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Versions: 10 11 12 13 14 15 16 17 18 RFC 2522

Network Working Group                                  P Karn (Qualcomm)
Internet Draft                                  W A Simpson (DayDreamer)
expires in six months                                           May 1997


               Photuris: Session Key Management Protocol
                     draft-simpson-photuris-12.txt


Status of this Memo

   This document is an Internet-Draft.  Internet Drafts are working doc-
   uments of the Internet Engineering Task Force (IETF), its Areas, and
   its Working Groups.  Note that other groups may also distribute work-
   ing documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months, and may be updated, replaced, or obsoleted by other documents
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   ence material, or to cite them other than as a ``working draft'' or
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   Distribution of this memo is unlimited.

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.












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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 the 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 defi-
   nitions, although it does mention some basic policy areas as required
   for the proper implementation and operation of the protocol mecha-
   nisms.

   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 cur-
   rent 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].


   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 asym-
                    metric 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 dis-
                    tributable.  As used in this document, this is dis-
                    tinguished from an asymmetric public/private key-
                    pair.  An example is a user password.





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   Security Association
                    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 infor-
                    mation such as sensitivity labelling.  For further
                    details, see [RFC-1825].

   Security Parameters Index (SPI)
                    A number that indicates the Security Association.
                    The number is relative to the IP Destination, which
                    is the SPI Owner.

   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 fre-
                    quently.

   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 (which are defined else-
                    where).  For example, AH-MD5 [RFC-1828] transforms
                    an IP datagram into a cryptographic hash, and ESP-
                    DES-CBC [RFC-1829] transforms plaintext to cipher-
                    text and back again.

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











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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 addition, supported exchange-schemes are offered by the Respon-
      der 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 establish 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 Associations.

   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 sepa-
      rate 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 may also be encrypted for party privacy protection
      using an exchange session-key based on the shared-secret.  This
      protects the identities of the parties, hides the security parame-
      ter values, and improves security for the exchange protocol and
      security transforms.  This (optional) facility is specified in
      companion documents.

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

      These exchanges may also be encrypted 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.







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   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 protection key)
      (encrypt message)
                                   <-   Identity_Response
                                           make SPI
                                           pick SPI attribute(s)
                                           identify self
                                           authenticate
                                           (make protection key)
                                           (encrypt message)

               [make SPI session-keys in each direction]



















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   SPI User                             SPI Owner
   ========                             =========
   SPI_Needed                     ->
      list SPI attribute(s)
      make integrity key
      authenticate
      (encrypt message)
                                   <-   SPI_Update
                                           make SPI
                                           pick SPI attribute(s)
                                           make SPI session-key(s)
                                           make integrity key
                                           authenticate
                                           (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 Associations

   A Photuris exchange between two parties results in a pair of SPI val-
   ues (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 Destina-
   tion 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 Cook-
   ies (and UDP Ports) keep the exchanges separate.  This results in
   more than one initial SPI for each Destination.

   To create multiple Security Associations 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 trans-
   mission.








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   Implementation Notes:

      The method used for SPI assignment is implementation dependent.
      The only requirement is that the SPI be unique for the IP Destina-
      tion.

      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 par-
      ticular order or range.


1.4.  LifeTimes

   The Photuris exchange results in two kinds of state, each with sepa-
   rate 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 multiple Security Associations that are
      established.  This state may be viewed as between users and nodes.

   The SPI LifeTimes may be shorter or longer than the Exchange Life-
   Time.  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, 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 min-
   utes).

   In addition, during a Photuris exchange, an Exchange TimeOut 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 results (default
   60 seconds).



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   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 Exchange TimeOut.

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

      1800000 + random(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 com-
   puted 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 is usually related to the speed of the
   link (typically 30 seconds to 30 minutes).

   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 by a few sec-
   onds.

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



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   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 imple-
   mentation 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].


2.  Protocol Details

   The Initiator begins a Photuris exchange under several circumstances:

   -  The Initiator has a datagram that it wishes to send with privacy,
      and has no current Photuris exchange state with the IP Destina-
      tion.  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-xxxx] 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-xxxx] Security
      Failures: Need Authentication (Type 40, Code 4), and has no cur-
      rent Photuris exchange state with the ICMP Source.

   -  The Initiator has received the ICMP message [RFC-xxxx] Security
      Failures: Need Authorization (Type 40, Code 5), that matches



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      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 previ-
   ously 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 mes-
   sage is received with an incorrect UDP checksum, it is silently dis-
   carded.

   Implementation Notes:

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

      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.





















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2.2.  Header Format

   All of the messages have a format similar to the following, as trans-
   mitted left to right in network order (most significant to least sig-
   nificant):

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Initiator-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Responder-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |
   +-+-+-+-+-+-+-+-+


   Initiator-Cookie 16 octets.

   Responder-Cookie 16 octets.

   Type             one octet.  Each message type has a unique value.
                    Initial values are assigned as follows:

                        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 mes-
   sages.







