    Internet Engineering Task Force (IETF)                            L. Zhu
    Request for Comments: 8062                                      P. Leach
    Obsoletes: 6112                                    Microsoft Corporation
    Updates: 4120, 4121, 4556                                     S. Hartman
    Category: Standards Track                              Hadron Industries
    ISSN: 2070-1721                                            S. Emery, Ed.
                                                                      Oracle
                                                               February 2017

                         Anonymity Support for Kerberos

    Abstract

       This document defines extensions to the Kerberos protocol to allow a
       Kerberos client to securely communicate with a Kerberos application
       service without revealing its identity, or without revealing more
       than its Kerberos realm.  It also defines extensions that allow a
       Kerberos client to obtain anonymous credentials without revealing its
       identity to the Kerberos Key Distribution Center (KDC).  This
       document updates RFCs 4120, 4121, and 4556.  This document obsoletes
       RFC 6112 and reclassifies that document as Historic.  RFC 6112
       contained errors, and the protocol described in that specification is
       not interoperable with any known implementation.  This specification
       describes a protocol that interoperates with multiple
       implementations.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc8062.

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    Copyright Notice

       Copyright (c) 2017 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (http://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

       This document may contain material from IETF Documents or IETF
       Contributions published or made publicly available before November
       10, 2008.  The person(s) controlling the copyright in some of this
       material may not have granted the IETF Trust the right to allow
       modifications of such material outside the IETF Standards Process.
       Without obtaining an adequate license from the person(s) controlling
       the copyright in such materials, this document may not be modified
       outside the IETF Standards Process, and derivative works of it may
       not be created outside the IETF Standards Process, except to format
       it for publication as an RFC or to translate it into languages other
       than English.

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    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
         1.1.  Changes since RFC 6112  . . . . . . . . . . . . . . . . .   4
       2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
       3.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   4
       4.  Protocol Description  . . . . . . . . . . . . . . . . . . . .   6
         4.1.  Anonymity Support in AS Exchange  . . . . . . . . . . . .   6
           4.1.1.  Anonymous PKINIT  . . . . . . . . . . . . . . . . . .   7
         4.2.  Anonymity Support in TGS Exchange . . . . . . . . . . . .   8
         4.3.  Subsequent Exchanges and Protocol Actions Common to AS
               and TGS for Anonymity Support . . . . . . . . . . . . . .  10
       5.  Interoperability Requirements . . . . . . . . . . . . . . . .  11
       6.  GSS-API Implementation Notes  . . . . . . . . . . . . . . . .  11
       7.  PKINIT Client Contribution to the Ticket Session Key  . . . .  12
         7.1.  Combining Two Protocol Keys . . . . . . . . . . . . . . .  14
       8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
       9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
       10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
         10.1.  Normative References . . . . . . . . . . . . . . . . . .  16
         10.2.  Informative References . . . . . . . . . . . . . . . . .  17
       Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  17
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

    1.  Introduction

       In certain situations, the Kerberos [RFC4120] client may wish to
       authenticate a server and/or protect communications without revealing
       the client's own identity.  For example, consider an application that
       provides read access to a research database and that permits queries
       by arbitrary requesters.  A client of such a service might wish to
       authenticate the service, to establish trust in the information
       received from it, but might not wish to disclose the client's
       identity to the service for privacy reasons.

       Extensions to Kerberos are specified in this document by which a
       client can authenticate the Key Distribution Center (KDC) and request
       an anonymous ticket.  The client can use the anonymous ticket to
       authenticate the server and protect subsequent client-server
       communications.

       By using the extensions defined in this specification, the client can
       request an anonymous ticket where the client may reveal the client's
       identity to the client's own KDC, or the client can hide the client's
       identity completely by using anonymous Public Key Cryptography for
       Initial Authentication in Kerberos (PKINIT) as defined in
       Section 4.1.  Using the returned anonymous ticket, the client remains

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       anonymous in subsequent Kerberos exchanges thereafter to KDCs on the
       cross-realm authentication path and to the server with which it
       communicates.

       In this specification, the client realm in the anonymous ticket is
       the anonymous realm name when anonymous PKINIT is used to obtain the
       ticket.  The client realm is the client's real realm name if the
       client is authenticated using the client's long-term keys.  Note that
       a membership in a realm can imply a member of the community
       represented by the realm.

