    Network Working Group                                            P. Karn
    Request for Comments: 2522                                      Qualcomm
    Category: Experimental                                        W. Simpson
                                                                  DayDreamer
                                                                  March 1999

                   Photuris: Session-Key Management Protocol

    Status of this Memo

       This document defines an Experimental Protocol for the Internet
       community.  It does not specify an Internet standard of any kind.
       Discussion and suggestions for improvement are requested.
       Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (1999).  Copyright (C) Philip Karn
       and William Allen Simpson (1994-1999).  All Rights Reserved.

    Abstract

       Photuris is a session-key management protocol intended for use with
       the IP Security Protocols (AH and ESP).  This document defines the
       basic protocol mechanisms.

    Karn & Simpson                Experimental                      [Page i]

    RFC 2522                   Photuris Protocol                  March 1999

    Table of Contents

         1.     Introduction ..........................................    1
            1.1       Terminology .....................................    1
            1.2       Protocol Overview ...............................    3
            1.3       Security Parameters .............................    5
            1.4       LifeTimes .......................................    6
               1.4.1  Exchange LifeTimes ..............................    6
               1.4.2  SPI LifeTimes ...................................    7
            1.5       Random Number Generation ........................    8

         2.     Protocol Details ......................................    9
            2.1       UDP .............................................    9
            2.2       Header Format ...................................   10
            2.3       Variable Precision Integers .....................   11
            2.4       Exchange-Schemes ................................   13
            2.5       Attributes ......................................   13

         3.     Cookie Exchange .......................................   14
               3.0.1  Send Cookie_Request .............................   14
               3.0.2  Receive Cookie_Request ..........................   15
               3.0.3  Send Cookie_Response ............................   15
               3.0.4  Receive Cookie_Response .........................   16
            3.1       Cookie_Request ..................................   17
            3.2       Cookie_Response .................................   18
            3.3       Cookie Generation ...............................   19
               3.3.1  Initiator Cookie ................................   19
               3.3.2  Responder Cookie ................................   20

         4.     Value Exchange ........................................   21
               4.0.1  Send Value_Request ..............................   21
               4.0.2  Receive Value_Request ...........................   22
               4.0.3  Send Value_Response .............................   22
               4.0.4  Receive Value_Response ..........................   23
            4.1       Value_Request ...................................   24
            4.2       Value_Response ..................................   25
            4.3       Offered Attribute List ..........................   26

         5.     Identification Exchange ...............................   28
               5.0.1  Send Identity_Request ...........................   29
               5.0.2  Receive Identity_Request ........................   29
               5.0.3  Send Identity_Response ..........................   30
               5.0.4  Receive Identity_Response .......................   30
            5.1       Identity_Messages ...............................   31
            5.2       Attribute Choices List ..........................   33
            5.3       Shared-Secret ...................................   34
            5.4       Identity Verification ...........................   34

    Karn & Simpson                Experimental                     [Page ii]

    RFC 2522                   Photuris Protocol                  March 1999

            5.5       Privacy-Key Computation .........................   36
            5.6       Session-Key Computation .........................   37

         6.     SPI Messages ..........................................   38
               6.0.1  Send SPI_Needed .................................   38
               6.0.2  Receive SPI_Needed ..............................   39
               6.0.3  Send SPI_Update .................................   39
               6.0.4  Receive SPI_Update ..............................   39
               6.0.5  Automated SPI_Updates ...........................   40
            6.1       SPI_Needed ......................................   41
            6.2       SPI_Update ......................................   43
               6.2.1  Creation ........................................   44
               6.2.2  Deletion ........................................   45
               6.2.3  Modification ....................................   45
            6.3       Validity Verification ...........................   45

         7.     Error Messages ........................................   46
            7.1       Bad_Cookie ......................................   47
            7.2       Resource_Limit ..................................   47
            7.3       Verification_Failure ............................   48
            7.4       Message_Reject ..................................   49

         8.     Public Value Exchanges ................................   50
            8.1       Modular Exponentiation Groups ...................   50
            8.2       Moduli Selection ................................   50
               8.2.1  Bootstrap Moduli ................................   51
               8.2.2  Learning Moduli .................................   51
            8.3       Generator Selection .............................   51
            8.4       Exponent Selection ..............................   52
            8.5       Defective Exchange Values .......................   53

         9.     Basic Exchange-Schemes ................................   54

         10.    Basic Key-Generation-Function .........................   55
            10.1      MD5 Hash ........................................   55

         11.    Basic Privacy-Method ..................................   55
            11.1      Simple Masking ..................................   55

         12.    Basic Validity-Method .................................   55
            12.1      MD5-IPMAC Check .................................   55

         13.    Basic Attributes ......................................   56
            13.1      Padding .........................................   56
            13.2      AH-Attributes ...................................   57
            13.3      ESP-Attributes ..................................   57
            13.4      MD5-IPMAC .......................................   58
               13.4.1 Symmetric Identification ........................   58

    Karn & Simpson                Experimental                    [Page iii]

    RFC 2522                   Photuris Protocol                  March 1999

               13.4.2 Authentication ..................................   59
            13.5      Organizational ..................................   60

         APPENDICES ...................................................   61

         A.     Automaton .............................................   61
            A.1       State Transition Table ..........................   62
            A.2       States ..........................................   65
               A.2.1  Initial .........................................   65
               A.2.2  Cookie ..........................................   66
               A.2.3  Value ...........................................   66
               A.2.4  Identity ........................................   66
               A.2.5  Ready ...........................................   66
               A.2.6  Update ..........................................   66

         B.     Use of Identification and Secrets .....................   67
            B.1       Identification ..................................   67
            B.2       Group Identity With Group Secret ................   67
            B.3       Multiple Identities With Group Secrets ..........   68
            B.4       Multiple Identities With Multiple Secrets .......   69

         OPERATIONAL CONSIDERATIONS ...................................   70

         SECURITY CONSIDERATIONS ......................................   70

         HISTORY ......................................................   71

         ACKNOWLEDGEMENTS .............................................   72

         REFERENCES ...................................................   73

         CONTACTS .....................................................   75

         COPYRIGHT ....................................................   76

    Karn & Simpson                Experimental                     [Page iv]

    RFC 2522                   Photuris Protocol                  March 1999

    1.  Introduction

       Photuris [Firefly] establishes short-lived session-keys between two
       parties, without passing the session-keys across the Internet.  These
       session-keys directly replace the long-lived secret-keys (such as
       passwords and passphrases) that have been historically configured for
       security purposes.

       The basic Photuris protocol utilizes these existing previously
       configured secret-keys for identification of the parties.  This is
       intended to speed deployment and reduce administrative configuration
       changes.

       This document is primarily intended for implementing the Photuris
       protocol.  It does not detail service and application interface
       definitions, although it does mention some basic policy areas
       required for the proper implementation and operation of the protocol
       mechanisms.

       Since the basic Photuris protocol is extensible, new data types and
       protocol behaviour should be expected.  The implementor is especially
       cautioned not to depend on values that appear in examples to be
       current or complete, since their purpose is primarily pedagogical.

    1.1.  Terminology

       In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
       "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
       described in [RFC-2119].

       byte             An 8-bit quantity; also known as "octet" in
                        standardese.

       exchange-value   The publically distributable value used to calculate
                        a shared-secret.  As used in this document, refers
                        to a Diffie-Hellman exchange, not the public part of
                        a public/private key-pair.

       private-key      A value that is kept secret, and is part of an
                        asymmetric public/private key-pair.

       public-key       A publically distributable value that is part of an
                        asymmetric public/private key-pair.

       secret-key       A symmetric key that is not publically
                        distributable.  As used in this document, this is
                        distinguished from an asymmetric public/private

    Karn & Simpson                Experimental                      [Page 1]

    RFC 2522                   Photuris Protocol                  March 1999

                        key-pair.  An example is a user password.

       Security Association (SA)
                        A collection of parameters describing the security
                        relationship between two nodes.  These parameters
                        include the identities of the parties, the transform
                        (including algorithm and algorithm mode), the key(s)
                        (such as a session-key, secret-key, or appropriate
                        public/private key-pair), and possibly other
                        information such as sensitivity labelling.

       Security Parameters Index (SPI)
                        A number that indicates a particular set of uni-
                        directional attributes used under a Security
                        Association, such as transform(s) and session-
                        key(s).  The number is relative to the IP
                        Destination, which is the SPI Owner, and is unique
                        per IP (Next Header) Protocol.  That is, the same
                        value MAY be used by multiple protocols to
                        concurrently indicate different Security Association
                        parameters.

       session-key      A key that is independently derived from a shared-
                        secret by the parties, and used for keying one
                        direction of traffic.  This key is changed
                        frequently.

       shared-secret    As used in this document, the calculated result of
                        the Photuris exchange.

       SPI Owner        The party that corresponds to the IP Destination;
                        the intended recipient of a protected datagram.

       SPI User         The party that corresponds to the IP Source; the
                        sender of a protected datagram.

       transform        A cryptographic manipulation of a particular set of
                        data.  As used in this document, refers to certain
                        well-specified methods (defined elsewhere).  For
                        example, AH-MD5 [RFC-1828] transforms an IP datagram
                        into a cryptographic hash, and ESP-DES-CBC [RFC-
                        1829] transforms plaintext to ciphertext and back
                        again.

    Karn & Simpson                Experimental                      [Page 2]

    RFC 2522                   Photuris Protocol                  March 1999

       Many of these terms are hierarchically related:

          Security Association (bi-directional)
           - one or more lists of Security Parameters (uni-directional)
            -- one or more Attributes
             --- may have a key
             --- may indicate a transform

       Implementors will find details of cryptographic hashing (such as
       MD5), encryption algorithms and modes (such as DES), digital
       signatures (such as DSS), and other algorithms in [Schneier95].

    1.2.  Protocol Overview

       The Photuris protocol consists of several simple phases:

       1. A "Cookie" Exchange guards against simple flooding attacks sent
          with bogus IP Sources or UDP Ports.  Each party passes a "cookie"
          to the other.

          In return, a list of supported Exchange-Schemes are offered by the
          Responder for calculating a shared-secret.

       2. A Value Exchange establishes a shared-secret between the parties.
          Each party passes an Exchange-Value to the other.  These values
          are used to calculate a shared-secret.  The Responder remains
          stateless until a shared-secret has been created.

          In addition, supported attributes are offered by each party for
          use in establishing new Security Parameters.

       3. An Identification Exchange identifies the parties to each other,
          and verifies the integrity of values sent in phases 1 and 2.

          In addition, the shared-secret provides a basis to generate
          separate session-keys in each direction, which are in turn used
          for conventional authentication or encryption.  Additional
          security attributes are also exchanged as needed.

          This exchange is masked for party privacy protection using a
          message privacy-key based on the shared-secret.  This protects the
          identities of the parties, hides the Security Parameter attribute
          values, and improves security for the exchange protocol and
          security transforms.

       4. Additional messages may be exchanged to periodically change the
          session-keys, and to establish new or revised Security Parameters.

    Karn & Simpson                Experimental                      [Page 3]

    RFC 2522                   Photuris Protocol                  March 1999

          These exchanges are also masked for party privacy protection in
          the same fashion as above.

       The sequence of message types and their purposes are summarized in
       the diagram below.  The first three phases (cookie, exchange, and
       identification) must be carried out in their entirety before any
       Security Association can be used.

       Initiator                            Responder
       =========                            =========
       Cookie_Request                 ->
                                       <-   Cookie_Response
                                               offer schemes
       Value_Request                  ->
          pick scheme
          offer value
          offer attributes
                                       <-   Value_Response
                                               offer value
                                               offer attributes

                 [generate shared-secret from exchanged values]

       Identity_Request               ->
          make SPI
          pick SPI attribute(s)
          identify self
          authenticate
          make privacy key(s)
          mask/encrypt message
                                       <-   Identity_Response
                                               make SPI
                                               pick SPI attribute(s)
                                               identify self
                                               authenticate
                                               make privacy key(s)
                                               mask/encrypt message

                   [make SPI session-keys in each direction]

    Karn & Simpson                Experimental                      [Page 4]

    RFC 2522                   Photuris Protocol                  March 1999

       SPI User                             SPI Owner
       ========                             =========
       SPI_Needed                     ->
          list SPI attribute(s)
          make validity key
          authenticate
          make privacy key(s)
          mask/encrypt message
                                       <-   SPI_Update
                                               make SPI
                                               pick SPI attribute(s)
                                               make SPI session-key(s)
                                               make validity key
                                               authenticate
                                               make privacy key(s)
                                               mask/encrypt message

       Either party may initiate an exchange at any time.  For example, the
       Initiator need not be a "caller" in a telephony link.

       The Initiator is responsible for recovering from all message losses
       by retransmission.

    1.3.  Security Parameters

       A Photuris exchange between two parties results in a pair of SPI
       values (one in each direction).  Each SPI is used in creating
       separate session-key(s) in each direction.

       The SPI is assigned by the entity controlling the IP Destination: the
       SPI Owner (receiver).  The parties use the combination of IP
       Destination, IP (Next Header) Protocol, and SPI to distinguish the
       correct Security Association.

       When both parties initiate Photuris exchanges concurrently, or one
       party initiates more than one Photuris exchange, the Initiator
       Cookies (and UDP Ports) keep the exchanges separate.  This results in
       more than one initial SPI for each Destination.

       To create multiple SPIs with different parameters, the parties may
       also send SPI_Updates.

       There is no requirement that all such outstanding SPIs be used.  The
       SPI User (sender) selects an appropriate SPI for each datagram
       transmission.

    Karn & Simpson                Experimental                      [Page 5]

    RFC 2522                   Photuris Protocol                  March 1999

       Implementation Notes:

          The method used for SPI assignment is implementation dependent.
          The only requirement is that the SPI be unique for the IP
          Destination and IP (Next Header) Protocol.

          However, selection of a cryptographically random SPI value can
          help prevent attacks that depend on a predicatable sequence of
          values.  The implementor MUST NOT expect SPI values to have a
          particular order or range.

    1.4.  LifeTimes

       The Photuris exchange results in two kinds of state, each with
       separate LifeTimes.

       1) The Exchange LifeTime of the small amount of state associated with
          the Photuris exchange itself.  This state may be viewed as between
          Internet nodes.

       2) The SPI LifeTimes of the individual SPIs that are established.
          This state may be viewed as between users and nodes.

       The SPI LifeTimes may be shorter or longer than the Exchange
       LifeTime.  These LifeTimes are not required to be related to each
       other.

       When an Exchange-Value expires (or is replaced by a newer value), any
       unexpired derived SPIs are not affected.  This is important to allow
       traffic to continue without interruption during new Photuris
       exchanges.

