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rfc:rfc2522

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

                                                            DayDreamer
                                                            March 1999
             Photuris: Session-Key Management Protocol

Status of this Memo

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

Copyright Notice

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

Abstract

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

Karn & Simpson Experimental [Page i] RFC 2522 Photuris Protocol March 1999

Table of Contents

   1.     Introduction ..........................................    1
      1.1       Terminology .....................................    1
      1.2       Protocol Overview ...............................    3
      1.3       Security Parameters .............................    5
      1.4       LifeTimes .......................................    6
         1.4.1  Exchange LifeTimes ..............................    6
         1.4.2  SPI LifeTimes ...................................    7
      1.5       Random Number Generation ........................    8
   2.     Protocol Details ......................................    9
      2.1       UDP .............................................    9
      2.2       Header Format ...................................   10
      2.3       Variable Precision Integers .....................   11
      2.4       Exchange-Schemes ................................   13
      2.5       Attributes ......................................   13
   3.     Cookie Exchange .......................................   14
         3.0.1  Send Cookie_Request .............................   14
         3.0.2  Receive Cookie_Request ..........................   15
         3.0.3  Send Cookie_Response ............................   15
         3.0.4  Receive Cookie_Response .........................   16
      3.1       Cookie_Request ..................................   17
      3.2       Cookie_Response .................................   18
      3.3       Cookie Generation ...............................   19
         3.3.1  Initiator Cookie ................................   19
         3.3.2  Responder Cookie ................................   20
   4.     Value Exchange ........................................   21
         4.0.1  Send Value_Request ..............................   21
         4.0.2  Receive Value_Request ...........................   22
         4.0.3  Send Value_Response .............................   22
         4.0.4  Receive Value_Response ..........................   23
      4.1       Value_Request ...................................   24
      4.2       Value_Response ..................................   25
      4.3       Offered Attribute List ..........................   26
   5.     Identification Exchange ...............................   28
         5.0.1  Send Identity_Request ...........................   29
         5.0.2  Receive Identity_Request ........................   29
         5.0.3  Send Identity_Response ..........................   30
         5.0.4  Receive Identity_Response .......................   30
      5.1       Identity_Messages ...............................   31
      5.2       Attribute Choices List ..........................   33
      5.3       Shared-Secret ...................................   34
      5.4       Identity Verification ...........................   34

Karn & Simpson Experimental [Page ii] RFC 2522 Photuris Protocol March 1999

      5.5       Privacy-Key Computation .........................   36
      5.6       Session-Key Computation .........................   37
   6.     SPI Messages ..........................................   38
         6.0.1  Send SPI_Needed .................................   38
         6.0.2  Receive SPI_Needed ..............................   39
         6.0.3  Send SPI_Update .................................   39
         6.0.4  Receive SPI_Update ..............................   39
         6.0.5  Automated SPI_Updates ...........................   40
      6.1       SPI_Needed ......................................   41
      6.2       SPI_Update ......................................   43
         6.2.1  Creation ........................................   44
         6.2.2  Deletion ........................................   45
         6.2.3  Modification ....................................   45
      6.3       Validity Verification ...........................   45
   7.     Error Messages ........................................   46
      7.1       Bad_Cookie ......................................   47
      7.2       Resource_Limit ..................................   47
      7.3       Verification_Failure ............................   48
      7.4       Message_Reject ..................................   49
   8.     Public Value Exchanges ................................   50
      8.1       Modular Exponentiation Groups ...................   50
      8.2       Moduli Selection ................................   50
         8.2.1  Bootstrap Moduli ................................   51
         8.2.2  Learning Moduli .................................   51
      8.3       Generator Selection .............................   51
      8.4       Exponent Selection ..............................   52
      8.5       Defective Exchange Values .......................   53
   9.     Basic Exchange-Schemes ................................   54
   10.    Basic Key-Generation-Function .........................   55
      10.1      MD5 Hash ........................................   55
   11.    Basic Privacy-Method ..................................   55
      11.1      Simple Masking ..................................   55
   12.    Basic Validity-Method .................................   55
      12.1      MD5-IPMAC Check .................................   55
   13.    Basic Attributes ......................................   56
      13.1      Padding .........................................   56
      13.2      AH-Attributes ...................................   57
      13.3      ESP-Attributes ..................................   57
      13.4      MD5-IPMAC .......................................   58
         13.4.1 Symmetric Identification ........................   58

Karn & Simpson Experimental [Page iii] RFC 2522 Photuris Protocol March 1999

         13.4.2 Authentication ..................................   59
      13.5      Organizational ..................................   60
   APPENDICES ...................................................   61
   A.     Automaton .............................................   61
      A.1       State Transition Table ..........................   62
      A.2       States ..........................................   65
         A.2.1  Initial .........................................   65
         A.2.2  Cookie ..........................................   66
         A.2.3  Value ...........................................   66
         A.2.4  Identity ........................................   66
         A.2.5  Ready ...........................................   66
         A.2.6  Update ..........................................   66
   B.     Use of Identification and Secrets .....................   67
      B.1       Identification ..................................   67
      B.2       Group Identity With Group Secret ................   67
      B.3       Multiple Identities With Group Secrets ..........   68
      B.4       Multiple Identities With Multiple Secrets .......   69
   OPERATIONAL CONSIDERATIONS ...................................   70
   SECURITY CONSIDERATIONS ......................................   70
   HISTORY ......................................................   71
   ACKNOWLEDGEMENTS .............................................   72
   REFERENCES ...................................................   73
   CONTACTS .....................................................   75
   COPYRIGHT ....................................................   76

Karn & Simpson Experimental [Page iv] RFC 2522 Photuris Protocol March 1999

1. Introduction

 Photuris [Firefly] establishes short-lived session-keys between two
 parties, without passing the session-keys across the Internet.  These
 session-keys directly replace the long-lived secret-keys (such as
 passwords and passphrases) that have been historically configured for
 security purposes.
 The basic Photuris protocol utilizes these existing previously
 configured secret-keys for identification of the parties.  This is
 intended to speed deployment and reduce administrative configuration
 changes.
 This document is primarily intended for implementing the Photuris
 protocol.  It does not detail service and application interface
 definitions, although it does mention some basic policy areas
 required for the proper implementation and operation of the protocol
 mechanisms.
 Since the basic Photuris protocol is extensible, new data types and
 protocol behaviour should be expected.  The implementor is especially
 cautioned not to depend on values that appear in examples to be
 current or complete, since their purpose is primarily pedagogical.