       The interaction with Generic Security Service Application Program
       Interface (GSS-API) is described after the protocol description.

       This specification replaces [RFC6112] to correct technical errors in
       that specification.  RFC 6112 is classified as Historic;
       implementation of RFC 6112 is NOT RECOMMENDED.  All known
       implementations comply with this specification and not RFC 6112.

    1.1.  Changes since RFC 6112

       In Section 7, the pepper2 string "KeyExchange" used in RFC 6112 is
       corrected to appear in all capital letters to comply with the string
       actually used by implementations.

       The requirement for the anonymous option to be used when an anonymous
       ticket is used in a Ticket-Granting Service (TGS) request is reduced
       from a MUST to a SHOULD.  At least one implementation does not
       require this; it is not necessary that both the anonymous option and
       anonymous ticket be used as an indicator of request type.

       The authorization data type name "AD-INITIAL-VERIFIED-CAS" used in
       RFC 6112 is corrected to appear as "AD_INITIAL_VERIFIED_CAS" in this
       document.

    2.  Conventions Used in This Document

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in [RFC2119].

    3.  Definitions

       The anonymous Kerberos realm name is defined as a well-known realm
       name based on [RFC6111], and the value of this well-known realm name
       is the literal "WELLKNOWN:ANONYMOUS".

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       The anonymous Kerberos principal name is defined as a well-known
       Kerberos principal name based on [RFC6111].  The value of the name-
       type field is KRB_NT_WELLKNOWN [RFC6111], and the value of the name-
       string field is a sequence of two KerberosString components:
       "WELLKNOWN" and "ANONYMOUS".

       The anonymous ticket flag is defined as bit 16 (with the first bit
       being bit 0) in the TicketFlags:

               TicketFlags     ::= KerberosFlags
                 -- anonymous(16)
                 -- TicketFlags and KerberosFlags are defined in [RFC4120]

       This is a new ticket flag that is used to indicate that a ticket is
       an anonymous one.

       An anonymous ticket is a ticket that has all of the following
       properties:

       o  The cname field contains the anonymous Kerberos principal name.

       o  The crealm field contains the client's realm name or the anonymous
          realm name.

       o  The anonymous ticket contains no information that can reveal the
          client's identity.  However, the ticket may contain the client
          realm, intermediate realms on the client's authentication path,
          and authorization data that may provide information related to the
          client's identity.  For example, an anonymous principal that is
          identifiable only as being in a particular group of users can be
          implemented using authorization data.  Such authorization data, if
          included in the anonymous ticket, would disclose that the client
          is a member of the group observed.

       o  The anonymous ticket flag is set.

       The anonymous KDC option is defined as bit 16 (with the first bit
       being bit 0) in the KDCOptions:

               KDCOptions      ::= KerberosFlags
                 -- anonymous(16)
                 -- KDCOptions and KerberosFlags are defined in [RFC4120]

       As described in Section 4, the anonymous KDC option is set to request
       an anonymous ticket in an Authentication Service (AS) request or a
       Ticket-Granting Service (TGS) request.

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    4.  Protocol Description

       In order to request an anonymous ticket, the client sets the
       anonymous KDC option in an AS request or a TGS request.

       The rest of this section is organized as follows: it first describes
       protocol actions specific to AS exchanges, then it describes those of
       TGS exchanges.  These are then followed by the description of
       protocol actions common to both AS and TGS and those in subsequent
       exchanges.

    4.1.  Anonymity Support in AS Exchange

       The client requests an anonymous ticket by setting the anonymous KDC
       option in an AS exchange.

       The Kerberos client can use the client's long-term keys, the client's
       X.509 certificates [RFC4556], or any other pre-authentication data to
       authenticate to the KDC and request an anonymous ticket in an AS
       exchange where the client's identity is known to the KDC.

       If the client in the AS request is anonymous, the anonymous KDC
       option MUST be set in the request.  Otherwise, the KDC MUST return a
       KRB-ERROR message with the code KDC_ERR_BADOPTION.

       If the client is anonymous and the KDC does not have a key to encrypt
       the reply (this can happen when, for example, the KDC does not
       support PKINIT [RFC4556]), the KDC MUST return an error message with
       the code KDC_ERR_NULL_KEY [RFC4120].