    1.4.1.  Exchange LifeTimes

       All retained exchange state of both parties has an associated
       Exchange LifeTime (ELT), and is subject to periodic expiration.  This
       depends on the physical and logistical security of the machine, and
       is typically in the range of 10 minutes to one day (default 30
       minutes).

       In addition, during a Photuris exchange, an Exchange TimeOut (ETO)
       limits the wait for the exchange to complete.  This timeout includes
       the packet round trips, and the time for completing the
       Identification Exchange calculations.  The time is bounded by both
       the maximum amount of calculation delay expected for the processing
       power of an unknown peer, and the minimum user expectation for

    Karn & Simpson                Experimental                      [Page 6]

    RFC 2522                   Photuris Protocol                  March 1999

       results (default 30 seconds).

       These Exchange LifeTimes and TimeOuts are implementation dependent
       and are not disclosed in any Photuris message.  The paranoid operator
       will have a fairly short Exchange LifeTime, but it MUST NOT be less
       than twice the ETO.

       To prevent synchronization between Photuris exchanges, the
       implementation SHOULD randomly vary each Exchange LifeTime within
       twice the range of seconds that are required to calculate a new
       Exchange-Value.  For example, when the Responder uses a base ELT of
       30 minutes, and takes 10 seconds to calculate the new Exchange-Value,
       the equation might be (in milliseconds):

          1790000 + urandom(20000)

       The Exchange-Scheme, Exchange-Values, and resulting shared-secret MAY
       be cached in short-term storage for the Exchange LifeTime.  When
       repetitive Photuris exchanges occur between the same parties, and the
       Exchange-Values are discovered to be unchanged, the previously
       calculated shared-secret can be used to rapidly generate new
       session-keys.

    1.4.2.  SPI LifeTimes

       Each SPI has an associated LifeTime, specified by the SPI owner
       (receiver).  This SPI LifeTime (SPILT) is usually related to the
       speed of the link (typically 2 to 30 minutes), but it MUST NOT be
       less than thrice the ETO.

       The SPI can also be deleted by the SPI Owner using the SPI_Update.
       Once the SPI has expired or been deleted, the parties cease using the
       SPI.

       To prevent synchronization between multiple Photuris exchanges, the
       implementation SHOULD randomly vary each SPI LifeTime.  For example,
       when the Responder uses a base SPILT of 5 minutes, and 30 seconds for
       the ETO, the equation might be (in milliseconds):

          285000 + urandom(30000)

       There is no requirement that a long LifeTime be accepted by the SPI
       User.  The SPI User might never use an established SPI, or cease
       using the SPI at any time.

       When more than one unexpired SPI is available to the SPI User for the
       same function, a common implementation technique is to select the SPI

    Karn & Simpson                Experimental                      [Page 7]

    RFC 2522                   Photuris Protocol                  March 1999

       with the greatest remaining LifeTime.  However, selecting randomly
       among a large number of SPIs might provide some defense against
       traffic analysis.

       To prevent resurrection of deleted or expired SPIs, SPI Owners SHOULD
       remember those SPIs, but mark them as unusable until the Photuris
       exchange shared-secret used to create them also expires and purges
       the associated state.

       When the SPI Owner detects an incoming SPI that has recently expired,
       but the associated exchange state has not yet been purged, the
       implementation MAY accept the SPI.  The length of time allowed is
       highly dependent on clock drift and variable packet round trip time,
       and is therefore implementation dependent.

    1.5.  Random Number Generation

       The security of Photuris critically depends on the quality of the
       secret random numbers generated by each party.  A poor random number
       generator at either party will compromise the shared-secret produced
       by the algorithm.

       Generating cryptographic quality random numbers on a general purpose
       computer without hardware assistance is a very tricky problem.  In
       general, this requires using a cryptographic hashing function to
       "distill" the entropy from a large number of semi-random external
       events, such as the timing of key strokes.  An excellent discussion
       can be found in [RFC-1750].

    Karn & Simpson                Experimental                      [Page 8]

    RFC 2522                   Photuris Protocol                  March 1999

    2.  Protocol Details

       The Initiator begins a Photuris exchange under several circumstances:

       -  The Initiator has a datagram that it wishes to send with
          confidentiality, and has no current Photuris exchange state with
          the IP Destination.  This datagram is discarded, and a
          Cookie_Request is sent instead.

       -  The Initiator has received the ICMP message [RFC-1812] Destination
          Unreachable: Communication Administratively Prohibited (Type 3,
          Code 13), and has no current Photuris exchange state with the ICMP
          Source.

       -  The Initiator has received the ICMP message [RFC-2521] Security
          Failures: Bad SPI (Type 40, Code 0), that matches current Photuris
          exchange state with the ICMP Source.

       -  The Initiator has received the ICMP message [RFC-2521] Security
          Failures: Need Authentication (Type 40, Code 4), and has no
          current Photuris exchange state with the ICMP Source.

       -  The Initiator has received the ICMP message [RFC-2521] Security
          Failures: Need Authorization (Type 40, Code 5), that matches
          current Photuris exchange state with the ICMP Source.

       When the event is an ICMP message, special care MUST be taken that
       the ICMP message actually includes information that matches a
       previously sent IP datagram.  Otherwise, this could provide an
       opportunity for a clogging attack, by stimulating a new Photuris
       Exchange.

    2.1.  UDP

       All Photuris messages use the User Datagram Protocol header [RFC-
       768].  The Initiator sends to UDP Destination Port 468.

       When replying to the Initiator, the Responder swaps the IP Source and
       Destination, and the UDP Source and Destination Ports.

       The UDP checksum MUST be correctly calculated when sent.  When a
       message is received with an incorrect UDP checksum, it is silently
       discarded.

    Karn & Simpson                Experimental                      [Page 9]

    RFC 2522                   Photuris Protocol                  March 1999

       Implementation Notes:

          It is expected that installation of Photuris will ensure that UDP
          checksum calculations are enabled for the computer operating
          system and later disabling by operators is prevented.

          Internet Protocol version 4 [RFC-791] restricts the maximum
          reassembled datagram to 576 bytes.

          When processing datagrams containing variable size values, the
          length must be checked against the overall datagram length.  An
          invalid size (too long or short) that causes a poorly coded
          receiver to abort could be used as a denial of service attack.

    2.2.  Header Format

       All of the messages have a format similar to the following, as
       transmitted left to right in network order (most significant to least
       significant):

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Initiator-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Responder-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Message    |
       +-+-+-+-+-+-+-+-+

       Initiator-Cookie  16 bytes.

       Responder-Cookie  16 bytes.

       Message          1 byte.  Each message type has a unique value.
                        Initial values are assigned as follows:

    Karn & Simpson                Experimental                     [Page 10]

    RFC 2522                   Photuris Protocol                  March 1999

                            0  Cookie_Request
                            1  Cookie_Response
                            2  Value_Request
                            3  Value_Response
                            4  Identity_Request
                            5  Secret_Response (optional)
                            6  Secret_Request (optional)
                            7  Identity_Response
                            8  SPI_Needed
                            9  SPI_Update
                           10  Bad_Cookie
                           11  Resource_Limit
                           12  Verification_Failure
                           13  Message_Reject

       Further details and differences are elaborated in the individual
       messages.

    2.3.  Variable Precision Integers

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Size              |             Value ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Size             2, 4, or 8 bytes.  The number of significant bits
                        used in the Value field.  Always transmitted most
                        significant byte first.

                        When the Size is zero, no Value field is present;
                        there are no significant bits.  This means "missing"
                        or "null".  It should not be confused with the value
                        zero, which includes an indication of the number of
                        significant bits.

                        When the most significant byte is in the range 0
                        through 254 (0xfe), the field is 2 bytes.  Both
                        bytes are used to indicate the size of the Value
                        field, which ranges from 1 to 65,279 significant
                        bits (in 1 to 8,160 bytes).

                        When the most significant byte is 255 (0xff), the
                        field is 4 bytes.  The remaining 3 bytes are added
                        to 65,280 to indicate the size of the Value field,
                        which is limited to 16,776,959 significant bits (in

    Karn & Simpson                Experimental                     [Page 11]

    RFC 2522                   Photuris Protocol                  March 1999

                        2,097,120 bytes).

                        When the most significant 2 bytes are 65,535
                        (0xffff), the field is 8 bytes.  The remaining 6
                        bytes are added to 16,776,960 to indicate the size
                        of the Value field.

       Value            0 or more bytes.  Always transmitted most
                        significant byte first.

                        The bits used are right justified within byte
                        boundaries; that is, any unused bits are in the most
                        significant byte.  When there are no unused bits, or
                        unused bits are zero filled, the value is assumed to
                        be an unsigned positive integer.

                        When the leading unused bits are ones filled, the
                        number is assumed to be a two's-complement negative
                        integer.  A negative integer will always have at
                        least one unused leading sign bit in the most
                        significant byte.

       Shortened forms SHOULD NOT be used when the Value includes a number
       of leading zero significant bits.  The Size SHOULD indicate the
       correct number of significant bits.

       Implementation Notes:

          Negative integers are not required to be supported, but are
          included for completeness.

          No more than 65,279 significant bits are required to be supported.
          Other ranges are vastly too long for these UDP messages, but are
          included for completeness.

    Karn & Simpson                Experimental                     [Page 12]

    RFC 2522                   Photuris Protocol                  March 1999

    2.4.  Exchange-Schemes

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Scheme             |             Size              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Value ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Scheme           2 bytes.  A unique value indicating the Exchange-
                        Scheme.  See the "Basic Exchange-Schemes" for
                        details.

       Size             2 bytes, ranging from 0 to 65,279.  See "Variable
                        Precision Integer".

       Value            0 or more bytes.  See "Variable Precision Integer".

       The Size MUST NOT be assumed to be constant for a particular Scheme.
       Multiple kinds of the same Scheme with varying Sizes MAY be present
       in any list of schemes.

       However, only one of each Scheme and Size combination will be present
       in any list of schemes.

    2.5.  Attributes

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Attribute   |    Length     |  Value(s) ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Attribute        1 byte.  A unique value indicating the kind of
                        attribute.  See the "Basic Attributes" for details.

                        When the value is zero (padding), no Length field is
                        present (always zero).

       Length           1 byte.  The size of the Value(s) field in bytes.

                        When the Length is zero, no Value(s) field is
                        present.

       Value(s)         0 or more bytes.  See the "Basic Attributes" for
                        details.

       The Length MUST NOT be assumed to be constant for a particular

    Karn & Simpson                Experimental                     [Page 13]

    RFC 2522                   Photuris Protocol                  March 1999

       Attribute.  Multiple kinds of the same Attribute with varying Lengths
       MAY be present in any list of attributes.

    3.  Cookie Exchange

       Initiator                            Responder
       =========                            =========
       Cookie_Request                 ->
                                       <-   Cookie_Response
                                               offer schemes

    3.0.1.  Send Cookie_Request

       The Initiator initializes local state, and generates a unique
       "cookie".  The Initiator-Cookie MUST be different in each new
       Cookie_Request between the same parties.  See "Cookie Generation" for
       details.

       -  If any previous exchange between the peer IP nodes has not expired
          in which this party was the Initiator, this Responder-Cookie is
          set to the most recent Responder-Cookie, and this Counter is set
          to the corresponding Counter.

          For example, a new Virtual Private Network (VPN) tunnel is about
          to be established to an existing partner.  The Counter is the same
          value received in the prior Cookie_Response, the Responder-Cookie
          remains the same, and a new Initiator-Cookie is generated.

       -  If the new Cookie_Request is in response to a message of a
          previous exchange in which this party was the Responder, this
          Responder-Cookie is set to the previous Initiator-Cookie, and this
          Counter is set to zero.

          For example, a Bad_Cookie message was received from the previous
          Initiator in response to SPI_Needed.  The Responder-Cookie is
          replaced with the Initiator-Cookie, and a new Initiator-Cookie is
          generated.  This provides bookkeeping to detect bogus Bad_Cookie
          messages.

          Also, can be used for bi-directional User, Transport, and Process
          oriented keying.  Such mechanisms are outside the scope of this
          document.

       -  Otherwise, this Responder-Cookie and Counter are both set to zero.

    Karn & Simpson                Experimental                     [Page 14]

    RFC 2522                   Photuris Protocol                  March 1999

          By default, the Initiator operates in the same manner as when all
          of its previous exchange state has expired.  The Responder will
          send a Resource_Limit when its own exchange state has not expired.

       The Initiator also starts a retransmission timer.  If no valid
       Cookie_Response arrives within the time limit, the same
       Cookie_Request is retransmitted for the remaining number of
       Retransmissions.  The Initiator-Cookie value MUST be the same in each
       such retransmission to the same IP Destination and UDP Port.

       When Retransmissions have been exceeded, if a Resource_Limit message
       has been received during the exchange, the Initiator SHOULD begin the
       Photuris exchange again by sending a new Cookie_Request with updated
       values.

    3.0.2.  Receive Cookie_Request

       On receipt of a Cookie_Request, the Responder determines whether
       there are sufficient resources to begin another Photuris exchange.

       -  When too many SPI values are already in use for this particular
          peer, or too many concurrent exchanges are in progress, or some
          other resource limit is reached, a Resource_Limit message is sent.

       -  When any previous exchange initiated by this particular peer has
          not exceeded the Exchange TimeOut, and the Responder-Cookie does
          not specify one of these previous exchanges, a Resource_Limit
          message is sent.

       Otherwise, the Responder returns a Cookie_Response.

       Note that the Responder creates no additional state at this time.

    3.0.3.  Send Cookie_Response

       The IP Source for the Initiator is examined.  If any previous
       exchange between the peer IP nodes has not expired, the response
       Counter is set to the most recent exchange Counter plus one (allowing
       for out of order retransmissions).  Otherwise, the response Counter
       is set to the request Counter plus one.

       If (through rollover of the Counter) the new Counter value is zero
       (modulo 256), the value is set to one.

       If this new Counter value matches some previous exchange initiated by
       this particular peer that has not yet exceeded the Exchange TimeOut,

    Karn & Simpson                Experimental                     [Page 15]

    RFC 2522                   Photuris Protocol                  March 1999

       the Counter is incremented again, until a unique Counter value is
       reached.

       Nota Bene:
          No more than 254 concurrent exchanges between the same two peers
          are supported.

       The Responder generates a unique cookie.  The Responder-Cookie value
       in each successive response SHOULD be different.  See "Cookie
       Generation" for details.

       The Exchange-Schemes available between the peers are listed in the
       Offered-Schemes.

    3.0.4.  Receive Cookie_Response

       The Initiator validates the Initiator-Cookie, and the Offered-
       Schemes.