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2.3.  Variable Precision Numbers

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


   Size             two, four, or eight octets.  The number of signifi-
                    cant bits used in the Value field.  Always transmit-
                    ted most significant octet 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 octet is in the range 0
                    through 254 (0xfe), the field is two octets.  Both
                    octets are used to indicate the size of the Value
                    field, which ranges from 1 to 65,279 significant
                    bits (in 1 to 8,160 octets).

                    When the most significant octet is 255 (0xff), the
                    field is four octets.  The remaining three octets
                    are added to 65,280 to indicate the size of the
                    Value field, which is limited to 16,776,959 signifi-
                    cant bits (in 2,097,120 octets).

                    When the most significant two octets are 65,535
                    (0xffff), the field is eight octets.  The remaining
                    six octets are added to 16,776,960 to indicate the
                    size of the Value field.  This is vastly too long
                    for these UDP messages, but is included for com-
                    pleteness.

   Value            Zero or more octets.  Always transmitted most sig-
                    nificant octet first.

                    The bits used are right justified within octet
                    boundaries; that is, any unused bits are in the most
                    significant octet.  Unused bits are zero filled.

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





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   Implementation Note:

      The numbers are assumed to be unsigned.


2.4.  Exchange Schemes

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


   Scheme           two octets.  A unique value indicating the exchange-
                    scheme.  See the "Exchange Scheme List".

   Size             two octets, ranging from 0 to 65,279.  See "Variable
                    Precision Number".

   Value            Zero or more octets.  See "Variable Precision Num-
                    ber".

   The Size MUST NOT be assumed to be constant for a particular Scheme.
   However, only one of each kind of Scheme will be present in any list
   of schemes.

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


2.5.  Attributes

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


   Type             one octet.  A unique value indicating the kind of
                    attribute.  See the "Attribute List" for details.

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

   Length           one octet.  The size of the Value(s) field in
                    octets.

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



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                    present.

   Value(s)         Zero or more octets.  See the "Attribute List" for
                    details.

   The Length MUST NOT be assumed to be constant for a particular Type.
   Multiple attributes of the same Type with varying Lengths MAY be pre-
   sent in any list of attributes.

   Support is required for the "MD5-KDpKp Symmetric Verification" (#3)
   and "MD5-KpDpKp Integrity" (#5) Attributes, and they SHOULD be pre-
   sent in every Offered-Attributes list.

   Implementation Note:

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


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

      For example, a Bad_Cookie message is received from the Initiator.
      That message has an Initiator-Cookie of A, and a Responder-Cookie
      of B.  The Responder-Cookie is replaced with A, and a new Initia-
      tor-Cookie C is generated.

   If any previous exchange between the peer IP nodes has not expired,



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   the Responder-Cookie is set to the most recent Responder-Cookie, and
   the request 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 B, and a new Initiator-Cookie C is generated.

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

      By default, the Initiator operates in the same manner as when all
      of its previous exchange state has expired.  The Responder will
      update the Counter appropriately when not all of its own exchange
      state has 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 Retrans-
   missions.  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 Bad_Cookie message has
   been received during the exchange, the Initiator SHOULD begin the
   Photuris exchange again by sending a new Cookie_Request.


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 mes-
      sage is sent.

   Otherwise, the Responder returns a Cookie_Response.

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








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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,
   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 Genera-
   tion" 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.

   -  Whenever an invalid/expired Initiator-Cookie is detected, the mes-
      sage is silently discarded.

   -  Whenever 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.



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   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 out-
      side the scope of this document.


3.1.  Cookie_Request

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


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

   Responder-Cookie 16 octets.  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 speci-
                    fied 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.



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

   Type             0

   Counter          one octet.  Indicates the number of the current
                    exchange.  Copied from a previous Cookie_Response.

                    When zero, no previous Responder is specified.



3.2.  Cookie_Response

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


   Initiator-Cookie 16 octets.  Copied from the Cookie_Request.

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

   Type             1

   Counter          one octet.  Indicates the number of the current
                    exchange.  Must be greater than zero.

   Offered-Schemes  A list of one or more exchange-schemes supported by
                    the Responder, beginning with most preferred.

                    Each scheme is four or more octets (see "Exchange
                    Scheme List").  Only one of each kind of Scheme may
                    be offered.  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 pro-
   tection 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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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 (typi-
   cally each 60 seconds), and MUST be changed whenever any precalcu-
   lated Responder Exchange-Value is changed.

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

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

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

   Once the Value_Response is sent, both Initiator and Responder cookies
   are cached to identify the exchange.


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 precalculated and used for multi-
   ple Responders.

   The IP Destination for the Responder is examined, and the attributes



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   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 message has
   been received during the exchange, the Initiator SHOULD begin the
   Photuris exchange again by sending a new Cookie_Request.


4.0.2.  Receive Value_Request

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

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

   -  Whenever 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 precalculated and used for multi-
   ple Initiators.

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









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   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 calcula-
      tion.  This is not usually a problem, as retransmission timeouts
      typically exceed calculation time.