1.1. Terminology

 In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
 "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
 described in [RFC-2119].
 byte             An 8-bit quantity; also known as "octet" in
                  standardese.
 exchange-value   The publically distributable value used to calculate
                  a shared-secret.  As used in this document, refers
                  to a Diffie-Hellman exchange, not the public part of
                  a public/private key-pair.
 private-key      A value that is kept secret, and is part of an
                  asymmetric public/private key-pair.
 public-key       A publically distributable value that is part of an
                  asymmetric public/private key-pair.
 secret-key       A symmetric key that is not publically
                  distributable.  As used in this document, this is
                  distinguished from an asymmetric public/private

Karn & Simpson Experimental [Page 1] RFC 2522 Photuris Protocol March 1999

                  key-pair.  An example is a user password.
 Security Association (SA)
                  A collection of parameters describing the security
                  relationship between two nodes.  These parameters
                  include the identities of the parties, the transform
                  (including algorithm and algorithm mode), the key(s)
                  (such as a session-key, secret-key, or appropriate
                  public/private key-pair), and possibly other
                  information such as sensitivity labelling.
 Security Parameters Index (SPI)
                  A number that indicates a particular set of uni-
                  directional attributes used under a Security
                  Association, such as transform(s) and session-
                  key(s).  The number is relative to the IP
                  Destination, which is the SPI Owner, and is unique
                  per IP (Next Header) Protocol.  That is, the same
                  value MAY be used by multiple protocols to
                  concurrently indicate different Security Association
                  parameters.
 session-key      A key that is independently derived from a shared-
                  secret by the parties, and used for keying one
                  direction of traffic.  This key is changed
                  frequently.
 shared-secret    As used in this document, the calculated result of
                  the Photuris exchange.
 SPI Owner        The party that corresponds to the IP Destination;
                  the intended recipient of a protected datagram.
 SPI User         The party that corresponds to the IP Source; the
                  sender of a protected datagram.
 transform        A cryptographic manipulation of a particular set of
                  data.  As used in this document, refers to certain
                  well-specified methods (defined elsewhere).  For
                  example, AH-MD5 [RFC-1828] transforms an IP datagram
                  into a cryptographic hash, and ESP-DES-CBC [RFC-
                  1829] transforms plaintext to ciphertext and back
                  again.

Karn & Simpson Experimental [Page 2] RFC 2522 Photuris Protocol March 1999

 Many of these terms are hierarchically related:
    Security Association (bi-directional)
     - one or more lists of Security Parameters (uni-directional)
      -- one or more Attributes
       --- may have a key
       --- may indicate a transform
 Implementors will find details of cryptographic hashing (such as
 MD5), encryption algorithms and modes (such as DES), digital
 signatures (such as DSS), and other algorithms in [Schneier95].

1.2. Protocol Overview

 The Photuris protocol consists of several simple phases:
 1. A "Cookie" Exchange guards against simple flooding attacks sent
    with bogus IP Sources or UDP Ports.  Each party passes a "cookie"
    to the other.
    In return, a list of supported Exchange-Schemes are offered by the
    Responder for calculating a shared-secret.
 2. A Value Exchange establishes a shared-secret between the parties.
    Each party passes an Exchange-Value to the other.  These values
    are used to calculate a shared-secret.  The Responder remains
    stateless until a shared-secret has been created.
    In addition, supported attributes are offered by each party for
    use in establishing new Security Parameters.
 3. An Identification Exchange identifies the parties to each other,
    and verifies the integrity of values sent in phases 1 and 2.
    In addition, the shared-secret provides a basis to generate
    separate session-keys in each direction, which are in turn used
    for conventional authentication or encryption.  Additional
    security attributes are also exchanged as needed.
    This exchange is masked for party privacy protection using a
    message privacy-key based on the shared-secret.  This protects the
    identities of the parties, hides the Security Parameter attribute
    values, and improves security for the exchange protocol and
    security transforms.
 4. Additional messages may be exchanged to periodically change the
    session-keys, and to establish new or revised Security Parameters.

Karn & Simpson Experimental [Page 3] RFC 2522 Photuris Protocol March 1999

    These exchanges are also masked for party privacy protection in
    the same fashion as above.
 The sequence of message types and their purposes are summarized in
 the diagram below.  The first three phases (cookie, exchange, and
 identification) must be carried out in their entirety before any
 Security Association can be used.
 Initiator                            Responder
 =========                            =========
 Cookie_Request                 ->
                                 <-   Cookie_Response
                                         offer schemes
 Value_Request                  ->
    pick scheme
    offer value
    offer attributes
                                 <-   Value_Response
                                         offer value
                                         offer attributes
           [generate shared-secret from exchanged values]
 Identity_Request               ->
    make SPI
    pick SPI attribute(s)
    identify self
    authenticate
    make privacy key(s)
    mask/encrypt message
                                 <-   Identity_Response
                                         make SPI
                                         pick SPI attribute(s)
                                         identify self
                                         authenticate
                                         make privacy key(s)
                                         mask/encrypt message
             [make SPI session-keys in each direction]

Karn & Simpson Experimental [Page 4] RFC 2522 Photuris Protocol March 1999

 SPI User                             SPI Owner
 ========                             =========
 SPI_Needed                     ->
    list SPI attribute(s)
    make validity key
    authenticate
    make privacy key(s)
    mask/encrypt message
                                 <-   SPI_Update
                                         make SPI
                                         pick SPI attribute(s)
                                         make SPI session-key(s)
                                         make validity key
                                         authenticate
                                         make privacy key(s)
                                         mask/encrypt message
 Either party may initiate an exchange at any time.  For example, the
 Initiator need not be a "caller" in a telephony link.
 The Initiator is responsible for recovering from all message losses
 by retransmission.