       -  When an invalid/expired Initiator-Cookie is detected, the message
          is silently discarded.

       -  When the variable length Offered-Schemes do not match the UDP
          Length, or all Offered-Schemes are obviously defective and/or
          insufficient for the purposes intended, the message is silently
          discarded; the implementation SHOULD log the occurance, and notify
          an operator as appropriate.

       -  Once a valid message has been received, later Cookie_Responses
          with matching Initiator-Cookies are also silently discarded, until
          a new Cookie_Request is sent.

       When the message is valid, an Exchange-Scheme is chosen from the list
       of Offered-Schemes.

       This Scheme-Choice may affect the next Photuris message sent.  By
       default, the next Photuris message is a Value_Request.

       Implementation Notes:

          Only the Initiator-Cookie is used to identify the exchange.  The
          Counter and Responder-Cookie will both be different from the
          Cookie_Request.

          Various proposals for extensions utilize the Scheme-Choice to
          indicate a different message sequence.  Such mechanisms are
          outside the scope of this document.

    Karn & Simpson                Experimental                     [Page 16]

    RFC 2522                   Photuris Protocol                  March 1999

    3.1.  Cookie_Request

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Initiator-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Responder-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Message    |    Counter    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Initiator-Cookie  16 bytes.  A randomized value that identifies the
                        exchange.  The value MUST NOT be zero.  See "Cookie
                        Generation" for details.

       Responder-Cookie  16 bytes.  Identifies a specific previous exchange.
                        Copied from a previous Cookie_Response.

                        When zero, no previous exchange is specified.

                        When non-zero, and the Counter is zero, contains the
                        Initiator-Cookie of a previous exchange.  The
                        specified party is requested to be the Responder in
                        this exchange, to retain previous party pairings.

                        When non-zero, and the Counter is also non-zero,
                        contains the Responder-Cookie of a previous
                        exchange.  The specified party is requested to be
                        the Responder in this exchange, to retain previous
                        party pairings.

       Message          0

       Counter          1 byte.  Indicates the number of previous exchanges.

                        When zero, the Responder-Cookie indicates the
                        Initiator of a previous exchange, or no previous
                        exchange is specified.

                        When non-zero, the Responder-Cookie indicates the
                        Responder to a previous exchange.  This value is set
                        to the Counter from the corresponding
                        Cookie_Response or from a Resource_Limit.

    Karn & Simpson                Experimental                     [Page 17]

    RFC 2522                   Photuris Protocol                  March 1999

    3.2.  Cookie_Response

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Initiator-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Responder-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Message    |    Counter    |  Offered-Schemes ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Initiator-Cookie  16 bytes.  Copied from the Cookie_Request.

       Responder-Cookie  16 bytes.  A randomized value that identifies the
                        exchange.  The value MUST NOT be zero.  See "Cookie
                        Generation" for details.

       Message          1

       Counter          1 byte.  Indicates the number of the current
                        exchange.  Must be greater than zero.

       Offered-Schemes  4 or more bytes.  A list of one or more Exchange-
                        Schemes supported by the Responder, ordered from
                        most to least preferable.  See the "Basic Exchange-
                        Schemes" for details.

                        Only one Scheme (#2) is required to be supported,
                        and SHOULD be present in every Offered-Schemes list.

                        More than one of each kind of Scheme may be offered,
                        but each is distinguished by its Size.  The end of
                        the list is indicated by the UDP Length.

    Karn & Simpson                Experimental                     [Page 18]

    RFC 2522                   Photuris Protocol                  March 1999

    3.3.  Cookie Generation

       The exact technique by which a Photuris party generates a cookie is
       implementation dependent.  The method chosen must satisfy some basic
       requirements:

       1. The cookie MUST depend on the specific parties.  This prevents an
          attacker from obtaining a cookie using a real IP address and UDP
          port, and then using it to swamp the victim with requests from
          randomly chosen IP addresses or ports.

       2. It MUST NOT be possible for anyone other than the issuing entity
          to generate cookies that will be accepted by that entity.  This
          implies that the issuing entity will use local secret information
          in the generation and subsequent verification of a cookie.  It
          must not be possible to deduce this secret information from any
          particular cookie.

       3. The cookie generation and verification methods MUST be fast to
          thwart attacks intended to sabotage CPU resources.

       A recommended technique is to use a cryptographic hashing function
       (such as MD5).

       An incoming cookie can be verified at any time by regenerating it
       locally from values contained in the incoming datagram and the local
       secret random value.

    3.3.1.  Initiator Cookie

       The Initiator secret value that affects its cookie SHOULD change for
       each new Photuris exchange, and is thereafter internally cached on a
       per Responder basis.  This provides improved synchronization and
       protection against replay attacks.

       An alternative is to cache the cookie instead of the secret value.
       Incoming cookies can be compared directly without the computational
       cost of regeneration.

       It is recommended that the cookie be calculated over the secret
       value, the IP Source and Destination addresses, and the UDP Source
       and Destination ports.

    Karn & Simpson                Experimental                     [Page 19]

    RFC 2522                   Photuris Protocol                  March 1999

       Implementation Notes:

          Although the recommendation includes the UDP Source port, this is
          very implementation specific.  For example, it might not be
          included when the value is constant.

          However, it is important that the implementation protect mutually
          suspicious users of the same machine from generating the same
          cookie.

    3.3.2.  Responder Cookie

       The Responder secret value that affects its cookies MAY remain the
       same for many different Initiators.  However, this secret SHOULD be
       changed periodically to limit the time for use of its cookies
       (typically each 60 seconds).

       The Responder-Cookie SHOULD include the Initiator-Cookie.  The
       Responder-Cookie MUST include the Counter (that is returned in the
       Cookie_Response).  This provides improved synchronization and
       protection against replay attacks.

       It is recommended that the cookie be calculated over the secret
       value, the IP Source and Destination addresses, its own UDP
       Destination port, the Counter, the Initiator-Cookie, and the
       currently Offered-Schemes.

       The cookie is not cached per Initiator to avoid saving state during
       the initial Cookie Exchange.  On receipt of a Value_Request
       (described later), the Responder regenerates its cookie for
       validation.

       Once the Value_Response is sent (also described later), both
       Initiator and Responder cookies are cached to identify the exchange.

       Implementation Notes:

          Although the recommendation does not include the UDP Source port,
          this is very implementation specific.  It might be successfully
          included in some variants.

          However, it is important that the UDP Source port not be included
          when matching existing Photuris exchanges for determining the
          appropriate Counter.

          The recommendation includes the Offered-Schemes to detect a
          dynamic change of scheme value between the Cookie_Response and

    Karn & Simpson                Experimental                     [Page 20]

    RFC 2522                   Photuris Protocol                  March 1999

          Value_Response.

          Some mechanism MAY be needed to detect a dynamic change of pre-
          calculated Responder Exchange-Value between the Value_Response and
          Identity_Response.  For example, change the secret value to render
          the cookie invalid, or explicitly mark the Photuris exchange state
          as expired.

    4.  Value Exchange

       Initiator                            Responder
       =========                            =========
       Value_Request                  ->
          pick scheme
          offer value
          offer attributes
                                       <-   Value_Response
                                               offer value
                                               offer attributes

                 [generate shared-secret from exchanged values]

    4.0.1.  Send Value_Request

       The Initiator generates an appropriate Exchange-Value for the
       Scheme-Choice.  This Exchange-Value may be pre-calculated and used
       for multiple Responders.

       The IP Destination for the Responder is examined, and the attributes
       available between the parties are listed in the Offered-Attributes.

       The Initiator also starts a retransmission timer.  If no valid
       Value_Response arrives within the time limit, the same Value_Request
       is retransmitted for the remaining number of Retransmissions.

       When Retransmissions have been exceeded, if a Bad_Cookie or
       Resource_Limit message has been received during the exchange, the
       Initiator SHOULD begin the Photuris exchange again by sending a new
       Cookie_Request.

    Karn & Simpson                Experimental                     [Page 21]

    RFC 2522                   Photuris Protocol                  March 1999

    4.0.2.  Receive Value_Request

       The Responder validates the Responder-Cookie, the Counter, the
       Scheme-Choice, the Exchange-Value, and the Offered-Attributes.

       -  When an invalid/expired Responder-Cookie is detected, a Bad_Cookie
          message is sent.

       -  When too many SPI values are already in use for this particular
          peer, or too many concurrent exchanges are in progress, or some
          other resource limit is reached, a Resource_Limit message is sent.

       -  When an invalid Scheme-Choice is detected, or the Exchange-Value
          is obviously defective, or the variable length Offered-Attributes
          do not match the UDP Length, the message is silently discarded;
          the implementation SHOULD log the occurance, and notify an
          operator as appropriate.

       When the message is valid, the Responder sets its Exchange timer to
       the Exchange TimeOut, and returns a Value_Response.

       The Responder keeps a copy of the incoming Value_Request cookie pair,
       and its Value_Response.  If a duplicate Value_Request is received, it
       merely resends its previous Value_Response, and takes no further
       action.

    4.0.3.  Send Value_Response

       The Responder generates an appropriate Exchange-Value for the
       Scheme-Choice.  This Exchange-Value may be pre-calculated and used
       for multiple Initiators.

       The IP Source for the Initiator is examined, and the attributes
       available between the parties are listed in the Offered-Attributes.

       Implementation Notes:

          At this time, the Responder begins calculation of the shared-
          secret.  Calculation of the shared-secret is executed in parallel
          to minimize delay.

          This may take a substantial amount of time.  The implementor
          should ensure that retransmission is not blocked by this
          calculation.  This is not usually a problem, as retransmission
          timeouts typically exceed calculation time.

    Karn & Simpson                Experimental                     [Page 22]

    RFC 2522                   Photuris Protocol                  March 1999

    4.0.4.  Receive Value_Response

       The Initiator validates the pair of Cookies, the Exchange-Value, and
       the Offered-Attributes.

       -  When an invalid/expired cookie is detected, the message is
          silently discarded.

       -  When the Exchange-Value is obviously defective, or the variable
          length Offered-Attributes do not match the UDP Length, the message
          is silently discarded; the implementation SHOULD log the
          occurance, and notify an operator as appropriate.

       -  Once a valid message has been received, later Value_Responses with
          both matching cookies are also silently discarded, until a new
          Cookie_Request is sent.

       When the message is valid, the Initiator begins its parallel
       computation of the shared-secret.

       When the Initiator completes computation, it sends an
       Identity_Request to the Responder.

    Karn & Simpson                Experimental                     [Page 23]

    RFC 2522                   Photuris Protocol                  March 1999

    4.1.  Value_Request

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Initiator-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Responder-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Message    |    Counter    |         Scheme-Choice         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                   Initiator-Exchange-Value                    ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Initiator-Offered-Attributes ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

       Initiator-Cookie  16 bytes.  Copied from the Cookie_Response.

       Responder-Cookie  16 bytes.  Copied from the Cookie_Response.

       Message          2

       Counter          1 byte.  Copied from the Cookie_Response.

       Scheme-Choice    2 bytes.  A value selected by the Initiator from the
                        list of Offered-Schemes in the Cookie_Response.

                        Only the Scheme is specified; the Size will match
                        the Initiator-Exchange-Value, and the Value(s) are
                        implicit.

       Initiator-Exchange-Value
                        Variable Precision Integer.  Provided by the
                        Initiator for calculating a shared-secret between
                        the parties.  The Value format is indicated by the
                        Scheme-Choice.

                        The field may be any integral number of bytes in
                        length, as indicated by its Size field.  It does not
                        require any particular alignment.  The 32-bit
                        alignment shown is for convenience in the
                        illustration.

    Karn & Simpson                Experimental                     [Page 24]

    RFC 2522                   Photuris Protocol                  March 1999

       Initiator-Offered-Attributes
                        4 or more bytes.  A list of Security Parameter
                        attributes supported by the Initiator.

                        The contents and usage of this list are further
                        described in "Offered Attributes List".  The end of
                        the list is indicated by the UDP Length.

    4.2.  Value_Response

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Initiator-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Responder-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Message    |                    Reserved                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                   Responder-Exchange-Value                    ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Responder-Offered-Attributes ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

       Initiator-Cookie  16 bytes.  Copied from the Value_Request.

       Responder-Cookie  16 bytes.  Copied from the Value_Request.

       Message          3

       Reserved         3 bytes.  For future use; MUST be set to zero when
                        transmitted, and MUST be ignored when received.

       Responder-Exchange-Value
                        Variable Precision Integer.  Provided by the
                        Responder for calculating a shared-secret between
                        the parties.  The Value format is indicated by the
                        current Scheme-Choice specified in the
                        Value_Request.

                        The field may be any integral number of bytes in

    Karn & Simpson                Experimental                     [Page 25]

    RFC 2522                   Photuris Protocol                  March 1999

                        length, as indicated by its Size field.  It does not
                        require any particular alignment.  The 32-bit
                        alignment shown is for convenience in the
                        illustration.

       Responder-Offered-Attributes
                        4 or more bytes.  A list of Security Parameter
                        attributes supported by the Responder.

                        The contents and usage of this list are further
                        described in "Offered Attributes List".  The end of
                        the list is indicated by the UDP Length.

    4.3.  Offered Attribute List

       This list includes those attributes supported by the party that are
       available to the other party.  The attribute formats are specified in
       the "Basic Attributes".

       The list is composed of two or three sections: Identification-
       Attributes, Authentication-Attributes, and (optional) Encapsulation-
       Attributes.  Within each section, the attributes are ordered from
       most to least preferable.

       The first section of the list includes methods of identification.  An
       Identity-Choice is selected from this list.

       The second section of the list begins with "AH-Attributes" (#1).  It
       includes methods of authentication, and other operational types.

       The third section of the list begins with "ESP-Attributes" (#2).  It
       includes methods of authentication, compression, encryption, and
       other operational types.  When no Encapsulation-Attributes are
       offered, the "ESP-Attributes" attribute itself is omitted from the
       list.

       Attribute-Choices are selected from the latter two sections of the
       list.

       Support is required for the "MD5-IPMAC" (#5) attribute for both
       "Symmetric Identification" and "Authentication" and they SHOULD be
       present in every Offered-Attributes list.

    Karn & Simpson                Experimental                     [Page 26]

    RFC 2522                   Photuris Protocol                  March 1999

       Implementation Notes:

          For example,

             "MD5-IPMAC" (Symmetric Identification),
             "AH-Attributes",
             "MD5-IPMAC" (Authentication).

          Since the offer is made by the prospective SPI User (sender),
          order of preference likely reflects the capabilities and
          engineering tradeoffs of a particular implementation.