4.0.4.  Receive Value_Response

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

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

   -  Whenever the Exchange-Value is obviously defective, or the vari-
      able 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 computa-
   tion of the shared-secret.

   When the Initiator completes computation, it sends an Iden-
   tity_Request to the Responder.

















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4.1.  Value_Request

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


   Initiator-Cookie 16 octets.  Copied from the Cookie_Response.

   Responder-Cookie 16 octets.  Copied from the Cookie_Response.

   Type             2

   Counter          one octet.  Copied from the Cookie_Response.

   Scheme-Choice    two octets.  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 number.  Provided by the Initia-
                    tor for calculating a shared-secret between the par-
                    ties.  The Value format is indicated by the Scheme-
                    Choice.

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





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


   Initiator-Cookie 16 octets.  Copied from the Value_Request.

   Responder-Cookie 16 octets.  Copied from the Value_Request.

   Type             3

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

   Responder-Exchange-Value
                    variable precision number.  Provided by the Respon-
                    der for calculating a shared-secret between the par-
                    ties.  The Value format is indicated by the current
                    Scheme-Choice as indicated by the Value_Request.

                    The field may be any integral number of octets in
                    length, as indicated by its Size field.  It does not



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                    require any particular alignment.  The 32-bit align-
                    ment shown is for convenience in the illustration.

   Responder-Offered-Attributes
                    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 "Attribute List", where mandatory attributes are also specified.

   The list is composed of two or three sections: Identification-
   Attributes, Authentication-Attributes, and (optional) Encapsulation-
   Attributes.  Within each section, the attributes are listed 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-KDpKp Symmetric Verification" (#3)
   and "MD5-KpDpKp Integrity" (#5) Attributes, and they SHOULD be pre-
   sent in every Offered-Attributes list.









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   Implementation Notes:

      Since the offer is made by the prospective SPI User (sender),
      order of preference likely reflects the capabilities and engineer-
      ing 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.


5.  Identification Exchange

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

               [make SPI session-keys in each direction]

   The exchange of messages is ordered, although the formats and mean-
   ings of the messages are identical in each direction.  The messages
   are easily distinguished by the parties themselves, by examining the
   Type 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



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      for the longest calculation.  This will be different for different
      processor speeds, different algorithms, and different length vari-
      ables.  Therefore, the method for estimating time is implementa-
      tion 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 dis-
      carded.


5.0.1.  Send Identity_Request

   The Initiator chooses an appropriate Identification, an SPI and SPI
   LifeTime, a set of Attributes for the SPI, calculates the Verifica-
   tion, and optionally encrypts the message for party privacy protec-
   tion (when a Privacy-Method is indicated by the Scheme-Choice).

   The Initiator also starts a retransmission timer.  If no valid Iden-
   tity_Response arrives within the time limit, its previous Iden-
   tity_Request is retransmitted for the remaining number of Retransmis-
   sions.

   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 Identification, the
   Verification, and the Attribute-Choices.

   -  Whenever an invalid/expired cookie is detected, a Bad_Cookie mes-
      sage is sent.

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

   -  Whenever the variable length Attribute-Choices do not match the
      UDP Length, or the attributes are not a subset of those in the
      Offered-Attributes, the message is silently discarded.

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



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   When the message is valid, the Responder sets its Exchange timer to
   the Exchange LifeTime (if this has not already been done for a previ-
   ous 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.


5.0.3.  Send Identity_Response

   The Responder chooses an appropriate Identification, an SPI and SPI
   LifeTime, a set of Attributes for the SPI, calculates the Verifica-
   tion, and optionally encrypts the message for party privacy protec-
   tion (when a Privacy-Method is indicated by the Scheme-Choice).

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

   The Responder sets its Update timer to half the value of its SPI
   LifeTime.  If no new Photuris exchange occurs within the time limit,
   and the Exchange timer has not expired, an SPI_Update is sent to cre-
   ate another SPI.

   At this time, the Responder begins the authentication and/or encryp-
   tion of user datagrams.


5.0.4.  Receive Identity_Response

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

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

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

   -  Whenever the variable length Attribute-Choices do not match the
      UDP Length, or the attributes are not a subset of those in the
      Offered-Attributes, the message is silently discarded.

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




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   -  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 previ-
   ous exchange).

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

   The Initiator sets its Update timer to half the value of its SPI
   LifeTime.  If no new Photuris exchange occurs within the time limit,
   and the Exchange timer has not expired, an SPI_Update is sent to cre-
   ate another SPI.

   At this time, the Initiator begins the authentication and/or encryp-
   tion of user datagrams.


































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5.1.  Identity_Messages

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Initiator-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Responder-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |                    LifeTime                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Security-Parameter-Index                    |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |        Identity-Choice        |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   ~                        Identification                         ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                         Verification                          ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attribute-Choices ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                             ... Padding           |   PadLength   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Initiator-Cookie 16 octets.  Copied from the Value_Request.

   Responder-Cookie 16 octets.  Copied from the Value_Request.