       When policy allows, the KDC issues an anonymous ticket.  If the
       client name in the request is the anonymous principal, the client
       realm (crealm) in the reply is the anonymous realm; otherwise, the
       client realm is the realm of the AS.  As specified by [RFC4120], the
       client name and the client realm in the EncTicketPart of the reply
       MUST match with the corresponding client name and the client realm of
       the KDC reply; the client MUST use the client name and the client
       realm returned in the KDC-REP in subsequent message exchanges when
       using the obtained anonymous ticket.

       The KDC MUST NOT reveal the client's identity in the authorization
       data of the returned ticket when populating the authorization data in
       a returned anonymous ticket.

       The AD_INITIAL_VERIFIED_CAS authorization data, as defined in
       [RFC4556], contains the issuer name of the client certificate.  This
       authorization is not applicable and MUST NOT be present in the
       returned anonymous ticket when anonymous PKINIT is used.  When the

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       client is authenticated (i.e., anonymous PKINIT is not used), if it
       is undesirable to disclose such information about the client's
       identity, the AD_INITIAL_VERIFIED_CAS authorization data SHOULD be
       removed from the returned anonymous ticket.

       The client can use the client's key to mutually authenticate with the
       KDC and request an anonymous Ticket-Granting Ticket (TGT) in the AS
       request.  In that case, the reply key is selected as normal,
       according to Section 3.1.3 of [RFC4120].

    4.1.1.  Anonymous PKINIT

       This sub-section defines anonymous PKINIT.

       As described earlier in this section, the client can request an
       anonymous ticket by authenticating to the KDC using the client's
       identity; alternatively, without revealing the client's identity to
       the KDC, the Kerberos client can request an anonymous ticket as
       follows: the client sets the client name as the anonymous principal
       in the AS exchange and provides PA_PK_AS_REQ pre-authentication data
       [RFC4556] where the signerInfos field of the SignedData [RFC5652] of
       the PA_PK_AS_REQ is empty, and the certificates field is absent.
       Because the anonymous client does not have an associated asymmetric
       key pair, the client MUST choose the Diffie-Hellman key agreement
       method by filling in the Diffie-Hellman domain parameters in the
       clientPublicValue [RFC4556].  This use of the anonymous client name
       in conjunction with PKINIT is referred to as "anonymous PKINIT".  If
       anonymous PKINIT is used, the realm name in the returned anonymous
       ticket MUST be the anonymous realm.

       Upon receiving the anonymous PKINIT request from the client, the KDC
       processes the request, according to Section 3.1.2 of [RFC4120].  The
       KDC skips the checks for the client's signature and the client's
       public key (such as the verification of the binding between the
       client's public key and the client name) but performs otherwise
       applicable checks and proceeds as normal, according to [RFC4556].
       For example, the AS MUST check if the client's Diffie-Hellman domain
       parameters are acceptable.  The Diffie-Hellman key agreement method
       MUST be used and the reply key is derived according to
       Section 3.2.3.1 of [RFC4556].  If the clientPublicValue is not
       present in the request, the KDC MUST return a KRB-ERROR with the code
       KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED [RFC4556].  If all goes
       well, an anonymous ticket is generated, according to Section 3.1.3 of
       [RFC4120], and PA_PK_AS_REP [RFC4556] pre-authentication data is
       included in the KDC reply, according to [RFC4556].  If the KDC does
       not have an asymmetric key pair, it MAY reply anonymously or reject
       the authentication attempt.  If the KDC replies anonymously, the

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       signerInfos field of the SignedData [RFC5652] of PA_PK_AS_REP in the
       reply is empty, and the certificates field is absent.  The server
       name in the anonymous KDC reply contains the name of the TGS.

       Upon receipt of the KDC reply that contains an anonymous ticket and
       PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then
       authenticate the KDC based on the KDC's signature in the
       PA_PK_AS_REP.  If the KDC's signature is missing in the KDC reply
       (the reply is anonymous), the client MUST reject the returned ticket
       if it cannot authenticate the KDC otherwise.

       A KDC that supports anonymous PKINIT MUST indicate the support of
       PKINIT, according to Section 3.4 of [RFC4556].  In addition, such a
       KDC MUST indicate support for anonymous PKINIT by including a padata
       element of padata-type PA_PKINIT_KX and empty padata-value when
       including PA-PK-AS-REQ in an error reply.