1.3. Security Parameters

 A Photuris exchange between two parties results in a pair of SPI
 values (one in each direction).  Each SPI is used in creating
 separate session-key(s) in each direction.
 The SPI is assigned by the entity controlling the IP Destination: the
 SPI Owner (receiver).  The parties use the combination of IP
 Destination, IP (Next Header) Protocol, and SPI to distinguish the
 correct Security Association.
 When both parties initiate Photuris exchanges concurrently, or one
 party initiates more than one Photuris exchange, the Initiator
 Cookies (and UDP Ports) keep the exchanges separate.  This results in
 more than one initial SPI for each Destination.
 To create multiple SPIs with different parameters, the parties may
 also send SPI_Updates.
 There is no requirement that all such outstanding SPIs be used.  The
 SPI User (sender) selects an appropriate SPI for each datagram
 transmission.

Karn & Simpson Experimental [Page 5] RFC 2522 Photuris Protocol March 1999

 Implementation Notes:
    The method used for SPI assignment is implementation dependent.
    The only requirement is that the SPI be unique for the IP
    Destination and IP (Next Header) Protocol.
    However, selection of a cryptographically random SPI value can
    help prevent attacks that depend on a predicatable sequence of
    values.  The implementor MUST NOT expect SPI values to have a
    particular order or range.

1.4. LifeTimes

 The Photuris exchange results in two kinds of state, each with
 separate LifeTimes.
 1) The Exchange LifeTime of the small amount of state associated with
    the Photuris exchange itself.  This state may be viewed as between
    Internet nodes.
 2) The SPI LifeTimes of the individual SPIs that are established.
    This state may be viewed as between users and nodes.
 The SPI LifeTimes may be shorter or longer than the Exchange
 LifeTime.  These LifeTimes are not required to be related to each
 other.
 When an Exchange-Value expires (or is replaced by a newer value), any
 unexpired derived SPIs are not affected.  This is important to allow
 traffic to continue without interruption during new Photuris
 exchanges.

1.4.1. Exchange LifeTimes

 All retained exchange state of both parties has an associated
 Exchange LifeTime (ELT), and is subject to periodic expiration.  This
 depends on the physical and logistical security of the machine, and
 is typically in the range of 10 minutes to one day (default 30
 minutes).
 In addition, during a Photuris exchange, an Exchange TimeOut (ETO)
 limits the wait for the exchange to complete.  This timeout includes
 the packet round trips, and the time for completing the
 Identification Exchange calculations.  The time is bounded by both
 the maximum amount of calculation delay expected for the processing
 power of an unknown peer, and the minimum user expectation for

Karn & Simpson Experimental [Page 6] RFC 2522 Photuris Protocol March 1999

 results (default 30 seconds).
 These Exchange LifeTimes and TimeOuts are implementation dependent
 and are not disclosed in any Photuris message.  The paranoid operator
 will have a fairly short Exchange LifeTime, but it MUST NOT be less
 than twice the ETO.
 To prevent synchronization between Photuris exchanges, the
 implementation SHOULD randomly vary each Exchange LifeTime within
 twice the range of seconds that are required to calculate a new
 Exchange-Value.  For example, when the Responder uses a base ELT of
 30 minutes, and takes 10 seconds to calculate the new Exchange-Value,
 the equation might be (in milliseconds):
    1790000 + urandom(20000)
 The Exchange-Scheme, Exchange-Values, and resulting shared-secret MAY
 be cached in short-term storage for the Exchange LifeTime.  When
 repetitive Photuris exchanges occur between the same parties, and the
 Exchange-Values are discovered to be unchanged, the previously
 calculated shared-secret can be used to rapidly generate new
 session-keys.

1.4.2. SPI LifeTimes

 Each SPI has an associated LifeTime, specified by the SPI owner
 (receiver).  This SPI LifeTime (SPILT) is usually related to the
 speed of the link (typically 2 to 30 minutes), but it MUST NOT be
 less than thrice the ETO.
 The SPI can also be deleted by the SPI Owner using the SPI_Update.
 Once the SPI has expired or been deleted, the parties cease using the
 SPI.
 To prevent synchronization between multiple Photuris exchanges, the
 implementation SHOULD randomly vary each SPI LifeTime.  For example,
 when the Responder uses a base SPILT of 5 minutes, and 30 seconds for
 the ETO, the equation might be (in milliseconds):
    285000 + urandom(30000)
 There is no requirement that a long LifeTime be accepted by the SPI
 User.  The SPI User might never use an established SPI, or cease
 using the SPI at any time.
 When more than one unexpired SPI is available to the SPI User for the
 same function, a common implementation technique is to select the SPI

Karn & Simpson Experimental [Page 7] RFC 2522 Photuris Protocol March 1999

 with the greatest remaining LifeTime.  However, selecting randomly
 among a large number of SPIs might provide some defense against
 traffic analysis.
 To prevent resurrection of deleted or expired SPIs, SPI Owners SHOULD
 remember those SPIs, but mark them as unusable until the Photuris
 exchange shared-secret used to create them also expires and purges
 the associated state.
 When the SPI Owner detects an incoming SPI that has recently expired,
 but the associated exchange state has not yet been purged, the
 implementation MAY accept the SPI.  The length of time allowed is
 highly dependent on clock drift and variable packet round trip time,
 and is therefore implementation dependent.