   Type             4 (Request) or 7 (Response)

   LifeTime         three octets.  The number of seconds remaining
                    before the indicated SPI expires.  Must be greater
                    than zero.

   Security-Parameter-Index
                    four octets.  The SPI to be used for incoming commu-
                    nications.

                    When zero, indicates that no SPI is created in this
                    direction.




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   Identity-Choice  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 octets 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 illustra-
                    tion.

   Identification   variable precision number, or alternative format
                    indicated by the Identity-Choice.  See the
                    "Attribute List" for details.

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

   Verification     variable precision number, or alternative format
                    indicated by the Identity-Choice.  The calculation
                    of the value is described in "Identity Verifica-
                    tion".

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

   Attribute-Choices
                    Zero or more octets.  A list of attributes for this
                    (non-zero) SPI, 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 remov-
                    ing the PadLength and Padding fields (UDP Length -
                    PadLength - 1).

   Padding          Zero or more octets.  Prior to (optional) encryp-
                    tion, it is filled to align the PadLength field at a
                    boundary appropriate to the Privacy-Method indicated
                    by the current Scheme-Choice.  The padding values
                    begin with the value 0, and count up to the number
                    of padding octets (zero relative).  For example, if
                    the PadLength is 5, the padding values are 0, 1, 2,
                    3, 4.



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                    After (optional) decryption, if the padding octets
                    are not the correct values for the PadLength, then
                    verification fails.

   PadLength        one octet.  The size of the Padding field in octets
                    (not including the PadLength field).  The value typ-
                    ically ranges from 0 to 7, but may be up to 255 to
                    permit hiding of the actual data length.

                    This field is always present, even when no Padding
                    is required.

   The portion of the message after the SPI MAY be encrypted for party
   privacy protection.  Such mechanisms are outside the scope of this
   document.

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


5.2.  Attribute Choices List

   This list specifies the attributes of a Security Association.  The
   attribute formats are specified in the "Attribute List".

   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",
      "DES-CBC",
      "AH-Attributes",
      "MD5-KpDpKp Integrity",

   would result in ESP with encryption (inside), and then AH authentica-
   tion (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
   Owner and SPI User generate the keys in the order listed.






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   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 sin-
      gle SPI.  The set of allowable combinations of attributes are
      dependent on implementation and operational policy.  Such consid-
      erations are outside the scope of this document.


5.3.  Shared-Secret

   The shared-secret is used in a number of calculations.  Regardless of
   the internal representation of the shared-secret, when used in calcu-
   lations it is in the same form as the Value part of a Variable Preci-
   sion Number:

    - most significant octet first.
    - bits used are right justified within octet boundaries.
    - any unused bits are in the most significant octet.
    - 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 Ver-
   ification value is calculated prior to (optional) encryption, and
   verified after (optional) decryption.

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

    - the computed shared-secret.
    - the data to be verified (as a concatenated sequence of octets).

   The resulting output value is stored in the Verification field.

   The Identity-Choice authentication function is calculated over the
   following concatenated data values:







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    + the Initiator Cookie,
    + the Responder Cookie,
    + the Responder Offered-Schemes,
    + the SPI Owner Exchange-Value,
    + the SPI Owner Offered-Attributes,
    + the SPI Owner Identification,
    + the SPI Owner secret-key,
    + the SPI User Exchange-Value,
    + the SPI User Offered-Attributes,
    + the SPI User Identification (response only),
    + the SPI User secret-key (response only),
    + the message Type, LifeTime and SPI fields,
    + the Attribute-Choices following the Verification field,
    + the Padding (if any),
    + the PadLength.

   Note that the order of the Exchange-Value and Offered-Attribute
   fields is different in each direction.  The Identification and SPI
   fields are also likely to be different in each direction.  Note also
   that the SPI User Identification and secret-key will be omitted in
   the Identity_Request.

   If the verification fails, the users are notified, and a Verifica-
   tion_Failure message is sent, without adding any Security Associa-
   tions.  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
      Security Associations.

      The exact details of the Identification and secret-keys that are
      included in the Verification calculation are dependent on the
      Identity-Choice, as described in the "Attribute List".

      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 Veri-
      fication_Failure message as failure of the verification computa-
      tion.

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





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5.5.  Session-Key Computation

   Each Security Association SPI has one or more session-keys.  These
   keys are generated based on the attributes of the Security Associa-
   tion.  See the "Attribute List" for details.

   The Attribute-Choice specified key generation function is used to
   create the SPI session-key for that particular attribute.  This func-
   tion is calculated over the following concatenated values:

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

   When a larger number of keying-bits are needed than are available
   from the specified function, these keying-bits are generated by
   duplicating the trailing shared-secret, and recalculating the func-
   tion.  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:

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

      To avoid keeping the secret-keys in memory after the initial veri-
      fication, it is often possible to precompute the function over the
      initial octets of the concatenated data values for each direction,
      and append the trailing copies of the shared-secret.

      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.