       When included in a KDC error, PA_PKINIT_KX indicates support for
       anonymous PKINIT.  As discussed in Section 7, when included in an
       AS-REP, PA_PKINIT_KX proves that the KDC and client both contributed
       to the session key for any use of Diffie-Hellman key agreement with
       PKINIT.

       Note that in order to obtain an anonymous ticket with the anonymous
       realm name, the client MUST set the client name as the anonymous
       principal in the request when requesting an anonymous ticket in an AS
       exchange.  Anonymous PKINIT is the only way via which an anonymous
       ticket with the anonymous realm as the client realm can be generated
       in this specification.

    4.2.  Anonymity Support in TGS Exchange

       The client requests an anonymous ticket by setting the anonymous KDC
       option in a TGS exchange, and in that request, the client can use a
       normal Ticket-Granting Ticket (TGT) with the client's identity, an
       anonymous TGT, or an anonymous cross-realm TGT.  If the client uses a
       normal TGT, the client's identity is known to the TGS.

       Note that the client can completely hide the client's identity in an
       AS exchange using anonymous PKINIT, as described in the previous
       section.

       If the ticket in the PA-TGS-REQ of the TGS request is an anonymous
       one, the anonymous KDC option SHOULD be set in the request.

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       When policy allows, the KDC issues an anonymous ticket.  If the
       ticket in the TGS request is an anonymous one, the client name and
       the client realm are copied from that ticket; otherwise, the ticket
       in the TGS request is a normal ticket, the returned anonymous ticket
       contains the client name as the anonymous principal and the client
       realm as the true realm of the client.  In all cases, according to
       [RFC4120], the client name and the client realm in the EncTicketPart
       of the reply MUST match with the corresponding client name and the
       client realm of the anonymous ticket in the reply; the client MUST
       use the client name and the client realm returned in the KDC-REP in
       subsequent message exchanges when using the obtained anonymous
       ticket.

       The TGS MUST NOT reveal the client's identity in the authorization
       data of the returned ticket.  When propagating authorization data in
       the ticket or in the enc-authorization-data field of the request, the
       TGS MUST ensure that the client confidentiality is not violated in
       the returned anonymous ticket.  The TGS MUST process the
       authorization data recursively, according to Section 5.2.6 of
       [RFC4120], beyond the container levels such that all embedded
       authorization elements are interpreted.  The TGS SHOULD NOT populate
       identity-based authorization data into an anonymous ticket in that
       such authorization data typically reveals the client's identity.  The
       specification of a new authorization data type MUST specify the
       processing rules of the authorization data when an anonymous ticket
       is returned.  If there is no processing rule defined for an
       authorization data element or the authorization data element is
       unknown, the TGS MUST process it when an anonymous ticket is returned
       as follows:

       o  If the authorization data element may reveal the client's
          identity, it MUST be removed unless otherwise specified.

       o  If the authorization data element that could reveal the client's
          identity is intended to restrict the use of the ticket or limit
          the rights otherwise conveyed in the ticket, it cannot be removed
          in order to hide the client's identity.  In this case, the
          authentication attempt MUST be rejected, and the TGS MUST return
          an error message with the code KDC_ERR_POLICY.  Note this is
          applicable to both critical and optional authorization data.

       o  If the authorization data element is unknown, the TGS MAY remove
          it, or transfer it into the returned anonymous ticket, or reject
          the authentication attempt, based on local policy for that
          authorization data type unless otherwise specified.  If there is
          no policy defined for a given unknown authorization data type, the
          authentication MUST be rejected.  The error code is KDC_ERR_POLICY
          when the authentication is rejected.

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       The AD_INITIAL_VERIFIED_CAS authorization data, as defined in
       [RFC4556], contains the issuer name of the client certificate.  If it
       is undesirable to disclose such information about the client's
       identity, the AD_INITIAL_VERIFIED_CAS authorization data SHOULD be
       removed from an anonymous ticket.

       The TGS encodes the name of the previous realm into the transited
       field, according to Section 3.3.3.2 of [RFC4120].  Based on local
       policy, the TGS MAY omit the previous realm, if the cross-realm TGT
       is an anonymous one, in order to hide the authentication path of the
       client.  The unordered set of realms in the transited field, if
       present, can reveal which realm may potentially be the realm of the
       client or the realm that issued the anonymous TGT.  The anonymous
       Kerberos realm name MUST NOT be present in the transited field of a
       ticket.  The true name of the realm that issued the anonymous ticket
       MAY be present in the transited field of a ticket.