1.5. Random Number Generation

 The security of Photuris critically depends on the quality of the
 secret random numbers generated by each party.  A poor random number
 generator at either party will compromise the shared-secret produced
 by the algorithm.
 Generating cryptographic quality random numbers on a general purpose
 computer without hardware assistance is a very tricky problem.  In
 general, this requires using a cryptographic hashing function to
 "distill" the entropy from a large number of semi-random external
 events, such as the timing of key strokes.  An excellent discussion
 can be found in [RFC-1750].

Karn & Simpson Experimental [Page 8] RFC 2522 Photuris Protocol March 1999

2. Protocol Details

 The Initiator begins a Photuris exchange under several circumstances:
  1. The Initiator has a datagram that it wishes to send with

confidentiality, and has no current Photuris exchange state with

    the IP Destination.  This datagram is discarded, and a
    Cookie_Request is sent instead.
  1. 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.
  1. The Initiator has received the ICMP message [RFC-2521] Security

Failures: Bad SPI (Type 40, Code 0), that matches current Photuris

    exchange state with the ICMP Source.
  1. The Initiator has received the ICMP message [RFC-2521] Security

Failures: Need Authentication (Type 40, Code 4), and has no

    current Photuris exchange state with the ICMP Source.
  1. The Initiator has received the ICMP message [RFC-2521] Security

Failures: Need Authorization (Type 40, Code 5), that matches

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

2.1. UDP

 All Photuris messages use the User Datagram Protocol header [RFC-
 768].  The Initiator sends to UDP Destination Port 468.
 When replying to the Initiator, the Responder swaps the IP Source and
 Destination, and the UDP Source and Destination Ports.
 The UDP checksum MUST be correctly calculated when sent.  When a
 message is received with an incorrect UDP checksum, it is silently
 discarded.

Karn & Simpson Experimental [Page 9] RFC 2522 Photuris Protocol March 1999

 Implementation Notes:
    It is expected that installation of Photuris will ensure that UDP
    checksum calculations are enabled for the computer operating
    system and later disabling by operators is prevented.
    Internet Protocol version 4 [RFC-791] restricts the maximum
    reassembled datagram to 576 bytes.
    When processing datagrams containing variable size values, the
    length must be checked against the overall datagram length.  An
    invalid size (too long or short) that causes a poorly coded
    receiver to abort could be used as a denial of service attack.

2.2. Header Format

 All of the messages have a format similar to the following, as
 transmitted left to right in network order (most significant to least
 significant):
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Initiator-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Responder-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Message    |
 +-+-+-+-+-+-+-+-+
 Initiator-Cookie  16 bytes.
 Responder-Cookie  16 bytes.
 Message          1 byte.  Each message type has a unique value.
                  Initial values are assigned as follows:

Karn & Simpson Experimental [Page 10] RFC 2522 Photuris Protocol March 1999

                      0  Cookie_Request
                      1  Cookie_Response
                      2  Value_Request
                      3  Value_Response
                      4  Identity_Request
                      5  Secret_Response (optional)
                      6  Secret_Request (optional)
                      7  Identity_Response
                      8  SPI_Needed
                      9  SPI_Update
                     10  Bad_Cookie
                     11  Resource_Limit
                     12  Verification_Failure
                     13  Message_Reject
 Further details and differences are elaborated in the individual
 messages.

2.3. Variable Precision Integers

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Size              |             Value ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Size             2, 4, or 8 bytes.  The number of significant bits
                  used in the Value field.  Always transmitted most
                  significant byte first.
                  When the Size is zero, no Value field is present;
                  there are no significant bits.  This means "missing"
                  or "null".  It should not be confused with the value
                  zero, which includes an indication of the number of
                  significant bits.
                  When the most significant byte is in the range 0
                  through 254 (0xfe), the field is 2 bytes.  Both
                  bytes are used to indicate the size of the Value
                  field, which ranges from 1 to 65,279 significant
                  bits (in 1 to 8,160 bytes).
                  When the most significant byte is 255 (0xff), the
                  field is 4 bytes.  The remaining 3 bytes are added
                  to 65,280 to indicate the size of the Value field,
                  which is limited to 16,776,959 significant bits (in

Karn & Simpson Experimental [Page 11] RFC 2522 Photuris Protocol March 1999

                  2,097,120 bytes).
                  When the most significant 2 bytes are 65,535
                  (0xffff), the field is 8 bytes.  The remaining 6
                  bytes are added to 16,776,960 to indicate the size
                  of the Value field.
 Value            0 or more bytes.  Always transmitted most
                  significant byte first.
                  The bits used are right justified within byte
                  boundaries; that is, any unused bits are in the most
                  significant byte.  When there are no unused bits, or
                  unused bits are zero filled, the value is assumed to
                  be an unsigned positive integer.
                  When the leading unused bits are ones filled, the
                  number is assumed to be a two's-complement negative
                  integer.  A negative integer will always have at
                  least one unused leading sign bit in the most
                  significant byte.
 Shortened forms SHOULD NOT be used when the Value includes a number
 of leading zero significant bits.  The Size SHOULD indicate the
 correct number of significant bits.
 Implementation Notes:
    Negative integers are not required to be supported, but are
    included for completeness.
    No more than 65,279 significant bits are required to be supported.
    Other ranges are vastly too long for these UDP messages, but are
    included for completeness.