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6.  SPI Messages

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

   The exchange of messages is not related to the Initiator and Respon-
   der.  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 an SPI_Needed.  This message is sent when a prospec-
   tive SPI User needs particular attributes for a datagram (such as
   privacy protection), 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 Verifica-
   tion, and optionally encrypts the message for party privacy protec-
   tion (when a Privacy-Method is indicated by the Scheme-Choice).


6.0.2.  Receive SPI_Needed

   The potential SPI Owner validates the pair of Cookies, the Verifica-
   tion, and the Attributes-Needed.

   -  Whenever an invalid/expired cookie is detected, a Bad_Cookie mes-
      sage is sent.

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

   -  Whenever the variable length Attributes-Needed do not match the
      UDP Length, or the attributes are not a subset of those in the



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      Offered-Attributes, the message is silently discarded.

   -  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 party SHOULD send an SPI_Update that
   includes the necessary attributes.


6.0.3.  Send SPI_Update

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

   The SPI Owner chooses an SPI and SPI LifeTime, a set of Attributes
   for the SPI, calculates the Verification, and optionally encrypts the
   message for party privacy protection (when a Privacy-Method is indi-
   cated by the Scheme-Choice).


6.0.4.  Receive SPI_Update

   The prospective SPI User validates the pair of Cookies, the Verifica-
   tion, and the Attributes-Needed.

   -  Whenever an invalid/expired cookie is detected, a Bad_Cookie mes-
      sage is sent.

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

   -  Whenever the variable length Attribute-Choices do not match the
      UDP Length, or the attributes are not a subset of those in the
      Offered-Attributes, the message is silently discarded.

   -  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, further actions are dependent on the value
   of the SPI LifeTime field, as described later.








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6.1.  SPI_Needed

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Initiator-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Responder-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |                    Reserved                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Reserved                            |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                                                               |
   ~                         Verification                          ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes-Needed ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                             ... Padding           |   PadLength   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Initiator-Cookie 16 octets.  Copied from the Value_Request.

   Responder-Cookie 16 octets.  Copied from the Value_Request.

   Type             8

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

   Verification     variable precision number, or other format indicated
                    by the Scheme-Choice.  The calculation of the value
                    is described in "Validity Verification".

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

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




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                    The contents and usage of this list are as previ-
                    ously described in "Attribute Choices List".  The
                    end of the list is indicated by the UDP Length after
                    removing the PadLength and Padding fields (UDP
                    Length - PadLength - 1).

   Padding          Zero or more octets.  Prior to (optional) encryp-
                    tion, it is filled to align the PadLength field at a
                    boundary appropriate to the Privacy-Method indicated
                    by the current Scheme-Choice.  The padding values
                    begin with the value 0, and count up to the number
                    of padding octets (zero relative).  For example, if
                    the PadLength is 5, the padding values are 0, 1, 2,
                    3, 4.

                    After (optional) decryption, if the padding octets
                    are not the correct values for the PadLength, then
                    verification fails.

   PadLength        one octet.  The size of the Padding field in octets
                    (not including the PadLength field).  The value typ-
                    ically ranges from 0 to 7, but may be up to 255 to
                    permit hiding of the actual data length.

                    This field is always present, even when no Padding
                    is required.

   The portion of the message after the SPI MAY be encrypted for party
   privacy protection, in the same fashion specified for Iden-
   tity_Messages.

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

















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6.2.  SPI_Update

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Initiator-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Responder-Cookie                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |                    LifeTime                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Security-Parameter-Index                    |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                                                               |
   ~                         Verification                          ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attribute-Choices ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                             ... Padding           |   PadLength   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Initiator-Cookie 16 octets.  Copied from the Value_Request.

   Responder-Cookie 16 octets.  Copied from the Value_Request.

   Type             9

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

   Security-Parameter-Index
                    four octets.  The SPI to be used for incoming commu-
                    nications.

                    This may be a new SPI value (for creation), or an
                    existing SPI value (for deletion).  The value zero
                    indicates all old SPIs for this IP Destination (used
                    for deletion).

   Verification     variable precision number, or other format indicated
                    by the Scheme-Choice.  The calculation of the value
                    is described in "Validity Verification".




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

   Attribute-Choices
                    Four or more octets.  A list of two or more
                    attributes for this SPI, selected from the list of
                    Offered-Attributes supported by the peer.

                    The contents and usage of this list are as previ-
                    ously described in "Attribute Choices List".  The
                    end of the list is indicated by the UDP Length after
                    removing the PadLength and Padding fields (UDP
                    Length - PadLength - 1).

   Padding          Zero or more octets.  Prior to (optional) encryp-
                    tion, it is filled to align the PadLength field at a
                    boundary appropriate to the Privacy-Method indicated
                    by the current Scheme-Choice.  The padding values
                    begin with the value 0, and count up to the number
                    of padding octets (zero relative).  For example, if
                    the PadLength is 5, the padding values are 0, 1, 2,
                    3, 4.

                    After (optional) decryption, if the padding octets
                    are not the correct values for the PadLength, then
                    verification fails.