    4.3.  Subsequent Exchanges and Protocol Actions Common to AS and TGS for
          Anonymity Support

       In both AS and TGS exchanges, the realm field in the KDC request is
       always the realm of the target KDC, not the anonymous realm when the
       client requests an anonymous ticket.

       Absent other information, the KDC MUST NOT include any identifier in
       the returned anonymous ticket that could reveal the client's identity
       to the server.

       Unless anonymous PKINIT is used, if a client requires anonymous
       communication, then the client MUST check to make sure that the
       ticket in the reply is actually anonymous by checking the presence of
       the anonymous ticket flag in the flags field of the EncKDCRepPart.
       This is because KDCs ignore unknown KDC options.  A KDC that does not
       understand the anonymous KDC option will not return an error but will
       instead return a normal ticket.

       The subsequent client and server communications then proceed as
       described in [RFC4120].

       Note that the anonymous principal name and realm are only applicable
       to the client in Kerberos messages, and the server cannot be
       anonymous in any Kerberos message per this specification.

       A server accepting an anonymous service ticket may assume that
       subsequent requests using the same ticket originate from the same
       client.  Requests with different tickets are likely to originate from
       different clients.

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       Upon receipt of an anonymous ticket, the transited policy check is
       performed in the same way as that of a normal ticket if the client's
       realm is not the anonymous realm; if the client realm is the
       anonymous realm, absent other information, any realm in the
       authentication path is allowed by the cross-realm policy check.

    5.  Interoperability Requirements

       Conforming implementations MUST support the anonymous principal with
       a non-anonymous realm, and they MAY support the anonymous principal
       with the anonymous realm using anonymous PKINIT.

    6.  GSS-API Implementation Notes

       GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to
       represent the anonymous identity.  In addition, Section 2.1.1 of
       [RFC1964] defines the single string representation of a Kerberos
       principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME.  The
       anonymous principal with the anonymous realm corresponds to the
       GSS-API anonymous principal.  A principal with the anonymous
       principal name and a non-anonymous realm is an authenticated
       principal; hence, such a principal does not correspond to the
       anonymous principal in GSS-API with the GSS_C_NT_ANONYMOUS name type.
       The [RFC1964] name syntax for GSS_KRB5_NT_PRINCIPAL_NAME MUST be used
       for importing the anonymous principal name with a non-anonymous realm
       name and for displaying and exporting these names.  In addition, this
       syntax must be used along with the name type GSS_C_NT_ANONYMOUS for
       displaying and exporting the anonymous principal with the anonymous
       realm.

       At the GSS-API [RFC2743] level, an initiator/client requests the use
       of an anonymous principal with the anonymous realm by asserting the
       "anonymous" flag when calling GSS_Init_Sec_Context().  The GSS-API
       implementation MAY provide implementation-specific means for
       requesting the use of an anonymous principal with a non-anonymous
       realm.

       GSS-API does not know or define "anonymous credentials", so the
       (printable) name of the anonymous principal will rarely be used by or
       relevant for the initiator/client.  The printable name is relevant
       for the acceptor/server when performing an authorization decision
       based on the initiator name that is returned from the acceptor side
       upon the successful security context establishment.

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       A GSS-API initiator MUST carefully check the resulting context
       attributes from the initial call to GSS_Init_Sec_Context() when
       requesting anonymity, because (as in the GSS-API tradition and for
       backwards compatibility) anonymity is just another optional context
       attribute.  It could be that the mechanism doesn't recognize the
       attribute at all or that anonymity is not available for some other
       reasons -- and in that case, the initiator MUST NOT send the initial
       security context token to the acceptor, because it will likely reveal
       the initiator's identity to the acceptor, something that can rarely
       be "undone".

       Portable initiators are RECOMMENDED to use default credentials
       whenever possible and request anonymity only through the input
       anon_req_flag [RFC2743] to GSS_Init_Sec_Context().

    7.  PKINIT Client Contribution to the Ticket Session Key

       The definition in this section was motivated by protocol analysis of
       anonymous PKINIT (defined in this document) in building secure
       channels [RFC6113] and subsequent channel bindings [RFC5056].  In
       order to enable applications of anonymous PKINIT to form secure
       channels, all implementations of anonymous PKINIT need to meet the
       requirements of this section.  There is otherwise no connection to
       the rest of this document.