Karn & Simpson Experimental [Page 12] RFC 2522 Photuris Protocol March 1999

2.4. Exchange-Schemes

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Scheme             |             Size              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Value ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Scheme           2 bytes.  A unique value indicating the Exchange-
                  Scheme.  See the "Basic Exchange-Schemes" for
                  details.
 Size             2 bytes, ranging from 0 to 65,279.  See "Variable
                  Precision Integer".
 Value            0 or more bytes.  See "Variable Precision Integer".
 The Size MUST NOT be assumed to be constant for a particular Scheme.
 Multiple kinds of the same Scheme with varying Sizes MAY be present
 in any list of schemes.
 However, only one of each Scheme and Size combination will be present
 in any list of schemes.

2.5. Attributes

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Attribute   |    Length     |  Value(s) ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Attribute        1 byte.  A unique value indicating the kind of
                  attribute.  See the "Basic Attributes" for details.
                  When the value is zero (padding), no Length field is
                  present (always zero).
 Length           1 byte.  The size of the Value(s) field in bytes.
                  When the Length is zero, no Value(s) field is
                  present.
 Value(s)         0 or more bytes.  See the "Basic Attributes" for
                  details.
 The Length MUST NOT be assumed to be constant for a particular

Karn & Simpson Experimental [Page 13] RFC 2522 Photuris Protocol March 1999

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

3. Cookie Exchange

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

3.0.1. Send Cookie_Request

 The Initiator initializes local state, and generates a unique
 "cookie".  The Initiator-Cookie MUST be different in each new
 Cookie_Request between the same parties.  See "Cookie Generation" for
 details.
  1. If any previous exchange between the peer IP nodes has not expired

in which this party was the Initiator, this Responder-Cookie is

    set to the most recent Responder-Cookie, and this Counter is set
    to the corresponding Counter.
    For example, a new Virtual Private Network (VPN) tunnel is about
    to be established to an existing partner.  The Counter is the same
    value received in the prior Cookie_Response, the Responder-Cookie
    remains the same, and a new Initiator-Cookie is generated.
  1. If the new Cookie_Request is in response to a message of a

previous exchange in which this party was the Responder, this

    Responder-Cookie is set to the previous Initiator-Cookie, and this
    Counter is set to zero.
    For example, a Bad_Cookie message was received from the previous
    Initiator in response to SPI_Needed.  The Responder-Cookie is
    replaced with the Initiator-Cookie, and a new Initiator-Cookie is
    generated.  This provides bookkeeping to detect bogus Bad_Cookie
    messages.
    Also, can be used for bi-directional User, Transport, and Process
    oriented keying.  Such mechanisms are outside the scope of this
    document.
  1. Otherwise, this Responder-Cookie and Counter are both set to zero.

Karn & Simpson Experimental [Page 14] RFC 2522 Photuris Protocol March 1999

    By default, the Initiator operates in the same manner as when all
    of its previous exchange state has expired.  The Responder will
    send a Resource_Limit when its own exchange state has not expired.
 The Initiator also starts a retransmission timer.  If no valid
 Cookie_Response arrives within the time limit, the same
 Cookie_Request is retransmitted for the remaining number of
 Retransmissions.  The Initiator-Cookie value MUST be the same in each
 such retransmission to the same IP Destination and UDP Port.
 When Retransmissions have been exceeded, if a Resource_Limit message
 has been received during the exchange, the Initiator SHOULD begin the
 Photuris exchange again by sending a new Cookie_Request with updated
 values.

3.0.2. Receive Cookie_Request

 On receipt of a Cookie_Request, the Responder determines whether
 there are sufficient resources to begin another Photuris exchange.
  1. 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.
  1. When any previous exchange initiated by this particular peer has

not exceeded the Exchange TimeOut, and the Responder-Cookie does

    not specify one of these previous exchanges, a Resource_Limit
    message is sent.
 Otherwise, the Responder returns a Cookie_Response.
 Note that the Responder creates no additional state at this time.

3.0.3. Send Cookie_Response

 The IP Source for the Initiator is examined.  If any previous
 exchange between the peer IP nodes has not expired, the response
 Counter is set to the most recent exchange Counter plus one (allowing
 for out of order retransmissions).  Otherwise, the response Counter
 is set to the request Counter plus one.
 If (through rollover of the Counter) the new Counter value is zero
 (modulo 256), the value is set to one.
 If this new Counter value matches some previous exchange initiated by
 this particular peer that has not yet exceeded the Exchange TimeOut,

Karn & Simpson Experimental [Page 15] RFC 2522 Photuris Protocol March 1999

 the Counter is incremented again, until a unique Counter value is
 reached.
 Nota Bene:
    No more than 254 concurrent exchanges between the same two peers
    are supported.
 The Responder generates a unique cookie.  The Responder-Cookie value
 in each successive response SHOULD be different.  See "Cookie
 Generation" for details.
 The Exchange-Schemes available between the peers are listed in the
 Offered-Schemes.

3.0.4. Receive Cookie_Response

 The Initiator validates the Initiator-Cookie, and the Offered-
 Schemes.
  1. When an invalid/expired Initiator-Cookie is detected, the message

is silently discarded.

  1. When the variable length Offered-Schemes do not match the UDP

Length, or all Offered-Schemes are obviously defective and/or

    insufficient for the purposes intended, the message is silently
    discarded; the implementation SHOULD log the occurance, and notify
    an operator as appropriate.
  1. Once a valid message has been received, later Cookie_Responses

with matching Initiator-Cookies are also silently discarded, until

    a new Cookie_Request is sent.
 When the message is valid, an Exchange-Scheme is chosen from the list
 of Offered-Schemes.
 This Scheme-Choice may affect the next Photuris message sent.  By
 default, the next Photuris message is a Value_Request.
 Implementation Notes:
    Only the Initiator-Cookie is used to identify the exchange.  The
    Counter and Responder-Cookie will both be different from the
    Cookie_Request.
    Various proposals for extensions utilize the Scheme-Choice to
    indicate a different message sequence.  Such mechanisms are
    outside the scope of this document.