   PadLength        one octet.  The size of the Padding field in octets
                    (not including the PadLength field).  The value typ-
                    ically ranges from 0 to 7, but may be up to 255 to
                    permit hiding of the actual data length.

                    This field is always present, even when no Padding
                    is required.

   The portion of the message after the SPI MAY be encrypted for party
   privacy protection, in the same fashion specified for Iden-
   tity_Messages.

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







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6.2.1.  Creation

   When the SPI LifeTime is greater than zero, the SPI_Update can be
   used to create a new Security Association.  Frequently, this message
   is used to create replacement SPIs as the LifeTime of an earlier SPI
   approaches expiration.

   In addition, this message allows more rapid SPI creation for high
   bandwidth applications.  The messages flow in the opposite direction
   from the primary traffic flow.

   The new session-keys are calculated in the same fashion as the Iden-
   tity_Messages.  Since the SPI value is always different than any pre-
   vious 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.

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

   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.


6.2.2.  Deletion

   When the SPI LifeTime is zero, the SPI_Update can be used to delete
   existing Security Associations.  This is especially useful when the
   application that needed them terminates, to prevent another applica-
   tion from replaying the datagrams.

   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.






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6.2.3.  Modification

   The SPI_Update cannot be used to modify existing Security Associa-
   tions, 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 (see "Exchange Scheme List").  The Veri-
   fication value is calculated prior to (optional) encryption, and ver-
   ified after (optional) decryption.

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

    - the computed shared-secret,
    - the data to be verified (as a concatenated sequence of octets).

   The resulting output value is stored in the Verification field.

   The Validity-Method authentication function is calculated over the
   following concatenated data values:

    + the Initiator Cookie,
    + the Responder Cookie,
    + the SPI Owner Identity Verification,
    + the SPI User Identity Verification,
    + the message Type, LifeTime and SPI fields,
    + the Attribute-Choices following the Verification field,
    + the Padding (if any),
    + the PadLength.

   Note that the order of the Identity Verification fields (from the
   Identity_Messages) is different in each direction.

   If the verification fails, the users are notified, and a Verifica-
   tion_Failure message is sent, without adding or deleting any Security
   Associations.  On success, normal operation begins with the authenti-
   cation and/or encryption of user datagrams.








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   Implementation Notes:

      This is distinct from any authentication method specified for
      Security Associations.

      The Identity Verification data includes both the Size and Value
      fields.  The Attribute-Choices data includes the Type, 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 retrans-
   mission timer is necessary.

   These messages are not encrypted for party privacy protection.

   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.


7.1.  Bad_Cookie

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

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

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

   Type             10

   This error message is sent when a Value_Request, Identity_Request,
   SPI_Needed, or SPI_Update is received, and the receiver's Cookie is
   invalid or the associated Exchange-Value has expired.

   During the Photuris exchange, when this error message is received, it
   has no immediate effect on the operation of the protocol phases.
   When Retransmissions have been exceeded, if this error message has
   been received, the Initiator SHOULD begin the Photuris exchange again
   by sending a new Cookie_Request.

   After the Photuris exchange has completed, when this error message is



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   received in response to an SPI_Needed or SPI_Update, the party SHOULD
   initiate a Photuris exchange by sending a new Cookie_Request.

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


7.2.  Resource_Limit

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

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

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

   Type             11

   This error message is sent when a Cookie_Request or SPI_Update 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, the implementation SHOULD double the
   retransmission timeout for sending another Cookie_Request.

   After the Photuris exchange has completed, when this error message is
   received in response to an SPI_Update, the implementation SHOULD NOT
   send another SPI_Update until it has deleted an existing SPI, or
   waited for a cached SPI entry to expire.


7.3.  Verification_Failure

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

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

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

   Type             12

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

   When this error message is received, the implementation SHOULD log
   the occurance, and notify an operator as appropriate.  However,



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   receipt has no effect on the operation of the protocol.


7.4.  Message_Reject

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

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

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

   Type             13

   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.


8.  Public Value Exchanges

   Photuris is based in principle on public-key cryptography, specifi-
   cally 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 expo-
   nentiation.  An Exchange-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.





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      (g**xy) mod p

   The generator (g) and modulus (p) are established by the Scheme-
   Choice (see "Exchange Scheme List" 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 Num-
   bers.  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 pro-
   cess can periodically generate new moduli.

   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 (pes-
   simistic) through 98 (optimistic) bit-equivalents of cryptographic
   strength.

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


8.2.1.  Bootstrap Moduli

   Each implementation is likely to use a fixed modulus during its boot-
   strap, 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 strong prime 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.






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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 expo-
   nents 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 strong).

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

   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 expo-
   nent for each different modulus.  Then, a D-H Exchange-Value is cal-
   culated for the given modulus, generator, and exponent.

   The exponent 0 will generate the public value 1, and exponent 1 will
   generate the public value g mod p.  Other small exponents such that



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      (g**x) < p

   will be easily visible.  These exponents do not qualify as secret.

   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 sev-
   eral 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 pri-
   vate-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.