       PKINIT is useful for constructing secure channels.  To ensure that an
       active attacker cannot create separate channels to the client and KDC
       with the same known key, it is desirable that neither the KDC nor the
       client unilaterally determine the ticket session key.  The specific
       reason why the ticket session key is derived jointly is discussed at
       the end of this section.  To achieve that end, a KDC conforming to
       this definition MUST encrypt a randomly generated key, called the
       "KDC contribution key", in the PA_PKINIT_KX padata (defined next in
       this section).  The KDC contribution key is then combined with the
       reply key to form the ticket session key of the returned ticket.
       These two keys are combined using the KRB-FX-CF2 operation defined in
       Section 7.1, where K1 is the KDC contribution key, K2 is the reply
       key, the input pepper1 is US-ASCII [ANSI.X3-4] string "PKINIT", and
       the input pepper2 is US-ASCII string "KEYEXCHANGE".

       PA_PKINIT_KX      147
         -- padata for PKINIT that contains an encrypted
         -- KDC contribution key.

       PA-PKINIT-KX  ::= EncryptedData -- EncryptionKey
         -- Contains an encrypted key randomly
         -- generated by the KDC (known as the KDC contribution key).
         -- Both EncryptedData and EncryptionKey are defined in [RFC4120]

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       The PA_PKINIT_KX padata MUST be included in the KDC reply when
       anonymous PKINIT is used; it SHOULD be included if PKINIT is used
       with the Diffie-Hellman key exchange but the client is not anonymous;
       it MUST NOT be included otherwise (e.g., when PKINIT is used with the
       public key encryption as the key exchange).

       The padata-value field of the PA-PKINIT-KX type padata contains the
       DER [X.680] [X.690] encoding of the Abstract Syntax Notation One
       (ASN.1) type PA-PKINIT-KX.  The PA-PKINIT-KX structure is an
       EncryptedData.  The cleartext data being encrypted is the DER-encoded
       KDC contribution key randomly generated by the KDC.  The encryption
       key is the reply key, and the key usage number is
       KEY_USAGE_PA_PKINIT_KX (44).

       The client then decrypts the KDC contribution key and verifies that
       the ticket session key in the returned ticket is the combined key of
       the KDC contribution key and the reply key as described above.  A
       conforming client MUST reject anonymous PKINIT authentication if the
       PA_PKINIT_KX padata is not present in the KDC reply or if the ticket
       session key of the returned ticket is not the combined key of the KDC
       contribution key and the reply key when PA-PKINIT-KX is present in
       the KDC reply.

       This protocol provides a binding between the party that generated the
       session key and the Diffie-Hellman exchange used to generate the
       reply key.  Hypothetically, if the KDC did not use PA-PKINIT-KX, the
       client and KDC would perform a Diffie-Hellman key exchange to
       determine a shared key, and that key would be used as a reply key.
       The KDC would then generate a ticket with a session key encrypting
       the reply with the Diffie-Helman agreement.  A man-in-the-middle
       (MITM) attacker would just decrypt the session key and ticket using
       the Diffie-Hellman key from the attacker-KDC Diffie-Hellman exchange
       and re-encrypt it using the key from the attacker-client Diffie-
       Hellman exchange, while keeping a copy of the session key and ticket.
       This protocol binds the ticket to the Diffie-Hellman exchange and
       prevents the MITM attack by requiring the session key to be created
       in a way that can be verified by the client.

    Zhu, et al.                  Standards Track                   [Page 13]

    RFC 8062               Kerberos Anonymity Support          February 2017

    7.1.  Combining Two Protocol Keys

       KRB-FX-CF2() combines two protocol keys based on the pseudo-random()
       function defined in [RFC3961].

