BADVERSION);
            endif
            if (packet.msg-type != KRB_PRIV) then
                    error_out(KRB_AP_ERR_MSG_TYPE);
            endif

            cleartext := decrypt(packet.enc-part) using negotiated key;
            if (decryption_error()) then
                    error_out(KRB_AP_ERR_BAD_INTEGRITY);
            endif

            if (safe_priv_common_checks_ok(cleartext)) then
                return(cleartext.DATA, PACKET_IS_GENUINE_AND_UNMODIFIED);
            else
                    return common_checks_error;
            endif

    A.18.  KRB_CRED generation
            invoke KRB_TGS; /* obtain tickets to be provided to peer */

            /* assemble packet: */
            packet.pvno := protocol version; /* 5 */
            packet.msg-type := message type; /* KRB_CRED */

            for (tickets[n] in tickets to be forwarded) do
                    packet.tickets[n] = tickets[n].ticket;
            done

            packet.enc-part.etype := encryption type;

            for (ticket[n] in tickets to be forwarded) do
                    body.ticket-info[n].key = tickets[n].session;
                    body.ticket-info[n].prealm = tickets[n].crealm;
                    body.ticket-info[n].pname = tickets[n].cname;
                    body.ticket-info[n].flags = tickets[n].flags;
                    body.ticket-info[n].authtime = tickets[n].authtime;
                    body.ticket-info[n].starttime = tickets[n].starttime;
                    body.ticket-info[n].endtime = tickets[n].endtime;
                    body.ticket-info[n].renew-till = tickets[n].renew-till;

    Kohl & Neuman                                                 [Page 110]

    RFC 1510                        Kerberos                  September 1993

                    body.ticket-info[n].srealm = tickets[n].srealm;
                    body.ticket-info[n].sname = tickets[n].sname;
                    body.ticket-info[n].caddr = tickets[n].caddr;
            done

            get system_time;
            body.timestamp, body.usec := system_time;

            if (using nonce) then
                    body.nonce := nonce;
            endif

            if (using s-address) then
                    body.s-address := sender host addresses;
            endif
            if (limited recipients) then
                    body.r-address := recipient host address;
            endif

            encode body into OCTET STRING;

            select encryption type;
            encrypt OCTET STRING into packet.enc-part.cipher
            using negotiated encryption key;

    A.19.  KRB_CRED verification
            receive packet;
            if (packet.pvno != 5) then
                    either process using other protocol spec
                    or error_out(KRB_AP_ERR_BADVERSION);
            endif
            if (packet.msg-type != KRB_CRED) then
                    error_out(KRB_AP_ERR_MSG_TYPE);
            endif

            cleartext := decrypt(packet.enc-part) using negotiated key;
            if (decryption_error()) then
                    error_out(KRB_AP_ERR_BAD_INTEGRITY);
            endif
            if ((packet.r-address is present or required) and
               (packet.s-address != O/S_sender(packet)) then
                /* O/S report of sender not who claims to have sent it */
                error_out(KRB_AP_ERR_BADADDR);
            endif
            if ((packet.r-address is present) and
                (packet.r-address != local_host_address)) then
                    /* was not sent to proper place */
                    error_out(KRB_AP_ERR_BADADDR);

    Kohl & Neuman                                                 [Page 111]

    RFC 1510                        Kerberos                  September 1993

            endif
            if (not in_clock_skew(packet.timestamp,packet.usec)) then
                    error_out(KRB_AP_ERR_SKEW);
            endif
            if (repeated(packet.timestamp,packet.usec,packet.s-address))
                    then error_out(KRB_AP_ERR_REPEAT);
            endif
            if (packet.nonce is required or present) and
               (packet.nonce != expected-nonce) then
                    error_out(KRB_AP_ERR_MODIFIED);
            endif

            for (ticket[n] in tickets that were forwarded) do
                    save_for_later(ticket[n],key[n],principal[n],
                                   server[n],times[n],flags[n]);
            return

    A.20.  KRB_ERROR generation

            /* assemble packet: */
            packet.pvno := protocol version; /* 5 */
            packet.msg-type := message type; /* KRB_ERROR */

            get system_time;
            packet.stime, packet.susec := system_time;
            packet.realm, packet.sname := server name;

            if (client time available) then
                    packet.ctime, packet.cusec := client_time;
            endif
            packet.error-code := error code;
            if (client name available) then
                    packet.cname, packet.crealm := client name;
            endif
            if (error text available) then
                    packet.e-text := error text;
            endif
            if (error data available) then
                    packet.e-data := error data;
            endif

    Kohl & Neuman                                                 [Page 112]