          However, the critical processing bottlenecks are frequently in the
          receiver.  The SPI Owner (receiver) may express its needs by
          choosing a less preferable attribute.

          The order may also be affected by operational policy and requested
          services for an application.  Such considerations are outside the
          scope of this document.

          The list may be divided into additional sections.  These sections
          will always follow the ESP-Attributes section, and are
          indistinguishable from unrecognized attributes.

          The authentication, compression, encryption and identification
          mechanisms chosen, as well as the encapsulation modes (if any),
          need not be the same in both directions.

    Karn & Simpson                Experimental                     [Page 27]

    RFC 2522                   Photuris Protocol                  March 1999

    5.  Identification Exchange

       Initiator                            Responder
       =========                            =========
       Identity_Request               ->
          make SPI
          pick SPI attribute(s)
          identify self
          authenticate
          make privacy key(s)
          mask/encrypt message
                                       <-   Identity_Response
                                               make SPI
                                               pick SPI attribute(s)
                                               identify self
                                               authenticate
                                               make privacy key(s)
                                               mask/encrypt message

                   [make SPI session-keys in each direction]

       The exchange of messages is ordered, although the formats and
       meanings of the messages are identical in each direction.  The
       messages are easily distinguished by the parties themselves, by
       examining the Message and Identification fields.

       Implementation Notes:

          The amount of time for the calculation may be dependent on the
          value of particular bits in secret values used in generating the
          shared-secret or identity verification.  To prevent analysis of
          these secret bits by recording the time for calculation, sending
          of the Identity_Messages SHOULD be delayed until the time expected
          for the longest calculation.  This will be different for different
          processor speeds, different algorithms, and different length
          variables.  Therefore, the method for estimating time is
          implementation dependent.

          Any authenticated and/or encrypted user datagrams received before
          the completion of identity verification can be placed on a queue
          pending completion of this step.  If verification succeeds, the
          queue is processed as though the datagrams had arrived subsequent
          to the verification.  If verification fails, the queue is
          discarded.

    Karn & Simpson                Experimental                     [Page 28]

    RFC 2522                   Photuris Protocol                  March 1999

    5.0.1.  Send Identity_Request

       The Initiator chooses an appropriate Identification, the SPI and
       SPILT, a set of Attributes for the SPI, calculates the Verification,
       and masks the message using the Privacy-Method indicated by the
       current Scheme-Choice.

       The Initiator also starts a retransmission timer.  If no valid
       Identity_Response arrives within the time limit, its previous
       Identity_Request is retransmitted for the remaining number of
       Retransmissions.

       When Retransmissions have been exceeded, if a Bad_Cookie message has
       been received during the exchange, the Initiator SHOULD begin the
       Photuris exchange again by sending a new Cookie_Request.

    5.0.2.  Receive Identity_Request

       The Responder validates the pair of Cookies, the Padding, the
       Identification, the Verification, and the Attribute-Choices.

       -  When an invalid/expired cookie is detected, a Bad_Cookie message
          is sent.

       -  After unmasking, when invalid Padding is detected, the variable
          length Attribute-Choices do not match the UDP Length, or an
          attribute was not in the Offered-Attributes, the message is
          silently discarded.

       -  When an invalid Identification is detected, or the message
          verification fails, a Verification_Failure message is sent.

       -  Whenever such a problem is detected, the Security Association is
          not established; the implementation SHOULD log the occurance, and
          notify an operator as appropriate.

       When the message is valid, the Responder sets its Exchange timer to
       the Exchange LifeTime (if this has not already been done for a
       previous exchange).  When its parallel computation of the shared-
       secret is complete, the Responder returns an Identity_Response.

       The Responder keeps a copy of the incoming Identity_Request values,
       and its Identity_Response.  If a duplicate Identity_Request is
       received, it merely resends its previous Identity_Response, and takes
       no further action.

    Karn & Simpson                Experimental                     [Page 29]

    RFC 2522                   Photuris Protocol                  March 1999

    5.0.3.  Send Identity_Response

       The Responder chooses an appropriate Identification, the SPI and
       SPILT, a set of Attributes for the SPI, calculates the Verification,
       and masks the message using the Privacy-Method indicated by the
       current Scheme-Choice.

       The Responder calculates the SPI session-keys in both directions.

       At this time, the Responder begins the authentication and/or
       encryption of user datagrams.

    5.0.4.  Receive Identity_Response

       The Initiator validates the pair of Cookies, the Padding, the
       Identification, the Verification, and the Attribute-Choices.

       -  When an invalid/expired cookie is detected, the message is
          silently discarded.

       -  After unmasking, when invalid Padding is detected, the variable
          length Attribute-Choices do not match the UDP Length, or an
          attribute was not in the Offered-Attributes, the message is
          silently discarded.

       -  When an invalid Identification is detected, or the message
          verification fails, a Verification_Failure message is sent.

       -  Whenever such a problem is detected, the Security Association is
          not established; the implementation SHOULD log the occurance, and
          notify an operator as appropriate.

       -  Once a valid message has been received, later Identity_Responses
          with both matching cookies are also silently discarded, until a
          new Cookie_Request is sent.

       When the message is valid, the Initiator sets its Exchange timer to
       the Exchange LifeTime (if this has not already been done for a
       previous exchange).

       The Initiator calculates the SPI session-keys in both directions.

       At this time, the Initiator begins the authentication and/or
       encryption of user datagrams.

    Karn & Simpson                Experimental                     [Page 30]

    RFC 2522                   Photuris Protocol                  March 1999

    5.1.  Identity_Messages

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Initiator-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Responder-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Message    |                    LifeTime                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Security-Parameters-Index                   |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |        Identity-Choice        |                               |
       + + + + + + + + + + + + + + + + +                               +
       |                                                               |
       ~                        Identification                         ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                         Verification                          ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Attribute-Choices ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                          ... Padding  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Initiator-Cookie  16 bytes.  Copied from the Value_Request.

       Responder-Cookie  16 bytes.  Copied from the Value_Request.

       Message          4 (Request) or 7 (Response)

       LifeTime         3 bytes.  The number of seconds remaining before the
                        indicated SPI expires.

                        When the SPI is zero, this field MUST be filled with
                        a random non-zero value.

       Security-Parameters-Index (SPI)
                        4 bytes.  The SPI to be used for incoming
                        communications.

                        When zero, indicates that no SPI is created in this

    Karn & Simpson                Experimental                     [Page 31]

    RFC 2522                   Photuris Protocol                  March 1999

                        direction.

       Identity-Choice  2 or more bytes.  An identity attribute is selected
                        from the list of Offered-Attributes sent by the
                        peer, and is used to calculate the Verification.

                        The field may be any integral number of bytes in
                        length, as indicated by its Length field.  It does
                        not require any particular alignment.  The 16-bit
                        alignment shown is for convenience in the
                        illustration.

       Identification   Variable Precision Integer, or alternative format
                        indicated by the Identity-Choice.  See the "Basic
                        Attributes" for details.

                        The field may be any integral number of bytes in
                        length.  It does not require any particular
                        alignment.  The 32-bit alignment shown is for
                        convenience in the illustration.

       Verification     Variable Precision Integer, or alternative format
                        indicated by the Identity-Choice.  The calculation
                        of the value is described in "Identity
                        Verification".

                        The field may be any integral number of bytes in
                        length.  It does not require any particular
                        alignment.  The 32-bit alignment shown is for
                        convenience in the illustration.

       Attribute-Choices
                        0 or more bytes.  When the SPI is non-zero, a list
                        of attributes selected from the list of Offered-
                        Attributes supported by the peer.

                        The contents and usage of this list are further
                        described in "Attribute Choices List".  The end of
                        the list is indicated by the UDP Length after
                        removing the Padding (UDP Length - last Padding
                        value).

       Padding          8 to 255 bytes.  This field is filled up to at least
                        a 128 byte boundary, measured from the beginning of
                        the message.  The number of pad bytes are chosen
                        randomly.

                        In addition, when a Privacy-Method indicated by the

    Karn & Simpson                Experimental                     [Page 32]

    RFC 2522                   Photuris Protocol                  March 1999

                        current Scheme-Choice requires the plaintext to be a
                        multiple of some number of bytes (the block size of
                        a block cipher), this field is adjusted as necessary
                        to the size required by the algorithm.

                        Self-Describing-Padding begins with the value 1.
                        Each byte contains the index of that byte.  Thus,
                        the final pad byte indicates the number of pad bytes
                        to remove.  For example, when the unpadded message
                        length is 120 bytes, the padding values might be 1,
                        2, 3, 4, 5, 6, 7, and 8.

       The portion of the message after the SPI field is masked using the
       Privacy-Method indicated by the current Scheme-Choice.

       The fields following the SPI are opaque.  That is, the values are set
       prior to masking (and optional encryption), and examined only after
       unmasking (and optional decryption).

    5.2.  Attribute Choices List

       This list specifies the attributes of the SPI.  The attribute formats
       are specified in the "Basic Attributes".

       The list is composed of one or two sections: Authentication-
       Attributes, and/or Encapsulation-Attributes.

       When sending from the SPI User to the SPI Owner, the attributes are
       processed in the order listed.  For example,

          "ESP-Attributes",
          "Deflate" (Compression),
          "XOR" (Encryption),
          "DES-CBC" (Encryption),
          "XOR" (Encryption),
          "AH-Attributes",
          "AH-Sequence",
          "MD5-IPMAC" (Authentication),

       would result in ESP with compression and triple encryption (inside),
       and then AH authentication with sequence numbers (outside) of the ESP
       payload.

       The SPI Owner will naturally process the datagram in the reverse
       order.

       This ordering also affects the order of key generation.  Both SPI

    Karn & Simpson                Experimental                     [Page 33]

    RFC 2522                   Photuris Protocol                  March 1999

       Owner and SPI User generate the keys in the order listed.

       Implementation Notes:

          When choices are made from the list of Offered-Attributes, it is
          not required that any Security Association include every kind of
          offered attribute in any single SPI, or that a separate SPI be
          created for every offered attribute.

          Some kinds of attributes may be included more than once in a
          single SPI.  The set of allowable combinations of attributes are
          dependent on implementation and operational policy.  Such
          considerations are outside the scope of this document.

          The list may be divided into additional sections.  This can occur
          only when both parties recognize the affected attributes.

          The authentication, compression, encryption and identification
          mechanisms chosen, as well as the encapsulation modes (if any),
          need not be the same in both directions.

    5.3.  Shared-Secret

       A shared-secret is used in a number of calculations.  Regardless of
       the internal representation of the shared-secret, when used in
       calculations it is in the same form as the Value part of a Variable
       Precision Integer:

        - most significant byte first.
        - bits used are right justified within byte boundaries.
        - any unused bits are in the most significant byte.
        - unused bits are zero filled.

       The shared-secret does not include a Size field.

    5.4.  Identity Verification

       These messages are authenticated using the Identity-Choice.  The
       Verification value is calculated prior to masking (and optional
       encryption), and verified after unmasking (and optional decryption).

       The Identity-Choice authentication function is supplied with two
       input values:

        - the sender (SPI Owner) verification-key,
        - the data to be verified (as a concatenated sequence of bytes).

    Karn & Simpson                Experimental                     [Page 34]

    RFC 2522                   Photuris Protocol                  March 1999

       The resulting output value is stored in the Verification field.

       The Identity-Choice verification data consists of the following
       concatenated values:

        + the Initiator Cookie,
        + the Responder Cookie,
        + the Message, LifeTime and SPI fields,
        + the Identity-Choice and Identification,
        + the SPI User Identity Verification (response only),
        + the Attribute-Choices following the Verification field,
        + the Padding,
        + the SPI Owner TBV,
        + the SPI Owner Exchange-Value,
        + the SPI Owner Offered-Attributes,
        + the SPI User TBV,
        + the SPI User Exchange-Value,
        + the SPI User Offered-Attributes,
        + the Responder Offered-Schemes.

       The TBV (Three Byte Value) consists of the Counter and Scheme-Choice
       fields from the Value_Request, or the Reserved field from the
       Value_Response, immediately preceding the associated Exchange-Value.

       Note that the order of the Exchange-Value and Offered-Attributes
       fields is different in each direction, and the Identification and SPI
       fields are also likely to be different in each direction.  Note also
       that the SPI User Identity Verification (from the Identity_Request)
       is present only in the Identity_Response.

       If the verification fails, the users are notified, and a
       Verification_Failure message is sent, without adding any SPI.  On
       success, normal operation begins with the authentication and/or
       encryption of user datagrams.

       Implementation Notes:

          This is distinct from any authentication method specified for the
          SPI.

          The exact details of the Identification and verification-key
          included in the Verification calculation are dependent on the
          Identity-Choice, as described in the "Basic Attributes".

          Each party may wish to keep their own trusted databases, such as
          the Pretty Good Privacy (PGP) web of trust, and accept only those
          identities found there.  Failure to find the Identification in
          either an internal or external database results in the same

    Karn & Simpson                Experimental                     [Page 35]

    RFC 2522                   Photuris Protocol                  March 1999

          Verification_Failure message as failure of the verification
          computation.

          The Exchange-Value data includes both the Size and Value fields.
          The Offered-Attributes and Attribute-Choices data includes the
          Attribute, Length and Value fields.

    5.5.  Privacy-Key Computation

       Identification Exchange messages are masked using the Privacy-Method
       indicated by the current Scheme-Choice.  Masking begins with the next
       field after the SPI, and continues to the end of the data indicated
       by the UDP Length, including the Padding.

       The Scheme-Choice specified Key-Generation-Function is used to create
       a special privacy-key for each message.  This function is calculated
       over the following concatenated values:

        + the SPI Owner Exchange-Value,
        + the SPI User Exchange-Value,
        + the Initiator Cookie,
        + the Responder Cookie,
        + the Message, LifeTime and SPI (or Reserved) fields,
        + the computed shared-secret.

       Since the order of the Exchange-Value fields is different in each
       direction, and the Message, LifeTime and SPI fields are also
       different in each direction, the resulting privacy-key will usually
       be different in each direction.

       When a larger number of keying-bits are needed than are available
       from one iteration of the specified Key-Generation-Function, more
       keying-bits are generated by duplicating the trailing shared-secret,
       and recalculating the function.  That is, the first iteration will
       have one trailing copy of the shared-secret, the second iteration
       will have two trailing copies of the shared-secret, and so forth.

       Implementation Notes:

          This is distinct from any encryption method specified for the SPI.

          The length of the Padding, and other details, are dependent on the
          Privacy-Method.  See the "Basic Privacy-Method" list for details.