Karn & Simpson Experimental [Page 16] RFC 2522 Photuris Protocol March 1999

3.1. Cookie_Request

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Initiator-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Responder-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Message    |    Counter    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Initiator-Cookie  16 bytes.  A randomized value that identifies the
                  exchange.  The value MUST NOT be zero.  See "Cookie
                  Generation" for details.
 Responder-Cookie  16 bytes.  Identifies a specific previous exchange.
                  Copied from a previous Cookie_Response.
                  When zero, no previous exchange is specified.
                  When non-zero, and the Counter is zero, contains the
                  Initiator-Cookie of a previous exchange.  The
                  specified party is requested to be the Responder in
                  this exchange, to retain previous party pairings.
                  When non-zero, and the Counter is also non-zero,
                  contains the Responder-Cookie of a previous
                  exchange.  The specified party is requested to be
                  the Responder in this exchange, to retain previous
                  party pairings.
 Message          0
 Counter          1 byte.  Indicates the number of previous exchanges.
                  When zero, the Responder-Cookie indicates the
                  Initiator of a previous exchange, or no previous
                  exchange is specified.
                  When non-zero, the Responder-Cookie indicates the
                  Responder to a previous exchange.  This value is set
                  to the Counter from the corresponding
                  Cookie_Response or from a Resource_Limit.

Karn & Simpson Experimental [Page 17] RFC 2522 Photuris Protocol March 1999

3.2. Cookie_Response

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Initiator-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Responder-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Message    |    Counter    |  Offered-Schemes ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Initiator-Cookie  16 bytes.  Copied from the Cookie_Request.
 Responder-Cookie  16 bytes.  A randomized value that identifies the
                  exchange.  The value MUST NOT be zero.  See "Cookie
                  Generation" for details.
 Message          1
 Counter          1 byte.  Indicates the number of the current
                  exchange.  Must be greater than zero.
 Offered-Schemes  4 or more bytes.  A list of one or more Exchange-
                  Schemes supported by the Responder, ordered from
                  most to least preferable.  See the "Basic Exchange-
                  Schemes" for details.
                  Only one Scheme (#2) is required to be supported,
                  and SHOULD be present in every Offered-Schemes list.
                  More than one of each kind of Scheme may be offered,
                  but each is distinguished by its Size.  The end of
                  the list is indicated by the UDP Length.

Karn & Simpson Experimental [Page 18] RFC 2522 Photuris Protocol March 1999

3.3. Cookie Generation

 The exact technique by which a Photuris party generates a cookie is
 implementation dependent.  The method chosen must satisfy some basic
 requirements:
 1. The cookie MUST depend on the specific parties.  This prevents an
    attacker from obtaining a cookie using a real IP address and UDP
    port, and then using it to swamp the victim with requests from
    randomly chosen IP addresses or ports.
 2. It MUST NOT be possible for anyone other than the issuing entity
    to generate cookies that will be accepted by that entity.  This
    implies that the issuing entity will use local secret information
    in the generation and subsequent verification of a cookie.  It
    must not be possible to deduce this secret information from any
    particular cookie.
 3. The cookie generation and verification methods MUST be fast to
    thwart attacks intended to sabotage CPU resources.
 A recommended technique is to use a cryptographic hashing function
 (such as MD5).
 An incoming cookie can be verified at any time by regenerating it
 locally from values contained in the incoming datagram and the local
 secret random value.

3.3.1. Initiator Cookie

 The Initiator secret value that affects its cookie SHOULD change for
 each new Photuris exchange, and is thereafter internally cached on a
 per Responder basis.  This provides improved synchronization and
 protection against replay attacks.
 An alternative is to cache the cookie instead of the secret value.
 Incoming cookies can be compared directly without the computational
 cost of regeneration.
 It is recommended that the cookie be calculated over the secret
 value, the IP Source and Destination addresses, and the UDP Source
 and Destination ports.

Karn & Simpson Experimental [Page 19] RFC 2522 Photuris Protocol March 1999

 Implementation Notes:
    Although the recommendation includes the UDP Source port, this is
    very implementation specific.  For example, it might not be
    included when the value is constant.
    However, it is important that the implementation protect mutually
    suspicious users of the same machine from generating the same
    cookie.

3.3.2. Responder Cookie

 The Responder secret value that affects its cookies MAY remain the
 same for many different Initiators.  However, this secret SHOULD be
 changed periodically to limit the time for use of its cookies
 (typically each 60 seconds).
 The Responder-Cookie SHOULD include the Initiator-Cookie.  The
 Responder-Cookie MUST include the Counter (that is returned in the
 Cookie_Response).  This provides improved synchronization and
 protection against replay attacks.
 It is recommended that the cookie be calculated over the secret
 value, the IP Source and Destination addresses, its own UDP
 Destination port, the Counter, the Initiator-Cookie, and the
 currently Offered-Schemes.
 The cookie is not cached per Initiator to avoid saving state during
 the initial Cookie Exchange.  On receipt of a Value_Request
 (described later), the Responder regenerates its cookie for
 validation.
 Once the Value_Response is sent (also described later), both
 Initiator and Responder cookies are cached to identify the exchange.
 Implementation Notes:
    Although the recommendation does not include the UDP Source port,
    this is very implementation specific.  It might be successfully
    included in some variants.
    However, it is important that the UDP Source port not be included
    when matching existing Photuris exchanges for determining the
    appropriate Counter.
    The recommendation includes the Offered-Schemes to detect a
    dynamic change of scheme value between the Cookie_Response and

Karn & Simpson Experimental [Page 20] RFC 2522 Photuris Protocol March 1999

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

4. Value Exchange

 Initiator                            Responder
 =========                            =========
 Value_Request                  ->
    pick scheme
    offer value
    offer attributes
                                 <-   Value_Response
                                         offer value
                                         offer attributes
           [generate shared-secret from exchanged values]

4.0.1. Send Value_Request

 The Initiator generates an appropriate Exchange-Value for the
 Scheme-Choice.  This Exchange-Value may be pre-calculated and used
 for multiple Responders.
 The IP Destination for the Responder is examined, and the attributes
 available between the parties are listed in the Offered-Attributes.
 The Initiator also starts a retransmission timer.  If no valid
 Value_Response arrives within the time limit, the same Value_Request
 is retransmitted for the remaining number of Retransmissions.
 When Retransmissions have been exceeded, if a Bad_Cookie or
 Resource_Limit message has been received during the exchange, the
 Initiator SHOULD begin the Photuris exchange again by sending a new
 Cookie_Request.