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

      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 pub-
      lic/private key-pair to be used in several closely spaced Photuris
      executions, when creating Security Associations with several dif-
      ferent nodes over a short period of time.

      Other precomputation suggestions are described in [BGMW93] and
      [Rooij94].











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9.  Exchange Scheme List

   Initial values are assigned as follows:

   (0)   Reserved.

   (1)   Reserved.

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

         The Privacy-Method is "not protected".

         The "SPI Messages" Validity-Method is "MD5-KDpKp".

   (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 sub-
         ject to duplication.  Other criteria, such as the IP Source and
         Destination of the Cookie_Request, are used to differentiate
         the particular Exchange-Schemes available.



10.  Validity Methods
10.1.  MD5-KDpKp

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

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

   where the key is the computed shared-secret.

   The leading key is not padded to any particular alignment.

   The datafill uses the same pad-with-length technique defined for
   md5fill.  The length includes the leading key and data.




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   The resulting Verification field is a 128-bit Variable Precision Num-
   ber (18 octets including Size).


11.  Attribute List

   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
      I       3* MD5-KDpKp Symmetric Verification
      X       5* MD5-KpDpKp Integrity
      E       8  DES-CBC
      X     255  Organizational

      A  AH-only Attribute-Choice
      E  ESP-only Attribute-Choice
      I  Identity-Choice
      X  dependent on list location
      *  feature must be supported (mandatory)

   Other attributes are specified in companion documents.


11.1.  Padding

   +-+-+-+-+-+-+-+-+
   |     Type      |
   +-+-+-+-+-+-+-+-+


   Type             0

   Each attribute may have value fields that are multiple octets.  To
   facilitate processing efficiency, these fields are aligned on inte-
   gral modulo 8 octet (64-bit) boundaries.

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

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







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11.2.  AH-Attributes

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type             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).


11.3.  ESP-Attributes

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type             2

   Length           0

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


11.4.  MD5-KDpKp Symmetric Verification

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type             3

   Length           0

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

   There is no required format or content for the Identification value.



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   The value may be a number or string of any kind.

   The authentication symmetric secret-key (as specified) is selected
   based on the contents of the Identification field.  All implementa-
   tions must support at least 62 octets.  The selected symmetric
   secret-key SHOULD provide at least 64-bits of cryptographic strength.

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

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

   where the key is the computed shared-secret.

   The leading key is not padded to any particular alignment.

   The datafill uses the same pad-with-length technique defined for
   md5fill.  The length includes the leading key and data.

   The resulting Verification field is a 128-bit Variable Precision Num-
   ber (18 octets including Size).

   For identity verification and session-key calculation, the authenti-
   cation symmetric secret-key is also used as the calculation secret-
   key.


11.5.  MD5-KpDpKp Integrity

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type             5

   Length           0

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

   MD5 [RFC-1321] is used as the SPI session-key generation function, as
   described in "Session-Key Computation".  The most significant
   496-bits (62 octets) of the generated hashes are used for the key.

   The remaining least significant 16-bits (2 octets) of the last hash
   are discarded.  When combined with other uses of key generation for



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   the same SPI, the next such attribute will begin with a new hash.

   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 attack described in
      [PO96].  This is also filled to the next 512-bit boundary, using
      the same pad-with-length technique defined for MD5.  The length
      includes the leading key and data.


11.6.  DES-CBC

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type             8

   Length           0

   May be selected as an ESP Attribute-Choice, pursuant to [RFC-1829] et
   sequitur.  The selected Exchange Scheme SHOULD provide at least
   56-bits of cryptographic strength.

   MD5 [RFC-1321] is used as the SPI session-key generation function, as
   described in "Session-Key Computation".  The most significant 64-bits
   (8 octets) of the generated hash are used for the key.  The least
   significant bit of each key octet is ignored (or set to parity when
   the implementation requires).

   If the key matches any of the weak, semi-weak or possibly weak keys
   [Schneier95, pages 280-282], that key is discarded; the next 64-bits
   of the generated hash are used instead, recursively.

   The remaining octets of the last hash are discarded.  When combined
   with other uses of key generation for the same SPI, the next such
   attribute will begin with a new hash.




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   Profile:

      When negotiated with Photuris, the transform differs slightly from
      [RFC-1829].

      The IV field is always 32-bits.

      The full 64-bit DES-CBC IV is generated from the 32-bit SPI field
      followed by (concatenated with) the 32-bit IV field.  The bit-wise
      complement of the 32-bit IV field is XOR'd with the first 32-bits
      (SPI field).

      The padding values begin with the value 0, and count up to the
      number of padding octets (zero relative).  For example, if the
      plaintext length is 41, the padding values are 0, 1, 2, 3, 4, and
      the following PadLength is 5.

      After decryption, if the padding octets are not the correct values
      for the PadLength, then the payload is discarded, and a "Decryp-
      tion Failed" error is indicated, as described in [RFC-xxxx].


11.7.  Organizational

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


   Type             255

   Length           >= 4

                    When the Length is four, no Value(s) field is pre-
                    sent.