          To avoid keeping the Exchange-Values in memory after the initial
          verification, it is often possible to pre-compute the function
          over the initial bytes of the concatenated data values for each

    Karn & Simpson                Experimental                     [Page 36]

    RFC 2522                   Photuris Protocol                  March 1999

          direction, and append the trailing copies of the shared-secret.

          The Exchange-Value data includes both the Size and Value fields.

    5.6.  Session-Key Computation

       Each SPI has one or more session-keys.  These keys are generated
       based on the attributes of the SPI.  See the "Basic Attributes" for
       details.

       The Scheme-Choice specified Key-Generation-Function is used to create
       the SPI session-key for that particular attribute.  This function is
       calculated over the following concatenated values:

        + the Initiator Cookie,
        + the Responder Cookie,
        + the SPI Owner generation-key,
        + the SPI User generation-key,
        + the message Verification field,
        + the computed shared-secret.

       Since the order of the generation-keys is different in each
       direction, and the Verification field is also likely to be different
       in each direction, the resulting session-key will usually be
       different in each direction.

       When a larger number of keying-bits are needed than are available
       from one iteration of the specified Key-Generation-Function, more
       keying-bits are generated by duplicating the trailing shared-secret,
       and recalculating the function.  That is, the first iteration will
       have one trailing copy of the shared-secret, the second iteration
       will have two trailing copies of the shared-secret, and so forth.

       Implementation Notes:

          This is distinct from any privacy-key generated for the Photuris
          exchange.  Different initialization data is used, and iterations
          are maintained separately.

          The exact details of the Verification field and generation-keys
          that are included in the session-key calculation are dependent on
          the Identity-Choices, as described in the "Basic Attributes".

          To avoid keeping the generation-keys in memory after the initial
          verification, it is often possible to pre-compute the function
          over the initial bytes of the concatenated data values for each
          direction, and append the trailing copies of the shared-secret.

    Karn & Simpson                Experimental                     [Page 37]

    RFC 2522                   Photuris Protocol                  March 1999

          When both authentication and encryption attributes are used for
          the same SPI, there may be multiple session-keys associated with
          the same SPI.  These session-keys are generated in the order of
          the Attribute-Choices list.

    6.  SPI Messages

       SPI User                             SPI Owner
       ========                             =========
       SPI_Needed                     ->
          list SPI attribute(s)
          make validity key
          authenticate
          make privacy key(s)
          mask/encrypt message
                                       <-   SPI_Update
                                               make SPI
                                               pick SPI attribute(s)
                                               make SPI session-key(s)
                                               make validity key
                                               authenticate
                                               make privacy key(s)
                                               mask/encrypt message

       The exchange of messages is not related to the Initiator and
       Responder.  Instead, either party may send one of these messages at
       any time.  The messages are easily distinguished by the parties.

    6.0.1.  Send SPI_Needed

       At any time after completion of the Identification Exchange, either
       party can send SPI_Needed.  This message is sent when a prospective
       SPI User needs particular attributes for a datagram (such as
       confidentiality), and no current SPI has those attributes.

       The prospective SPI User selects from the intersection of attributes
       that both parties have previously offered, calculates the
       Verification, and masks the message using the Privacy-Method
       indicated by the current Scheme-Choice.

    Karn & Simpson                Experimental                     [Page 38]

    RFC 2522                   Photuris Protocol                  March 1999

    6.0.2.  Receive SPI_Needed

       The potential SPI Owner validates the pair of Cookies, the Padding,
       the Verification, and the Attributes-Needed.

       -  When an invalid/expired cookie is detected, a Bad_Cookie message
          is sent.

       -  When too many SPI values are already in use for this particular
          peer, or some other resource limit is reached, a Resource_Limit
          message is sent.

       -  After unmasking, when invalid Padding is detected, the variable
          length Attributes-Needed do not match the UDP Length, or an
          attribute was not in the Offered-Attributes, the message is
          silently discarded.

       -  When the message verification fails, a Verification_Failure
          message is sent.

       -  Whenever such a problem is detected, the SPI is not established;
          the implementation SHOULD log the occurance, and notify an
          operator as appropriate.

       When the message is valid, the party SHOULD send SPI_Update with the
       necessary attributes.

       If an existing SPI has those attributes, that SPI is returned in the
       SPI_Update with the remaining SPILT.

    6.0.3.  Send SPI_Update

       At any time after completion of the Identification Exchange, either
       party can send SPI_Update.  This message has effect in only one
       direction, from the SPI Owner to the SPI User.

       The SPI Owner chooses the SPI and SPILT, a set of Attributes for the
       SPI, calculates the Verification, and masks the message using the
       Privacy-Method indicated by the current Scheme-Choice.

    6.0.4.  Receive SPI_Update

       The prospective SPI User validates the pair of Cookies, the Padding,
       the Verification, and the Attributes-Needed.

       -  When an invalid/expired cookie is detected, a Bad_Cookie message

    Karn & Simpson                Experimental                     [Page 39]

    RFC 2522                   Photuris Protocol                  March 1999

          is sent.

       -  After unmasking, when invalid Padding is detected, the variable
          length Attribute-Choices do not match the UDP Length, an attribute
          was not in the Offered-Attributes, or the message modifies an
          existing SPI, the message is silently discarded.

       -  When the message verification fails, a Verification_Failure
          message is sent.

       -  Whenever such a problem is detected, the SPI is not established;
          the implementation SHOULD log the occurance, and notify an
          operator as appropriate.

       When the message is valid, further actions are dependent on the value
       of the LifeTime field, as described later.

    6.0.5.  Automated SPI_Updates

       Each SPI requires replacement under several circumstances:

       -  the volume of data processed (inhibiting probability
          cryptanalysis),

       -  exhaustion of available anti-replay Sequence Numbers,

       -  and expiration of the LifeTime.

       In general, a determination is made upon receipt of a datagram.  If
       the transform specific processing finds that refreshment is needed,
       an automated SPI_Update is triggered.

       In addition, automated SPI_Updates allow rapid SPI refreshment for
       high bandwidth applications in a high delay environment.  The update
       messages flow in the opposite direction from the primary traffic,
       conserving bandwidth and avoiding service interruption.

       When creating each SPI, the implementation MAY optionally set an
       Update TimeOut (UTO); by default, to half the value of the LifeTime
       (SPILT/2).  This time is highly dynamic, and adjustable to provide an
       automated SPI_Update long before transform specific processing.  If
       no new Photuris exchange occurs within the time limit, and the
       current exchange state has not expired, an automated SPI_Update is
       sent.

    Karn & Simpson                Experimental                     [Page 40]

    RFC 2522                   Photuris Protocol                  March 1999

    6.1.  SPI_Needed

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Initiator-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Responder-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Message    |                  Reserved-LT                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Reserved-SPI                          |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                                                               |
       ~                         Verification                          ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Attributes-Needed ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                          ... Padding  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Initiator-Cookie  16 bytes.  Copied from the Value_Request.

       Responder-Cookie  16 bytes.  Copied from the Value_Request.

       Message          8

       Reserved-LT      3 bytes.  For future use; MUST be filled with a
                        random non-zero value when transmitted, and MUST be
                        ignored when received.

       Reserved-SPI     4 bytes.  For future use; MUST be set to zero when
                        transmitted, and MUST be ignored when received.

       Verification     Variable Precision Integer, or other format
                        indicated by the current Scheme-Choice.  The
                        calculation of the value is described in "Validity
                        Verification".

                        The field may be any integral number of bytes in
                        length.  It does not require any particular
                        alignment.  The 32-bit alignment shown is for
                        convenience in the illustration.

    Karn & Simpson                Experimental                     [Page 41]

    RFC 2522                   Photuris Protocol                  March 1999

       Attributes-Needed
                        4 or more bytes.  A list of two or more attributes,
                        selected from the list of Offered-Attributes
                        supported by the peer.

                        The contents and usage of this list are as
                        previously described in "Attribute Choices List".
                        The end of the list is indicated by the UDP Length
                        after removing the Padding (UDP Length - last
                        Padding value).

       Padding          8 or more bytes.  The message is padded in the same
                        fashion specified for Identification Exchange
                        messages.

       The portion of the message after the SPI field is masked using the
       Privacy-Method indicated by the current Scheme-Choice.

       The fields following the SPI are opaque.  That is, the values are set
       prior to masking (and optional encryption), and examined only after
       unmasking (and optional decryption).

    Karn & Simpson                Experimental                     [Page 42]

    RFC 2522                   Photuris Protocol                  March 1999

    6.2.  SPI_Update

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Initiator-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Responder-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Message    |                    LifeTime                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Security-Parameters-Index                   |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                                                               |
       ~                         Verification                          ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Attribute-Choices ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                          ... Padding  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Initiator-Cookie  16 bytes.  Copied from the Value_Request.

       Responder-Cookie  16 bytes.  Copied from the Value_Request.

       Message          9

       LifeTime         3 bytes.  The number of seconds remaining before the
                        indicated SPI expires.  The value zero indicates
                        deletion of the indicated SPI.

       Security-Parameters-Index (SPI)
                        4 bytes.  The SPI to be used for incoming
                        communications.

                        This may be a new SPI value (for creation), or an
                        existing SPI value (for deletion).  The value zero
                        indicates special processing.

       Verification     Variable Precision Integer, or other format
                        indicated by the current Scheme-Choice.  The
                        calculation of the value is described in "Validity
                        Verification".

    Karn & Simpson                Experimental                     [Page 43]

    RFC 2522                   Photuris Protocol                  March 1999

                        The field may be any integral number of bytes in
                        length.  It does not require any particular
                        alignment.  The 32-bit alignment shown is for
                        convenience in the illustration.

       Attribute-Choices
                        0 or more bytes.  When the SPI and SPILT are non-
                        zero, a list of attributes selected from the list of
                        Offered-Attributes supported by the peer.

                        The contents and usage of this list are as
                        previously described in "Attribute Choices List".
                        The end of the list is indicated by the UDP Length
                        after removing the Padding (UDP Length - last
                        Padding value).

       Padding          8 or more bytes.  The message is padded in the same
                        fashion specified for Identification Exchange
                        messages.

       The portion of the message after the SPI field is masked using the
       Privacy-Method indicated by the current Scheme-Choice.

       The fields following the SPI are opaque.  That is, the values are set
       prior to masking (and optional encryption), and examined only after
       unmasking (and optional decryption).

    6.2.1.  Creation

       When the LifeTime is non-zero, and the SPI is also non-zero, the
       SPI_Update can be used to create a new SPI.  When the SPI is zero,
       the SPI_Update is silently discarded.

       The new session-keys are calculated in the same fashion as the
       Identity_Messages.  Since the SPI value is always different than any
       previous SPI during the Exchange LifeTime of the shared-secret, the
       resulting session-keys will necessarily be different from all others
       used in the same direction.

       No retransmission timer is necessary.  Success is indicated by the
       peer use of the new SPI.

       Should all creation attempts fail, eventually the peer will find that
       all existing SPIs have expired, and will begin the Photuris exchange
       again by sending a new Cookie_Request.  When appropriate, this
       Cookie_Request MAY include a Responder-Cookie to retain previous
       party pairings.

    Karn & Simpson                Experimental                     [Page 44]

    RFC 2522                   Photuris Protocol                  March 1999

    6.2.2.  Deletion

       When the LifeTime is zero, the SPI_Update can be used to delete a
       single existing SPI.  When the SPI is also zero, the SPI_Update will
       delete all existing SPIs related to this Security Association, and
       mark the Photuris exchange state as expired.  This is especially
       useful when the application that needed them terminates.

       No retransmission timer is necessary.  This message is advisory, to
       reduce the number of ICMP Security Failures messages.

       Should any deletion attempts fail, the peer will learn that the
       deleted SPIs are invalid through the normal ICMP Security Failures
       messages, and will initiate a Photuris exchange by sending a new
       Cookie_Request.

    6.2.3.  Modification

       The SPI_Update cannot be used to modify existing SPIs, such as
       lengthen an existing SPI LifeTime, resurrect an expired SPI, or
       add/remove an Attribute-Choice.

       On receipt, such an otherwise valid message is silently discarded.

    6.3.  Validity Verification

       These messages are authenticated using the Validity-Method indicated
       by the current Scheme-Choice.  The Verification value is calculated
       prior to masking (and optional encryption), and verified after
       unmasking (and optional decryption).

       The Validity-Method authentication function is supplied with two
       input values:

        - the sender (SPI Owner) verification-key,
        - the data to be verified (as a concatenated sequence of bytes).

       The resulting output value is stored in the Verification field.

       The Validity-Method verification data consists of the following
       concatenated values:

    Karn & Simpson                Experimental                     [Page 45]

    RFC 2522                   Photuris Protocol                  March 1999

        + the Initiator Cookie,
        + the Responder Cookie,
        + the Message, LifeTime and SPI (or Reserved) fields,
        + the SPI Owner Identity Verification,
        + the SPI User Identity Verification,
        + the Attribute-Choices following the Verification field,
        + the Padding.

       Note that the order of the Identity Verification fields (from the
       Identity_Messages) is different in each direction, and the Message,
       LifeTime and SPI fields are also likely to be different in each
       direction.

       If the verification fails, the users are notified, and a
       Verification_Failure message is sent, without adding or deleting any
       SPIs.  On success, normal operation begins with the authentication
       and/or encryption of user datagrams.

       Implementation Notes:

          This is distinct from any authentication method specified for the
          SPI.

          The Identity Verification data includes both the Size and Value
          fields.  The Attribute-Choices data includes the Attribute, Length
          and Value fields.

    7.  Error Messages

       These messages are issued in response to Photuris state loss or other
       problems.  A message has effect in only one direction.  No
       retransmission timer is necessary.

       These messages are not masked.

       The receiver checks the Cookies for validity.  Special care MUST be
       taken that the Cookie pair in the Error Message actually match a pair
       currently in use, and that the protocol is currently in a state where
       such an Error Message might be expected.  Otherwise, these messages
       could provide an opportunity for a denial of service attack.  Invalid
       messages are silently discarded.

    Karn & Simpson                Experimental                     [Page 46]

    RFC 2522                   Photuris Protocol                  March 1999

    7.1.  Bad_Cookie

       For the format of the 33 byte message, see "Header Format".  There
       are no additional fields.

       Initiator-Cookie  16 bytes.  Copied from the offending message.

       Responder-Cookie  16 bytes.  Copied from the offending message.

       Message          10

       This error message is sent when a Value_Request, Identity_Request,
       SPI_Needed, or SPI_Update is received, and the receiver specific
       Cookie is invalid or the associated exchange state has expired.

       During the Photuris exchange, when this error message is received, it
       has no immediate effect on the operation of the protocol phases.
       Later, when Retransmissions have been exceeded, and this error
       message has been received, the Initiator SHOULD begin the Photuris
       exchange again by sending a new Cookie_Request with the Responder-
       Cookie and Counter updated appropriately.