       Given two input keys, K1 and K2, where K1 and K2 can be of two
       different enctypes, the output key of KRB-FX-CF2(), K3, is derived as
       follows:

        KRB-FX-CF2(protocol key, protocol key, octet string,
                  octet string)  ->  (protocol key)

        PRF+(K1, pepper1) -> octet-string-1
        PRF+(K2, pepper2) -> octet-string-2
        KRB-FX-CF2(K1, K2, pepper1, pepper2) ->
               random-to-key(octet-string-1 ^ octet-string-2)

       Where ^ denotes the exclusive-OR operation.  PRF+() is defined as
       follows:

       PRF+(protocol key, octet string) -> (octet string)

       PRF+(key, shared-info) -> pseudo-random( key,  1 || shared-info ) ||
                    pseudo-random( key, 2 || shared-info ) ||
                    pseudo-random( key, 3 || shared-info ) || ...

       Here the counter value 1, 2, 3, and so on are encoded as a one-octet
       integer.  The pseudo-random() operation is specified by the enctype
       of the protocol key.  PRF+() uses the counter to generate enough bits
       as needed by the random-to-key() [RFC3961] function for the
       encryption type specified for the resulting key; unneeded bits are
       removed from the tail.

    8.  Security Considerations

       Since KDCs ignore unknown options, a client requiring anonymous
       communication needs to make sure that the returned ticket is actually
       anonymous.  This is because a KDC that does not understand the
       anonymous option would not return an anonymous ticket.

       By using the mechanism defined in this specification, the client does
       not reveal the client's identity to the server, but the client's
       identity may be revealed to the KDC of the server principal (when the
       server principal is in a different realm than that of the client) and
       any KDC on the cross-realm authentication path.  The Kerberos client
       MUST verify the ticket being used is indeed anonymous before
       communicating with the server, otherwise, the client's identity may
       be revealed unintentionally.

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    RFC 8062               Kerberos Anonymity Support          February 2017

       In cases where specific server principals must not have access to the
       client's identity (for example, an anonymous poll service), the KDC
       can define the server-principal-specific policy that ensures any
       normal service ticket can NEVER be issued to any of these server
       principals.

       If the KDC that issued an anonymous ticket were to maintain records
       of the association of identities to an anonymous ticket, then someone
       obtaining such records could breach the anonymity.  Additionally, the
       implementations of most (for now all) KDCs respond to requests at the
       time that they are received.  Traffic analysis on the connection to
       the KDC will allow an attacker to match client identities to
       anonymous tickets issued.  Because there are plaintext parts of the
       tickets that are exposed on the wire, such matching by a third-party
       observer is relatively straightforward.  A service that is
       authenticated by the anonymous principals may be able to infer the
       identity of the client by examining and linking quasi-static protocol
       information such as the IP address from which a request is received
       or by linking multiple uses of the same anonymous ticket.

       Two mechanisms, the FAST facility with the hide-client-names option
       in [RFC6113] and the Kerberos5 starttls option [RFC6251], protect the
       client identity so that an attacker would never be able to observe
       the client identity sent to the KDC.  Transport- or network-layer
       security between the client and the server will help prevent tracking
       of a particular ticket to link a ticket to a user.  In addition,
       clients can limit how often a ticket is reused to minimize ticket
       linking.

       The client's real identity is not revealed when the client is
       authenticated as the anonymous principal.  Application servers MAY
       reject the authentication in order to, for example, prevent
       information disclosure or as part of Denial-of-Service (DoS)
       prevention.  Application servers MUST avoid accepting anonymous
       credentials in situations where they must record the client's
       identity, for example, when there must be an audit trail.

    9.  IANA Considerations

       This document defines an 'anonymous' Kerberos well-known name and an
       'anonymous' Kerberos well-known realm based on [RFC6111].  IANA has
       updated these two entries in the "Well-Known Kerberos Principal
       Names" and "Well-Known Kerberos Realm Names" registries,
       respectively, to refer to this document.

       In addition, IANA has updated the reference for PA_PKINIT_KX (147) in
       the "Pre-authentication and Typed Data" registry to refer to this
       document.

    Zhu, et al.                  Standards Track                   [Page 15]

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    10.  References

    10.1.  Normative References

       [ANSI.X3-4]
                  American National Standards Institute, "Coded Character
                  Set - 7-bit American Standard Code for Information
                  Interchange", ANSI X3-4, 1986.

       [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
                  RFC 1964, DOI 10.17487/RFC1964, June 1996,
                  <http://www.rfc-editor.org/info/rfc1964>.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <http://www.rfc-editor.org/info/rfc2119>.