Karn & Simpson Experimental [Page 21] RFC 2522 Photuris Protocol March 1999

4.0.2. Receive Value_Request

 The Responder validates the Responder-Cookie, the Counter, the
 Scheme-Choice, the Exchange-Value, and the Offered-Attributes.
  1. When an invalid/expired Responder-Cookie is detected, a Bad_Cookie

message is sent.

  1. 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.
  1. When an invalid Scheme-Choice is detected, or the Exchange-Value

is obviously defective, or the variable length Offered-Attributes

    do not match the UDP Length, the message is silently discarded;
    the implementation SHOULD log the occurance, and notify an
    operator as appropriate.
 When the message is valid, the Responder sets its Exchange timer to
 the Exchange TimeOut, and returns a Value_Response.
 The Responder keeps a copy of the incoming Value_Request cookie pair,
 and its Value_Response.  If a duplicate Value_Request is received, it
 merely resends its previous Value_Response, and takes no further
 action.

4.0.3. Send Value_Response

 The Responder generates an appropriate Exchange-Value for the
 Scheme-Choice.  This Exchange-Value may be pre-calculated and used
 for multiple Initiators.
 The IP Source for the Initiator is examined, and the attributes
 available between the parties are listed in the Offered-Attributes.
 Implementation Notes:
    At this time, the Responder begins calculation of the shared-
    secret.  Calculation of the shared-secret is executed in parallel
    to minimize delay.
    This may take a substantial amount of time.  The implementor
    should ensure that retransmission is not blocked by this
    calculation.  This is not usually a problem, as retransmission
    timeouts typically exceed calculation time.

Karn & Simpson Experimental [Page 22] RFC 2522 Photuris Protocol March 1999

4.0.4. Receive Value_Response

 The Initiator validates the pair of Cookies, the Exchange-Value, and
 the Offered-Attributes.
  1. When an invalid/expired cookie is detected, the message is

silently discarded.

  1. When the Exchange-Value is obviously defective, or the variable

length Offered-Attributes do not match the UDP Length, the message

    is silently discarded; the implementation SHOULD log the
    occurance, and notify an operator as appropriate.
  1. Once a valid message has been received, later Value_Responses with

both matching cookies are also silently discarded, until a new

    Cookie_Request is sent.
 When the message is valid, the Initiator begins its parallel
 computation of the shared-secret.
 When the Initiator completes computation, it sends an
 Identity_Request to the Responder.

Karn & Simpson Experimental [Page 23] RFC 2522 Photuris Protocol March 1999

4.1. Value_Request

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Initiator-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Responder-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Message    |    Counter    |         Scheme-Choice         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                   Initiator-Exchange-Value                    ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Initiator-Offered-Attributes ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Initiator-Cookie  16 bytes.  Copied from the Cookie_Response.
 Responder-Cookie  16 bytes.  Copied from the Cookie_Response.
 Message          2
 Counter          1 byte.  Copied from the Cookie_Response.
 Scheme-Choice    2 bytes.  A value selected by the Initiator from the
                  list of Offered-Schemes in the Cookie_Response.
                  Only the Scheme is specified; the Size will match
                  the Initiator-Exchange-Value, and the Value(s) are
                  implicit.
 Initiator-Exchange-Value
                  Variable Precision Integer.  Provided by the
                  Initiator for calculating a shared-secret between
                  the parties.  The Value format is indicated by the
                  Scheme-Choice.
                  The field may be any integral number of bytes in
                  length, as indicated by its Size field.  It does not
                  require any particular alignment.  The 32-bit
                  alignment shown is for convenience in the
                  illustration.

Karn & Simpson Experimental [Page 24] RFC 2522 Photuris Protocol March 1999

 Initiator-Offered-Attributes
                  4 or more bytes.  A list of Security Parameter
                  attributes supported by the Initiator.
                  The contents and usage of this list are further
                  described in "Offered Attributes List".  The end of
                  the list is indicated by the UDP Length.

4.2. Value_Response

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Initiator-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Responder-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Message    |                    Reserved                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                   Responder-Exchange-Value                    ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Responder-Offered-Attributes ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Initiator-Cookie  16 bytes.  Copied from the Value_Request.
 Responder-Cookie  16 bytes.  Copied from the Value_Request.
 Message          3
 Reserved         3 bytes.  For future use; MUST be set to zero when
                  transmitted, and MUST be ignored when received.
 Responder-Exchange-Value
                  Variable Precision Integer.  Provided by the
                  Responder for calculating a shared-secret between
                  the parties.  The Value format is indicated by the
                  current Scheme-Choice specified in the
                  Value_Request.
                  The field may be any integral number of bytes in

Karn & Simpson Experimental [Page 25] RFC 2522 Photuris Protocol March 1999

                  length, as indicated by its Size field.  It does not
                  require any particular alignment.  The 32-bit
                  alignment shown is for convenience in the
                  illustration.
 Responder-Offered-Attributes
                  4 or more bytes.  A list of Security Parameter
                  attributes supported by the Responder.
                  The contents and usage of this list are further
                  described in "Offered Attributes List".  The end of
                  the list is indicated by the UDP Length.