       When this error message is received in response to SPI_Needed, the
       exchange state SHOULD NOT be marked as expired, but the party SHOULD
       initiate a Photuris exchange by sending a new Cookie_Request.

       When this error message is received in response to SPI_Update, the
       exchange state SHOULD NOT be marked as expired, and no further action
       is taken.  A new exchange will be initiated later when needed by the
       peer to send authenticated and/or encrypted data.

       Existing SPIs are not deleted.  They expire normally, and are purged
       sometime later.

    7.2.  Resource_Limit

       For the format of the 34 byte message, see "Cookie_Request".  There
       are no additional fields.

       Initiator-Cookie  16 bytes.  Copied from the offending message.

       Responder-Cookie  16 bytes.  Copied from the offending message.

                        Special processing is applied to a Cookie_Request.
                        When the offending message Responder-Cookie and
                        Counter were both zero, and an existing exchange has
                        not yet been purged, this field is replaced with the

    Karn & Simpson                Experimental                     [Page 47]

    RFC 2522                   Photuris Protocol                  March 1999

                        Responder-Cookie from the existing exchange.

       Message          11

       Counter          1 byte.  Copied from the offending message.

                        When zero, the Responder-Cookie indicates the
                        Initiator of a previous exchange, or no previous
                        exchange is specified.

                        When non-zero, the Responder-Cookie indicates the
                        Responder to a previous exchange.  This value is set
                        to the Counter from the corresponding
                        Cookie_Response.

       This error message is sent when a Cookie_Request, Value_Request or
       SPI_Needed is received, and too many SPI values are already in use
       for that peer, or some other Photuris resource is unavailable.

       During the Photuris exchange, when this error message is received in
       response to a Cookie_Request or Value_Request, the implementation
       SHOULD double the retransmission timeout (as usual) for sending
       another Cookie_Request or Value_Request.  Otherwise, it has no
       immediate effect on the operation of the protocol phases.  Later,
       when Retransmissions have been exceeded, and this error message has
       been received, the Initiator SHOULD begin the Photuris exchange again
       by sending a new Cookie_Request with the Responder-Cookie and Counter
       updated appropriately.

       When this error message is received in response to SPI_Needed, the
       implementation SHOULD NOT send another SPI_Needed until one of the
       existing SPIs associated with this exchange is deleted or has
       expired.

    7.3.  Verification_Failure

       For the format of the 33 byte message, see "Header Format".  There
       are no additional fields.

       Initiator-Cookie  16 bytes.  Copied from the offending message.

       Responder-Cookie  16 bytes.  Copied from the offending message.

       Message          12

       This error message is sent when an Identity_Message, SPI_Needed or
       SPI_Update is received, and verification fails.

    Karn & Simpson                Experimental                     [Page 48]

    RFC 2522                   Photuris Protocol                  March 1999

       When this error message is received, the implementation SHOULD log
       the occurance, and notify an operator as appropriate.  However,
       receipt has no effect on the operation of the protocol.

    7.4.  Message_Reject

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Initiator-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                       Responder-Cookie                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Message    |  Bad-Message  |             Offset            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Initiator-Cookie  16 bytes.  Copied from the offending message.

       Responder-Cookie  16 bytes.  Copied from the offending message.

       Message          13

       Bad-Message      1 byte.  Indicates the Message number of the
                        offending message.

       Offset           2 bytes.  The number of bytes from the beginning of
                        the offending message where the unrecognized field
                        starts.  The minimum value is 32.

       This error message is sent when an optional Message type is received
       that is not supported, or an optional format of a supported Message
       is not recognized.

       When this error message is received, the implementation SHOULD log
       the occurance, and notify an operator as appropriate.  However,
       receipt has no effect on the operation of the protocol.

    Karn & Simpson                Experimental                     [Page 49]

    RFC 2522                   Photuris Protocol                  March 1999

    8.  Public Value Exchanges

       Photuris is based in principle on public-key cryptography,
       specifically Diffie-Hellman key exchange.  Exchange of public D-H
       Exchange-Values based on private-secret values results in a mutual
       shared-secret between the parties.  This shared-secret can be used on
       its own, or to generate a series of session-keys for authentication
       and encryption of subsequent traffic.

       This document assumes familiarity with the Diffie-Hellman public-key
       algorithm.  A good description can be found in [Schneier95].

    8.1.  Modular Exponentiation Groups

       The original Diffie-Hellman technique [DH76] specified modular
       exponentiation.  A public-value is generated using a generator (g),
       raised to a private-secret exponent (x), modulo a prime (p):

          (g**x) mod p.

       When these public-values are exchanged between parties, the parties
       can calculate a shared-secret value between themselves:

          (g**xy) mod p.

       The generator (g) and modulus (p) are established by the Scheme-
       Choice (see the "Basic Exchange-Schemes" for details).  They are
       offered in the Cookie_Response, and one pair is chosen in the
       Value_Request.

       The private exponents (x) and (y) are kept secret by the parties.
       Only the public-value result of the modular exponentiation with (x)
       or (y) is sent as the Initiator and Responder Exchange-Value.

       These public-values are represented in single Variable Precision
       Integers.  The Size of these Exchange-Values will match the Size of
       the modulus (p).

    8.2.  Moduli Selection

       Each implementation proposes one or more moduli in its Offered-
       Schemes.  Every implementation MUST support up to 1024-bit moduli.

       For any particular Photuris node, these moduli need not change for
       significant periods of time; likely days or weeks.  A background
       process can periodically generate new moduli.

    Karn & Simpson                Experimental                     [Page 50]

    RFC 2522                   Photuris Protocol                  March 1999

          For 512-bit moduli, current estimates would provide 64
          (pessimistic) bit-equivalents of cryptographic strength.

          For 1024-bit moduli, current estimates would range from 80
          (pessimistic) through 98 (optimistic) bit-equivalents of
          cryptographic strength.

       These estimates are used when choosing moduli that are appropriate
       for the desired Security Parameter attributes.

    8.2.1.  Bootstrap Moduli

       Each implementation is likely to use a fixed modulus during its
       bootstrap, until it can generate another modulus in the background.
       As the bootstrap modulus will be widely distributed, and reused
       whenever the machine reinitializes, it SHOULD be a "safe" prime (p =
       2q+1) to provide the greatest long-term protection.

       Implementors are encouraged to generate their own bootstrap moduli,
       and to change bootstrap moduli in successive implementation releases.

    8.2.2.  Learning Moduli

       As Photuris exchanges are initiated, new moduli will be learned from
       the Responder Offered-Schemes.  The Initiator MAY cache these moduli
       for its own use.

       Before offering any learned modulus, the implementation MUST perform
       at least one iteration of probable primality verification.  In this
       fashion, many processors will perform verification in parallel as
       moduli are passed around.

       When primality verification failures are found, the failed moduli
       SHOULD be retained for some (implementation dependent) period of
       time, to avoid re-learning and re-testing after subsequent exchanges.

    8.3.  Generator Selection

       The generator (g) should be chosen such that the private-secret
       exponents will generate all possible public-values, evenly
       distributed throughout the range of the modulus (p), without cycling
       through a smaller subset.  Such a generator is called a "primitive
       root" (which is trivial to find when p is "safe").

       Only one generator (2) is required to be supported.

    Karn & Simpson                Experimental                     [Page 51]

    RFC 2522                   Photuris Protocol                  March 1999

       Implementation Notes:

          One useful technique is to select the generator, and then limit
          the modulus selection sieve to primes with that generator:

             2   when p (mod 24) = 11.
             3   when p (mod 12) = 5.
             5   when p (mod 10) = 3 or 7.

          The required generator (2) improves efficiency in multiplication
          performance.  It is usable even when it is not a primitive root,
          as it still covers half of the space of possible residues.

    8.4.  Exponent Selection

       Each implementation generates a separate random private-secret
       exponent for each different modulus.  Then, a D-H Exchange-Value is
       calculated for the given modulus, generator, and exponent.

       This specification recommends that the exponent length be at least
       twice the desired cryptographic strength of the longest session-key
       needed by the strongest offered-attribute.

       Based on the estimates in "Moduli Selection" (above):

          For 512-bit moduli, exponent lengths of 128 bits (or more) are
          recommended.

          For 1024-bit moduli, exponent lengths of 160 to 256 bits (or more)
          are recommended.

       Although the same exponent and Exchange-Value may be used with
       several parties whenever the same modulus and generator are used, the
       exponent SHOULD be changed at random intervals.  A background process
       can periodically destroy the old values, generate a new random
       private-secret exponent, and recalculate the Exchange-Value.

       Implementation Notes:

          The size of the exponent is entirely implementation dependent, is
          unknown to the other party, and can be easily changed.

          Since these operations involve several time-consuming modular
          exponentiations, moving them to the "background" substantially
          improves the apparent execution speed of the Photuris protocol.
          It also reduces CPU loading sufficiently to allow a single
          public/private key-pair to be used in several closely spaced

    Karn & Simpson                Experimental                     [Page 52]

    RFC 2522                   Photuris Protocol                  March 1999

          Photuris executions, when creating Security Associations with
          several different nodes over a short period of time.

          Other pre-computation suggestions are described in [BGMW93, LL94,
          Rooij94].

    8.5.  Defective Exchange Values

       Some exponents do not qualify as secret.  The exponent 0 will
       generate the Exchange-Value 1, and the exponent 1 will generate the
       Exchange-Value g.  Small exponents will be easily visible and SHOULD
       be avoided where:

          g**x < p.

       Depending on the structure of the moduli, certain exponents can be
       used for sub-group confinement attacks.  For "safe" primes (p =
       2q+1), these exponents are p-1 and (p-1)/2, which will generate the
       Exchange-Values 1 and p-1 respectively.

       When an implementation chooses a random exponent, the resulting
       Exchange-Value is examined.  If the Exchange-Value is represented in
       less than half the number of significant bits in the modulus, then a
       new random exponent MUST be chosen.

          For 512-bit moduli, Exchange-Values of 2**256 or greater are
          required.

          For 1024-bit moduli, Exchange-Values of 2**512 or greater are
          required.

       In addition, if the resulting Exchange-Value is p-1, then a new
       random exponent MUST be chosen.

       Upon receipt of an Exchange-Value that fails to meet the
       requirements, the Value Exchange message is silently discarded.

       Implementation Notes:

          Avoidance of small exponents can be assured by setting at least
          one bit in the most significant half of the exponent.

    Karn & Simpson                Experimental                     [Page 53]

    RFC 2522                   Photuris Protocol                  March 1999

    9.  Basic Exchange-Schemes

       Initial values are assigned as follows:

       (0)   Reserved.

       (1)   Reserved.

       (2)   Implementation Required.  Any modulus (p) with a recommended
             generator (g) of 2.  When the Exchange-Scheme Size is non-zero,
             the modulus is contained in the Exchange-Scheme Value field in
             the list of Offered-Schemes.

             An Exchange-Scheme Size of zero is invalid.

             Key-Generation-Function     "MD5 Hash"
             Privacy-Method              "Simple Masking"
             Validity-Method             "MD5-IPMAC Check"

             This combination of features requires a modulus with at least
             64-bits of cryptographic strength.

       (3)   Exchange-Schemes 3 to 255 are intended for future well-known
             published schemes.

       (256)  Exchange-Schemes 256 to 32767 are intended for vendor-specific
             unpublished schemes.  Implementors wishing a number MUST
             request the number from the authors.

       (32768)
             Exchange-Schemes 32768 to 65535 are available for cooperating
             parties to indicate private schemes, regardless of vendor
             implementation.  These numbers are not reserved, and are
             subject to duplication.  Other criteria, such as the IP Source
             and Destination of the Cookie_Request, are used to
             differentiate the particular Exchange-Schemes available.

    Karn & Simpson                Experimental                     [Page 54]

    RFC 2522                   Photuris Protocol                  March 1999

    10.  Basic Key-Generation-Function
    10.1.  MD5 Hash

       MD5 [RFC-1321] is used as a pseudo-random-function for generating the
       key(s).  The key(s) begin with the most significant bits of the hash.
       MD5 is iterated as needed to generate the requisite length of key
       material.

       When an individual key does not use all 128-bits of the last hash,
       any remaining unused (least significant) bits of the last hash are
       discarded.  When combined with other uses of key generation for the
       same purpose, the next key will begin with a new hash iteration.

    11.  Basic Privacy-Method
    11.1.  Simple Masking

       As described in "Privacy-Key Computation", sufficient privacy-key
       material is generated to match the message length, beginning with the
       next field after the SPI, and including the Padding.  The message is
       masked by XOR with the privacy-key.

    12.  Basic Validity-Method
    12.1.  MD5-IPMAC Check

       As described in "Validity Verification", the Verification field value
       is the MD5 [RFC-1321] hash over the concatenation of

          MD5( key, keyfill, data, datafill, key, md5fill )

       where the key is the computed verification-key.

       The keyfill and datafill use the same pad-with-length technique
       defined for md5fill.  This padding and length is implicit, and does
       not appear in the datagram.

       The resulting Verification field is a 128-bit Variable Precision
       Integer (18 bytes including Size).  When used in calculations, the
       Verification data includes both the Size and Value fields.

    Karn & Simpson                Experimental                     [Page 55]

    RFC 2522                   Photuris Protocol                  March 1999

    13.  Basic Attributes

       Implementors wishing a number MUST request the number from the
       authors.  Initial values are assigned as follows:

         Use    Type
          -       0* padding
          -       1* AH-Attributes
          -       2+ ESP-Attributes
         AEI      5* MD5-IPMAC
         AEIX   255+ Organizational

         A      AH Attribute-Choice
          E     ESP Attribute-Choice
           I    Identity-Choice
            X   dependent on list location
             +  feature must be recognized even when not supported
             *  feature must be supported (mandatory)

       Other attributes are specified in companion documents.

    13.1.  Padding

       +-+-+-+-+-+-+-+-+
       |   Attribute   |
       +-+-+-+-+-+-+-+-+

       Attribute        0

       Each attribute may have value fields that are multiple bytes.  To
       facilitate processing efficiency, these fields are aligned on
       integral modulo 8 byte (64-bit) boundaries.

       Padding is accomplished by insertion of 1 to 7 Attribute 0 padding
       bytes before the attribute that needs alignment.

       No padding is used after the final attribute in a list.

    Karn & Simpson                Experimental                     [Page 56]

    RFC 2522                   Photuris Protocol                  March 1999

    13.2.  AH-Attributes

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Attribute   |    Length     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Attribute        1

       Length           0

       When a list of Attributes is specified, this Attribute begins the
       section of the list which applies to the Authentication Header (AH).