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

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




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   Value(s)         Zero or more octets.  The details are implementation
                    specific.

   Some implementors might not need nor want to publish their propri-
   etary 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 rep-
   resent 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
   WP   = Want Privacy

   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

   TO+  = Timeout with counter > 0
   TO-  = Timeout with counter expired



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   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 imple-
   mented in any convenient order.  The state may be followed by a let-
   ter, which indicates an explanatory footnote.  The dash ('-') indi-
   cates 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
    WP   |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  |    -         2         2         4         4
    RRL  |    -       brto/1    brto/2      3         4
    RVF  |    -         -         -         -         -
         |
     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
    WP   |    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
         |
     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
   ------+-----------------------------
    WP   |    -         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
         |
    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 mes-
   sage has been received.


A.2.1.  Initial

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


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.




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   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 Life-
   Time 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 con-
   figuration, an implementation MUST allow configuration of an arbi-
   trary stream of octets.


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 partic-
   ular 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 administra-
   tion comprising a Virtual Private Network over the Internet.

   Nota Bene:
      The passwords used in these examples do not meet the "MD5-KDpKp
      Symmetric Verification" 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 Identi-
   fication field is "Tiny VPN 1995 November" and the SPI Owner secret-



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   key is "abracadabra".  The SPI User is considered unknown (despite
   the fact that only one possible user has been configured).  Thus, the
   SPI User Identification and SPI User secret-key are omitted from the
   Identity Verification calculation.

   When the Responder sends its Identity_Response, the SPI Owner Identi-
   fication 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, with completely identical configuration, the addition
   of the SPI User fields in the response calculation is highly likely
   to produce a different Verification value.  In turn, the different
   Verification values affect the calculation of SPI session-keys that
   are highly likely to be different in each direction.


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 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".  The SPI User is considered unknown
   (despite the fact that the mobile user has only a single configura-
   tion).  Thus, the SPI User Identification and SPI User secret-key are
   omitted from the Identity Verification calculation.

   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 ini-
   tial 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 cooperat-
   ing 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 Identi-
   fication field is "Apple" and the SPI Owner secret-key is "all for
   one".  The SPI User is unknown (many more destination parties could
   be configured).  Thus, the SPI User Identification and SPI User
   secret-key are omitted from the Identity Verification calculation.




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   When the Responder sends its Identity_Response, finding that the spe-
   cial 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 Identifi-
   cation is "Apple" (taken from the request), and the SPI User 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: 10 seconds.

   Exchange TimeOut (ETO)
      Default: 60 seconds.  Minimum: Retransmissions * IRTO.

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

   SPI LifeTime (SPILT)
      Default: 5 minutes.  Minimum: 2 * ELT.

   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 Tel-
   net.  Such considerations are outside the scope of this document.
















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Security Considerations

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

   Therefore, only minimal background cryptologic discussion and ratio-
   nale 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 subse-
   quent 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.


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 suggested the cookie exchange rate limitation
   counter.  Also, he developed the first complete independent implemen-
   tation (circa October 1995).

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

   Hilarie Orman suggested adding secret "nonces" to session-key genera-
   tion (now removed to a separate document), and provided extensive



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   review of the protocol details.

   Bart Preneel and Paul C van Oorschot in [PO96] suggested adding
   padding between the data and trailing key when hashing for authenti-
   cation.

   Niels Provos developed a third independent implementation (circa May
   1997), ported to AIX, Linux, OpenBSD, and Solaris.

   Bill Sommerfeld suggested using the Cookie values on successive
   exchanges to provide bi-directional user-oriented keying, and includ-
   ing the authentication secret-key in the session-key generation.

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

   International interoperability testing provided the impetus for many
   of the implementation notes herein.

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


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 Cryptogra-
            phy", IEEE Transactions on Information Theory, v IT-22 n 6
            pp 644-654, November 1976.

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

   [Firefly]
            "Photuris" is the latin name for the firefly.  "Firefly" is
            in turn the name for the USA National Security



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            Administration's (classified) key exchange protocol for the
            STU-III secure telephone.  Informed speculation has it that
            Firefly is based on very similar design principles.

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

   [PO96]   Bart Preneel, 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, August 1980.

   [RFC-1321]
            Rivest, R., "The MD5 Message-Digest Algorithm", RFC-1321,
            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-1825]
            Atkinson, R., "Security Architecture for the Internet Proto-
            col", Naval Research Laboratory, July 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 Trans-
            form", July 1995.

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

   [RFC-xxxx]
            Karn, P., and Simpson, W., "ICMP Security Failures Mes-
            sages", draft-ietf-ipsec-icmp-fail-01.txt, work in progress.




Simpson & Karn            expires in six months                [Page 68]


DRAFT                       Photuris Protocol                   May 1997


   [Rooij94]
            P. de Rooij, "Efficient exponentiation using precomputation
            and vector addition chains", Advances in Cryptology -- Euro-
            crypt '94, Lecture Notes in Computer 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@majordomo.soscorp.com 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)
          bsimpson@MorningStar.com














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