File: rfc2522.txt

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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]











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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.




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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



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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



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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.

















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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



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   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.




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      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,



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   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.



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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.




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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.














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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.








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   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



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      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.









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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.





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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.





























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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.




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   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



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                    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.







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   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.
























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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.







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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.





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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.






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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



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                    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



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                    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



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   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).



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   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



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      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



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      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.



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      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.










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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



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      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.






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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.




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   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).






























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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".




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                    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.



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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:








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    + 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.








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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



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                    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.



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   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.











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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.



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      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.



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   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



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      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.









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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.















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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.











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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.












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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.







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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:




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    + 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.







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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.


















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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



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   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.










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   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


















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   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


















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   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.









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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.

















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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



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   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



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   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



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   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.



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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



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   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



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   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



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               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



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               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)



















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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.























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