    13.3.  ESP-Attributes

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Attribute   |    Length     |  PayloadType  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Attribute        2

       Length           1

       PayloadType      1 byte.  Indicates the contents of the ESP Transform
                        Data field, using the IP Next Header (Protocol)
                        value.  Up-to-date values of the IP Next Header
                        (Protocol) are specified in the most recent
                        "Assigned Numbers" [RFC-1700].

                        For example, when encrypting an entire IP datagram,
                        this field will contain the value 4, indicating IP-
                        in-IP encapsulation.

       When a list of Attributes is specified, this Attribute begins the
       section of the list which applies to the Encapsulating Security
       Payload (ESP).

       When listed as an Offered-Attribute, the PayloadType is set to 255.

       When selected as an Attribute-Choice, the PayloadType is set to the
       actual value to be used.

    Karn & Simpson                Experimental                     [Page 57]

    RFC 2522                   Photuris Protocol                  March 1999

    13.4.  MD5-IPMAC

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Attribute   |    Length     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Attribute        5

       Length           0

    13.4.1.  Symmetric Identification

       When selected as an Identity-Choice, the immediately following
       Identification field contains an unstructured Variable Precision
       Integer.  Valid Identifications and symmetric secret-keys are
       preconfigured by the parties.

       There is no required format or content for the Identification value.
       The value may be a number or string of any kind.  See "Use of
       Identification and Secrets" for details.

       The symmetric secret-key (as specified) is selected based on the
       contents of the Identification field.  All implementations MUST
       support at least 62 bytes.  The selected symmetric secret-key SHOULD
       provide at least 64-bits of cryptographic strength.

       As described in "Identity Verification", the Verification field value
       is the MD5 [RFC-1321] hash over the concatenation of:

          MD5( key, keyfill, data, datafill, key, md5fill )

       where the key is the computed verification-key.

       The keyfill and datafill use the same pad-with-length technique
       defined for md5fill.  This padding and length is implicit, and does
       not appear in the datagram.

       The resulting Verification field is a 128-bit Variable Precision
       Integer (18 bytes including Size).  When used in calculations, the
       Verification data includes both the Size and Value fields.

       For both "Identity Verification" and "Validity Verification", the
       verification-key is the MD5 [RFC-1321] hash of the following
       concatenated values:

    Karn & Simpson                Experimental                     [Page 58]

    RFC 2522                   Photuris Protocol                  March 1999

        + the symmetric secret-key,
        + the computed shared-secret.

       For "Session-Key Computation", the symmetric secret-key is used
       directly as the generation-key.

       Regardless of the internal representation of the symmetric secret-
       key, when used in calculations it is in the same form as the Value
       part of a Variable Precision Integer:

        - most significant byte first.
        - bits used are right justified within byte boundaries.
        - any unused bits are in the most significant byte.
        - unused bits are zero filled.

       The symmetric secret-key does not include a Size field.

    13.4.2.  Authentication

       May be selected as an AH or ESP Attribute-Choice, pursuant to [RFC-
       1828] et sequitur.  The selected Exchange-Scheme SHOULD provide at
       least 64-bits of cryptographic strength.

       As described in "Session-Key Computation", the most significant 384-
       bits (48 bytes) of the Key-Generation-Function iterations are used
       for the key.

       Profile:

          When negotiated with Photuris, the transform differs slightly from
          [RFC-1828].

          The form of the authenticated message is:

             MD5( key, keyfill, datagram, datafill, key, md5fill )

          where the key is the SPI session-key.

          The additional datafill protects against the (impractical) attack
          described in [PO96].  The keyfill and datafill use the same pad-
          with-length technique defined for md5fill.  This padding and
          length is implicit, and does not appear in the datagram.

    Karn & Simpson                Experimental                     [Page 59]

    RFC 2522                   Photuris Protocol                  March 1999

    13.5.  Organizational

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Attribute   |    Length     |              OUI
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              ...      |     Kind      |  Value(s) ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Attribute        255

       Length           >= 4

                        When the Length is four, no Value(s) field is
                        present.

       OUI              3 bytes.  The vendor's Organizationally Unique
                        Identifier, assigned by IEEE 802 or IANA (see [RFC-
                        1700] for contact details).  The bits within the
                        byte are in canonical order, and the most
                        significant byte is transmitted first.

       Kind             1 byte.  Indicates a sub-type for the OUI.  There is
                        no standardization for this field.  Each OUI
                        implements its own values.

       Value(s)         0 or more bytes.  The details are implementation
                        specific.

       Some implementors might not need nor want to publish their
       proprietary algorithms and attributes.  This OUI mechanism is
       available to specify these without encumbering the authors with
       proprietary number requests.

    Karn & Simpson                Experimental                     [Page 60]

    RFC 2522                   Photuris Protocol                  March 1999

    A.  Automaton

       An example automaton is provided to illustrate the operation of the
       protocol.  It is incomplete and non-deterministic; many of the
       Good/Bad semantic decisions are policy-based or too difficult to
       represent in tabular form.  Where conflicts appear between this
       example and the text, the text takes precedence.

       The finite-state automaton is defined by events, actions and state
       transitions.  Events include reception of external commands such as
       expiration of a timer, and reception of datagrams from a peer.
       Actions include the starting of timers and transmission of datagrams
       to the peer.

       Events

       DU13 = Communication Administratively Prohibited
       SF0  = Bad SPI
       SF4  = Need Authentication
       SF5  = Need Authorization
       WC   = Want Confidentiality

       RCQ+ = Receive Cookie_Request (Good)
       RCQ- = Receive Cookie_Request (Bad)
       RCR+ = Receive Cookie_Response (Good)
       RCR- = Receive Cookie_Response (Bad)

       RVQ+ = Receive Value_Request (Good)
       RVQ- = Receive Value_Request (Bad)
       RVR+ = Receive Value_Response (Good)
       RVR- = Receive Value_Response (Bad)

       RIQ+ = Receive Identity_Request (Good)
       RIQ- = Receive Identity_Request (Bad)
       RIR+ = Receive Identity_Response (Good)
       RIR- = Receive Identity_Response (Bad)

       RUN+ = Receive SPI_Needed (Good)
       RUN- = Receive SPI_Needed (Bad)
       RUM+ = Receive SPI_Update (Good)
       RUM- = Receive SPI_Update (Bad)

       RBC  = Receive Bad Cookie
       RRL  = Receive Resource Limit
       RVF  = Receive Verification Failure
       RMR  = Receive Message Reject

       TO+  = Timeout with counter > 0

    Karn & Simpson                Experimental                     [Page 61]

    RFC 2522                   Photuris Protocol                  March 1999

       TO-  = Timeout with counter expired
       UTO  = Update TimeOut
       XTO  = Exchange TimeOut

       Actions

       scq  = Send Cookie_Request
       scr  = Send Cookie_Response

       svq  = Send Value_Request
       svr  = Send Value_Response

       siq  = Send Identity_Request
       sir  = Send Identity_Response

       sum  = Send SPI_Update

       se*  = Send error message (see text)
       sbc  = Send Bad Cookie
       srl  = Send Resource Limit
       svf  = Send Verification Failure

       brto = Backoff Retransmission TimeOut
       buto = Backoff Update TimeOut
       rto  = Set Retransmission TimeOut
       uto  = Set Update TimeOut
       xto  = Set Exchange TimeOut

       log  = log operator message

    A.1.  State Transition Table

       States are indicated horizontally, and events are read vertically.
       State transitions and actions are represented in the form
       action/new-state.  Multiple actions are separated by commas, and may
       continue on succeeding lines as space requires; multiple actions may
       be implemented in any convenient order.  The state may be followed by
       a letter, which indicates an explanatory footnote.  The dash ('-')
       indicates an illegal transition.

    Karn & Simpson                Experimental                     [Page 62]

    RFC 2522                   Photuris Protocol                  March 1999

       Initiator

             |    0         1         2         3         4
             | Initial    Cookie  CookieBad   Value    ValueBad
       ------+--------------------------------------------------
        DU13 |rto,scq/1 rto,scq/1 rto,scq/1     3         4
        SF0  |rto,scq/1     1         2         3         4
        SF4  |rto,scq/1     1         2         3         4
        SF5  |rto,scq/1     1         2         3         4
        WC   |rto,scq/1     1         2         3         4
             |
        RCR+ |    -     rto,svq/3 rto,svq/3     3         4
        RCR- |    0         1         2         3         4
        RVR+ |    -         -         -     rto,siq/5 rto,siq/5
        RVR- |    0         1         2         3         4
        RIR+ |    -         -         -         -         -
        RIR- |    0         1         2         3         4
             |
        RUN+ |    -         -         -         -         -
        RUN- |  sbc/0     sbc/1     sbc/2     sbc/3     sbc/4
        RUM+ |    -         -         -         -         -
        RUM- |  sbc/0     sbc/1     sbc/2     sbc/3     sbc/4
             |
        RBC  |    -         -         -         4         4
        RRL  |    -       brto/2    brto/2    brto/4    brto/4
        RVF  |    -         -         -         -         -
        RMR  |    -         -         -         -         -
             |
         TO+ |    -       scq/1     scq/2     svq/3     svq/4
         TO- |    -         0       scq/1       0       scq/1
        UTO  |    -         -         -         -         -
        XTO  |    -         0         0         0         0

    Karn & Simpson                Experimental                     [Page 63]

    RFC 2522                   Photuris Protocol                  March 1999

       Initiator

             |    5         6         8
             |Identity IdentityBad  Update
       ------+-----------------------------
        DU13 |    5         6         8
        SF0  |    5         6     rto,scq/1
        SF4  |    5         6     rto,scq/1
        SF5  |    5         6     rto,scq/1
        WC   |    5         6       sun/8
             |
        RCR+ |    5         6         8
        RCR- |    5         6         8
        RVR+ |    5         6         8
        RVR- |    5         6         8
        RIR+ |  uto/8     uto/8       8
        RIR- |  svf/5     svf/6       8
             |
        RUN+ |    -         -       sum/8
        RUN- |  sbc/5     sbc/6     se*/8
        RUM+ |    -         -         8
        RUM- |  sbc/5     sbc/6     se*/8
             |
        RBC  |    6         6     rto,scq/1
        RRL  |    5         6       buto/8
        RVF  |  log/5     log/6     log/8
        RMR  |  log/5     log/6     log/8
             |
         TO+ |  sim/5     sim/6       -
         TO- |    0       scq/1       -
        UTO  |    -         -       sum/8
        XTO  |    0         0         0

    Karn & Simpson                Experimental                     [Page 64]

    RFC 2522                   Photuris Protocol                  March 1999

       Responder

             |    0         7         8
             | Initial    Ready     Update
       ------+-----------------------------
        WC   |    -         7       sun/8
             |
        RCQ+ |  scr/0     scr/7     scr/8
        RCQ- |  srl/0     srl/7     srl/8
        RVQ+ |xto,svr/7   svr/7     svr/8
        RVQ- |  sbc/0     sbc/7     sbc/8
        RIQ+ |    -     uto,sir/8   sir/8
        RIQ- |  sbc/0     se*/7     se*/8
             |
        RUN+ |    -         -       sum/8
        RUN- |  sbc/0     sbc/7     se*/8
        RUM+ |    -         -         8
        RUM- |  sbc/0     sbc/7     se*/8
             |
        RBC  |    -         7     rto,scq/1
        RRL  |    -         -       buto/8
        RVF  |    -         -       log/8
        RMR  |    -         -       log/8
             |
        UTO  |    -         -       sum/8
        XTO  |    -         0         0

    A.2.  States

       Following is a more detailed description of each automaton state.

       The "Bad" version of a state is to indicate that the Bad_Cookie or
       Resource_Limit message has been received.

    A.2.1.  Initial

       The Initial state is fictional, in that there is no state between the
       parties.

    Karn & Simpson                Experimental                     [Page 65]

    RFC 2522                   Photuris Protocol                  March 1999

    A.2.2.  Cookie

       In the Cookie state, the Initiator has sent a Cookie_Request, and is
       waiting for a Cookie_Response.  Both the Restart and Exchange timers
       are running.

       Note that the Responder has no Cookie state.

    A.2.3.  Value

       In the Value state, the Initiator has sent its Exchange-Value, and is
       waiting for an Identity_Message.  Both the Restart and Exchange
       timers are running.

    A.2.4.  Identity

       In the Identity state, the Initiator has sent an Identity_Request,
       and is waiting for an Identity_Response in reply.  Both the Restart
       and Exchange timers are running.

    A.2.5.  Ready

       In the Ready state, the Responder has sent its Exchange-Value, and is
       waiting for an Identity_Message.  The Exchange timer is running.

    A.2.6.  Update

       In the Update state, each party has concluded the Photuris exchange,
       and is unilaterally updating expiring SPIs until the Exchange
       LifeTime expires.  Both the Update and Exchange timers are running.

    Karn & Simpson                Experimental                     [Page 66]

    RFC 2522                   Photuris Protocol                  March 1999

    B.  Use of Identification and Secrets

       Implementation of the base protocol requires support for operator
       configuration of participant identities and associated symmetric
       secret-keys.

       The form of the Identification and Secret fields is not constrained
       to be a readable string.  In addition to a simpler quoted string
       configuration, an implementation MUST allow configuration of an
       arbitrary stream of bytes.

    B.1.  Identification

       Typically, the Identification is a user name, a site name, a Fully
       Qualified Domain Name, or an email address which contains a user name
       and a domain name.  Examples include:

          user
          node.site.
          user@node.site.
          rcmd@node.site.
          "Mundane Name" <user@node.site>

       There is no requirement that the domain name match any of the
       particular IP addresses in use by the parties.

    B.2.  Group Identity With Group Secret

       A simple configuration approach could use a single Identity and
       Secret, distributed to all the participants in the trusted group.
       This might be appropriate between routers under a single
       administration comprising a Virtual Private Network over the
       Internet.

       Nota Bene:
          The passwords used in these examples do not meet the "MD5-IPMAC
          Symmetric Identification" recommendation for at least 64-bits of
          cryptographic strength.

       The administrator configures each router with the same username and
       password:

          identity local "Tiny VPN 1995 November" "abracadabra"
          identity remote "Tiny VPN 1995 November" "abracadabra"

       When the Initiator sends its Identity_Request, the SPI Owner

    Karn & Simpson                Experimental                     [Page 67]

    RFC 2522                   Photuris Protocol                  March 1999

       Identification field is "Tiny VPN 1995 November" and the SPI Owner
       secret-key is "abracadabra".