       [RFC2743]  Linn, J., "Generic Security Service Application Program
                  Interface Version 2, Update 1", RFC 2743,
                  DOI 10.17487/RFC2743, January 2000,
                  <http://www.rfc-editor.org/info/rfc2743>.

       [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
                  Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February
                  2005, <http://www.rfc-editor.org/info/rfc3961>.

       [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
                  Kerberos Network Authentication Service (V5)", RFC 4120,
                  DOI 10.17487/RFC4120, July 2005,
                  <http://www.rfc-editor.org/info/rfc4120>.

       [RFC4556]  Zhu, L. and B. Tung, "Public Key Cryptography for Initial
                  Authentication in Kerberos (PKINIT)", RFC 4556,
                  DOI 10.17487/RFC4556, June 2006,
                  <http://www.rfc-editor.org/info/rfc4556>.

       [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
                  RFC 5652, DOI 10.17487/RFC5652, September 2009,
                  <http://www.rfc-editor.org/info/rfc5652>.

       [RFC6111]  Zhu, L., "Additional Kerberos Naming Constraints",
                  RFC 6111, DOI 10.17487/RFC6111, April 2011,
                  <http://www.rfc-editor.org/info/rfc6111>.

       [RFC6112]  Zhu, L., Leach, P., and S. Hartman, "Anonymity Support for
                  Kerberos", RFC 6112, DOI 10.17487/RFC6112, April 2011,
                  <http://www.rfc-editor.org/info/rfc6112>.

    Zhu, et al.                  Standards Track                   [Page 16]

    RFC 8062               Kerberos Anonymity Support          February 2017

       [X.680]    International Telecommunications Union, "Information
                  technology - Abstract Syntax Notation One (ASN.1):
                  Specification of Basic Notation", ITU-T Recommendation
                  X.680, ISO/IEC International Standard 8824-1:1998, 1997.

       [X.690]    International Telecommunications Union, "Information
                  technology - ASN.1 encoding rules: Specification of Basic
                  Encoding Rules (BER), Canonical Encoding Rules (CER) and
                  Distinguished Encoding Rules (DER)", ITU-T Recommendation
                  X.690, ISO/IEC International Standard 8825-1:1998, 1997.

    10.2.  Informative References

       [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
                  Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
                  <http://www.rfc-editor.org/info/rfc5056>.

       [RFC6113]  Hartman, S. and L. Zhu, "A Generalized Framework for
                  Kerberos Pre-Authentication", RFC 6113,
                  DOI 10.17487/RFC6113, April 2011,
                  <http://www.rfc-editor.org/info/rfc6113>.

       [RFC6251]  Josefsson, S., "Using Kerberos Version 5 over the
                  Transport Layer Security (TLS) Protocol", RFC 6251,
                  DOI 10.17487/RFC6251, May 2011,
                  <http://www.rfc-editor.org/info/rfc6251>.

    Acknowledgments

       JK Jaganathan helped edit early draft revisions of RFC 6112.

       Clifford Neuman contributed the core notions of this document.

       Ken Raeburn reviewed the document and provided suggestions for
       improvements.

       Martin Rex wrote the text for the GSS-API considerations.

       Nicolas Williams reviewed the GSS-API considerations section and
       suggested ideas for improvements.

       Sam Hartman and Nicolas Williams were great champions of this work.

       Miguel Garcia and Phillip Hallam-Baker reviewed the document and
       provided helpful suggestions.

    Zhu, et al.                  Standards Track                   [Page 17]

    RFC 8062               Kerberos Anonymity Support          February 2017

       In addition, the following individuals made significant
       contributions: Jeffrey Altman, Tom Yu, Chaskiel M. Grundman, Love
       Hornquist Astrand, Jeffrey Hutzelman, and Olga Kornievskaia.

       Greg Hudson and Robert Sparks provided helpful text in this document.

    Authors' Addresses

       Larry Zhu
       Microsoft Corporation
       One Microsoft Way
       Redmond, WA  98052
       United States of America

       Email: larry.zhu@microsoft.com

       Paul Leach
       Microsoft Corporation
       One Microsoft Way
       Redmond, WA  98052
       United States of America

       Email: pauljleach@msn.com

       Sam Hartman
       Hadron Industries

       Email: hartmans-ietf@mit.edu

       Shawn Emery (editor)
       Oracle

       Email: shawn.emery@gmail.com

    Zhu, et al.                  Standards Track                   [Page 18]