       When the Responder sends its Identity_Response, the SPI Owner
       Identification field is "Tiny VPN 1995 November" and the SPI Owner
       secret-key is "abracadabra".  The SPI User Identification is "Tiny
       VPN 1995 November" (taken from the request), and the SPI User
       secret-key is "abracadabra".

       Note that even in the face of implementations with very poor random
       number generation yielding the same random numbers for both parties
       at every step, and with this completely identical configuration, the
       addition of the SPI User Verification field in the response
       calculation is highly likely to produce a different Verification
       value (see "Identity Verification").  In turn, the different
       Verification values affect the calculation of SPI session-keys that
       are highly likely to be different in each direction (see "Session-Key
       Computation").

    B.3.  Multiple Identities With Group Secrets

       A more robust configuration approach could use a separate Identity
       and Secret for each party, distributed to the participants in the
       trusted group.  This might be appropriate for authenticated firewall
       traversal.

       An administrator has one or more networks, and a number of mobile
       users.  It is desirable to restrict access to authorized external
       users.  The example boundary router is 10.0.0.1.

       The administrator gives each user a different username and password,
       together with a group username and password for the router.

       The administrator configures (in part):

          identity local "199511@router.site" "FalDaRah"
          identity remote "Happy_Wanderer@router.site" "FalDaRee"

       Each mobile user adds commands to tunnel and authenticate.

          route addprivate 10.0.0.0/8 tunnel 10.0.0.1
          secure 10.0.0.1 authenticate-only
          identity local "Happy_Wanderer@router.site" "FalDaRee"
          identity remote "199511@router.site" "FalDaRah"
          identity remote "199512@router.site" "FalDaHaHaHaHaHaHa"

       When the mobile Initiator sends its Identity_Request, the SPI Owner

    Karn & Simpson                Experimental                     [Page 68]

    RFC 2522                   Photuris Protocol                  March 1999

       Identification field is "Happy_Wanderer@router.site" and the SPI
       Owner secret-key is "FalDaRee".

       When the firewall Responder sends its Identity_Response, the SPI
       Owner Identification field is "199511@router.site" and the SPI Owner
       secret-key is "FalDaRah".  The SPI User Identification field is
       "Happy_Wanderer@router.site" (taken from the request), and the SPI
       User secret-key is "FalDaRee".

       In this example, the mobile user is already prepared for a monthly
       password changeover, where the router might identify itself as
       "199512@router.site".

    B.4.  Multiple Identities With Multiple Secrets

       Greater security might be achieved through configuration of a pair of
       secrets between each party.  As before, one secret is used for
       initial contact to any member of the group, but another secret is
       used between specific parties.  Compromise of one secret or pair of
       secrets does not affect any other member of the group.  This might be
       appropriate between the routers forming a boundary between
       cooperating Virtual Private Networks that establish local policy for
       each VPN member access.

       One administrator configures:

          identity local "Apple" "all for one"
          identity local "Apple-Baker" "Apple to Baker" "Baker"
          identity remote "Baker" "one for all"
          identity remote "Baker-Apple" "Baker to Apple"

       Another configures:

          identity local "Baker" "one for all"
          identity local "Baker-Apple" "Baker to Apple" "Apple"
          identity remote "Apple" "all for one"
          identity remote "Apple-Baker" "Apple to Baker"

       When the Initiator sends its Identity_Request, the SPI Owner
       Identification field is "Apple" and the SPI Owner secret-key is "all
       for one".

       When the Responder sends its Identity_Response, finding that the
       special pairing exists for "Apple" (in this example, indicated by a
       third field), the SPI Owner Identification field is "Baker-Apple" and
       the SPI Owner secret-key is "Baker to Apple".  The SPI User
       Identification is "Apple" (taken from the request), and the SPI User

    Karn & Simpson                Experimental                     [Page 69]

    RFC 2522                   Photuris Protocol                  March 1999

       secret-key is "all for one".

    Operational Considerations

       The specification provides only a few configurable parameters, with
       defaults that should satisfy most situations.

       Retransmissions
          Default: 3.

       Initial Retransmission TimeOut (IRTO)
          Default: 5 seconds.

       Exchange TimeOut (ETO)
          Default: 30 seconds.  Minimum: Retransmissions * IRTO.

       Exchange LifeTime (ELT)
          Default: 30 minutes.  Minimum: 2 * ETO.

       SPI LifeTime (SPILT)
          Default: 5 minutes.  Minimum: 3 * ETO.

       Each party configures a list of known identities and symmetric
       secret-keys.

       In addition, each party configures local policy that determines what
       access (if any) is granted to the holder of a particular identity.
       For example, the party might allow anonymous FTP, but prohibit
       Telnet.  Such considerations are outside the scope of this document.

    Security Considerations

       Photuris was based on currently available tools, by experienced
       network protocol designers with an interest in cryptography, rather
       than by cryptographers with an interest in network protocols.  This
       specification is intended to be readily implementable without
       requiring an extensive background in cryptology.

       Therefore, only minimal background cryptologic discussion and
       rationale is included in this document.  Although some review has
       been provided by the general cryptologic community, it is anticipated
       that design decisions and tradeoffs will be thoroughly analysed in
       subsequent dissertations and debated for many years to come.

       Cryptologic details are reserved for separate documents that may be
       more readily and timely updated with new analysis.

    Karn & Simpson                Experimental                     [Page 70]

    RFC 2522                   Photuris Protocol                  March 1999

    History

       The initial specification of Photuris, now called version 1 (December
       1994 to March 1995), was based on a short list of design
       requirements, and simple experimental code by Phil Karn.  Only one
       modular exponentiation form was used, with a single byte index of
       pre-specified group parameters.  The transform attributes were
       selected during the public value exchange.  Party privacy was
       protected in the identification signature exchange with standard ESP
       transforms.

       Upon submission for review by the IP Security Working Group, a large
       number of features were demanded.  A mere 254 future group choices
       were not deemed enough; it was expanded to two bytes (and renamed
       schemes), and was expanded again to carry variable parameters.  The
       transform attributes were made variable length to accomodate optional
       parameters.  Every other possible parameter was made negotiable.
       Some participants were unable to switch modes on the UDP sockets to
       use standard ESP transforms for only some messages, and party privacy
       was integrated into the protocol.  The message headers were
       reorganized, and selection of transform attributes was delayed until
       the identification exchange.  An additional update message phase was
       added.

       Version 2 (July 1995 to December 1995) specification stability was
       achieved in November 1995 by moving most parameters into separate
       documents for later discussion, and leaving only a few mandatory
       features in the base specification.  Within a month, multiple
       interoperable implementations were produced.

       Unfortunately, in a fit of demagoguery, the IP Security Working Group
       decided in a straw poll to remove party privacy protection, and the
       Working Group chair terminated the meeting without allowing further
       discussion.  Because the identification exchange messages required
       privacy to function correctly, the messages were reorganized again.
       Party privacy and other optional schemes were split into a separate
       document.

       The implementors established a separate discussion group.  Version 3
       (April 1996 to June 1997) enjoyed a long period of specification
       stability and multiple implementations on half a dozen platforms.

       Meanwhile, the IP Security Working Group has developed a competing
       specification with large numbers of negotiable parameters.  Also, the
       PPP Extensions Working Group has deployed link security transforms.

       Version 4 (July 1997 onward) attempts to maintain a semblance of
       interface compatibility with these other efforts.  Minor changes are

    Karn & Simpson                Experimental                     [Page 71]

    RFC 2522                   Photuris Protocol                  March 1999

       specified in transform padding format and key generation.  More than
       one value is permitted per scheme, giving greater latitude in choice
       for future extensions.  The opportunity is taken to return party
       privacy to the base document, and make small semantic changes in
       automated updates and error recovery.  All ESP transform attributes
       are moved to separate documents, to (hopefully) avoid future
       incompatible changes to the base document.

    Acknowledgements

          Thou shalt make no law restricting the size of integers that may
          be multiplied together, nor the number of times that an integer
          may be multiplied by itself, nor the modulus by which an integer
          may be reduced.  [Prime Commandment]

       Phil Karn was principally responsible for the design of the protocol
       phases, particularly the "cookie" anti-clogging defense, developed
       the initial testing implementation, and provided much of the design
       rationale text (now removed to a separate document).

       William Simpson was responsible for the packet formats and
       attributes, additional message types, editing and formatting.  All
       such mistakes are his responsibility.

       This protocol was later discovered to have many elements in common
       with the Station-To-Station authentication protocol [DOW92].

       Angelos Keromytis developed the first completely independent
       implementation (circa October 1995).  Also, he suggested the cookie
       exchange rate limitation counter.

       Paul C van Oorschot suggested signing both the public exponents and
       the shared-secret, to provide an authentication-only version of
       identity verification.  Also, he provided text regarding moduli,
       generator, and exponent selection (now removed to a separate
       document).

       Hilarie Orman suggested adding secret "nonces" to session-key
       generation for asymmetric public/private-key identity methods (now
       removed to a separate document), and provided extensive review of the
       protocol details.

       Bart Preneel and Paul C van Oorschot in [PO96] recommended padding
       between the data and trailing key when hashing for authentication.

       Niels Provos developed another independent implementation (circa May
       1997), ported to AIX, Linux, OpenBSD, and Solaris.  Also, he made

    Karn & Simpson                Experimental                     [Page 72]

    RFC 2522                   Photuris Protocol                  March 1999

       suggestions regarding automated update, and listing multiple moduli
       per scheme.

       Bill Sommerfeld suggested including the authentication symmetric
       secret-keys in the session-key generation, and using the Cookie
       values on successive exchanges to provide bi-directional user-
       oriented keying (now removed to a separate document).

       Oliver Spatscheck developed the second independent implementation
       (circa December 1995) for the Xkernel.

       International interoperability testing between early implementors
       provided the impetus for many of the implementation notes herein, and
       numerous refinements in the semantics of the protocol messages.

       Randall Atkinson, Steven Bellovin, Wataru Hamada, James Hughes, Brian
       LaMacchia, Cheryl Madson, Lewis McCarthy, Perry Metzger, Bob Quinn,
       Ron Rivest, Rich Schroeppel, and Norman Shulman provided useful
       critiques of earlier versions of this document.

       Special thanks to the Center for Information Technology Integration
       (CITI) for providing computing resources.

    References

       [BGMW93]    E. Brickell, D. Gordon, K. McCurley, and D. Wilson, "Fast
                   Exponentiation with Precomputation (Extended Abstract)",
                   Advances in Cryptology -- Eurocrypt '92, Lecture Notes in
                   Computer Science 658 (1993), Springer-Verlag, 200-207.

                   Also U.S. Patent #5,299,262, E.F. Brickell, D.M. Gordon,
                   K.S. McCurley, "Method for exponentiating in
                   cryptographic systems", 29 Mar 1994.

       [DH76]      Diffie, W., and Hellman, H.E., "New Directions in
                   Cryptography", IEEE Transactions on Information Theory, v
                   IT-22 n 6 pp 644-654, November 1976.

       [DOW92]     Whitfield Diffie, Paul C van Oorshot, and Michael J
                   Wiener, "Authentication and Authenticated Key Exchanges",
                   Designs, Codes and Cryptography, v 2 pp 107-125, Kluwer
                   Academic Publishers, 1992.

       [Firefly]   "Photuris" is the latin name for the firefly.  "Firefly"
                   is in turn the name for the USA National Security
                   Administration's (classified) key exchange protocol for
                   the STU-III secure telephone.  Informed speculation has

    Karn & Simpson                Experimental                     [Page 73]

    RFC 2522                   Photuris Protocol                  March 1999

                   it that Firefly is based on very similar design
                   principles.

       [LL94]      Lim, C.H., Lee, P.J., "More flexible exponentiation with
                   precomputation", Advances in Cryptology -- Crypto '94,
                   Lecture Notes in Computer Science 839 (1994), Springer-
                   Verlag, pages 95-107.

       [Prime Commandment]
                   A derivation of an apocryphal quote from the usenet list
                   sci.crypt.

       [PO96]      Bart Preneel, and Paul C van Oorshot, "On the security of
                   two MAC algorithms", Advances in Cryptology -- Eurocrypt
                   '96, Lecture Notes in Computer Science 1070 (May 1996),
                   Springer-Verlag, pages 19-32.

       [RFC-768]   Postel, J., "User Datagram Protocol", STD 6,
                   USC/Information Sciences Institute, August 1980.

       [RFC-791]   Postel, J., "Internet Protocol", STD 5, USC/Information
                   Sciences Institute, September 1981.

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

       [RFC-1700]  Reynolds, J., and Postel, J., "Assigned Numbers", STD 2,
                   USC/Information Sciences Institute, October 1994.

       [RFC-1812]  Baker, F., Editor, "Requirements for IP Version 4
                   Routers", Cisco Systems, June 1995.

       [RFC-1828]  Metzger, P., Simpson, W., "IP Authentication using Keyed
                   MD5", July 1995.

       [RFC-1829]  Karn, P., Metzger, P., Simpson, W., "The ESP DES-CBC
                   Transform", July 1995.

       [RFC-2119]  Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, Harvard University, March
                   1997.

       [RFC-2521]  Karn, P., and Simpson, W., "ICMP Security Failures
                   Messages", March 1999.

       [Rooij94]   P. de Rooij, "Efficient exponentiation using
                   precomputation and vector addition chains", Advances in
                   Cryptology -- Eurocrypt '94, Lecture Notes in Computer

    Karn & Simpson                Experimental                     [Page 74]

    RFC 2522                   Photuris Protocol                  March 1999

                   Science, Springer-Verlag, pages 403-415.

       [Schneier95]
                   Schneier, B., "Applied Cryptography Second Edition", John
                   Wiley & Sons, New York, NY, 1995.  ISBN 0-471-12845-7.

    Contacts

       Comments about this document should be discussed on the
       photuris@adk.gr mailing list.

       Questions about this document can also be directed to:

          Phil Karn
          Qualcomm, Inc.
          6455 Lusk Blvd.
          San Diego, California  92121-2779

              karn@qualcomm.com
              karn@unix.ka9q.ampr.org (preferred)

          William Allen Simpson
          DayDreamer
          Computer Systems Consulting Services
          1384 Fontaine
          Madison Heights, Michigan  48071

              wsimpson@UMich.edu
              wsimpson@GreenDragon.com (preferred)

    Karn & Simpson                Experimental                     [Page 75]

    RFC 2522                   Photuris Protocol                  March 1999

    Full Copyright Statement

       Copyright (C) The Internet Society (1999).  Copyright (C) Philip Karn
       and William Allen Simpson (1994-1999).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards (in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed), or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       (BUT NOT LIMITED TO) ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Karn & Simpson                Experimental                     [Page 76]