4.3. Offered Attribute List

 This list includes those attributes supported by the party that are
 available to the other party.  The attribute formats are specified in
 the "Basic Attributes".
 The list is composed of two or three sections: Identification-
 Attributes, Authentication-Attributes, and (optional) Encapsulation-
 Attributes.  Within each section, the attributes are ordered from
 most to least preferable.
 The first section of the list includes methods of identification.  An
 Identity-Choice is selected from this list.
 The second section of the list begins with "AH-Attributes" (#1).  It
 includes methods of authentication, and other operational types.
 The third section of the list begins with "ESP-Attributes" (#2).  It
 includes methods of authentication, compression, encryption, and
 other operational types.  When no Encapsulation-Attributes are
 offered, the "ESP-Attributes" attribute itself is omitted from the
 list.
 Attribute-Choices are selected from the latter two sections of the
 list.
 Support is required for the "MD5-IPMAC" (#5) attribute for both
 "Symmetric Identification" and "Authentication" and they SHOULD be
 present in every Offered-Attributes list.

Karn & Simpson Experimental [Page 26] RFC 2522 Photuris Protocol March 1999

 Implementation Notes:
    For example,
       "MD5-IPMAC" (Symmetric Identification),
       "AH-Attributes",
       "MD5-IPMAC" (Authentication).
    Since the offer is made by the prospective SPI User (sender),
    order of preference likely reflects the capabilities and
    engineering tradeoffs of a particular implementation.
    However, the critical processing bottlenecks are frequently in the
    receiver.  The SPI Owner (receiver) may express its needs by
    choosing a less preferable attribute.
    The order may also be affected by operational policy and requested
    services for an application.  Such considerations are outside the
    scope of this document.
    The list may be divided into additional sections.  These sections
    will always follow the ESP-Attributes section, and are
    indistinguishable from unrecognized attributes.
    The authentication, compression, encryption and identification
    mechanisms chosen, as well as the encapsulation modes (if any),
    need not be the same in both directions.

Karn & Simpson Experimental [Page 27] RFC 2522 Photuris Protocol March 1999

5. Identification Exchange

 Initiator                            Responder
 =========                            =========
 Identity_Request               ->
    make SPI
    pick SPI attribute(s)
    identify self
    authenticate
    make privacy key(s)
    mask/encrypt message
                                 <-   Identity_Response
                                         make SPI
                                         pick SPI attribute(s)
                                         identify self
                                         authenticate
                                         make privacy key(s)
                                         mask/encrypt message
             [make SPI session-keys in each direction]
 The exchange of messages is ordered, although the formats and
 meanings of the messages are identical in each direction.  The
 messages are easily distinguished by the parties themselves, by
 examining the Message and Identification fields.
 Implementation Notes:
    The amount of time for the calculation may be dependent on the
    value of particular bits in secret values used in generating the
    shared-secret or identity verification.  To prevent analysis of
    these secret bits by recording the time for calculation, sending
    of the Identity_Messages SHOULD be delayed until the time expected
    for the longest calculation.  This will be different for different
    processor speeds, different algorithms, and different length
    variables.  Therefore, the method for estimating time is
    implementation dependent.
    Any authenticated and/or encrypted user datagrams received before
    the completion of identity verification can be placed on a queue
    pending completion of this step.  If verification succeeds, the
    queue is processed as though the datagrams had arrived subsequent
    to the verification.  If verification fails, the queue is
    discarded.

Karn & Simpson Experimental [Page 28] RFC 2522 Photuris Protocol March 1999

5.0.1. Send Identity_Request

 The Initiator chooses an appropriate Identification, the SPI and
 SPILT, a set of Attributes for the SPI, calculates the Verification,
 and masks the message using the Privacy-Method indicated by the
 current Scheme-Choice.
 The Initiator also starts a retransmission timer.  If no valid
 Identity_Response arrives within the time limit, its previous
 Identity_Request is retransmitted for the remaining number of
 Retransmissions.
 When Retransmissions have been exceeded, if a Bad_Cookie message has
 been received during the exchange, the Initiator SHOULD begin the
 Photuris exchange again by sending a new Cookie_Request.

5.0.2. Receive Identity_Request

 The Responder validates the pair of Cookies, the Padding, the
 Identification, the Verification, and the Attribute-Choices.
  1. When an invalid/expired cookie is detected, a Bad_Cookie message

is sent.

  1. After unmasking, when invalid Padding is detected, the variable

length Attribute-Choices do not match the UDP Length, or an

    attribute was not in the Offered-Attributes, the message is
    silently discarded.
  1. When an invalid Identification is detected, or the message

verification fails, a Verification_Failure message is sent.

  1. Whenever such a problem is detected, the Security Association is

not established; the implementation SHOULD log the occurance, and

    notify an operator as appropriate.
 When the message is valid, the Responder sets its Exchange timer to
 the Exchange LifeTime (if this has not already been done for a
 previous exchange).  When its parallel computation of the shared-
 secret is complete, the Responder returns an Identity_Response.
 The Responder keeps a copy of the incoming Identity_Request values,
 and its Identity_Response.  If a duplicate Identity_Request is
 received, it merely resends its previous Identity_Response, and takes
 no further action.

Karn & Simpson Experimental [Page 29] RFC 2522 Photuris Protocol March 1999

5.0.3. Send Identity_Response

 The Responder chooses an appropriate Identification, the SPI and
 SPILT, a set of Attributes for the SPI, calculates the Verification,
 and masks the message using the Privacy-Method indicated by the
 current Scheme-Choice.
 The Responder calculates the SPI session-keys in both directions.
 At this time, the Responder begins the authentication and/or
 encryption of user datagrams.

5.0.4. Receive Identity_Response

 The Initiator validates the pair of Cookies, the Padding, the
 Identification, the Verification, and the Attribute-Choices.
  1. When an invalid/expired cookie is detected, the message is

silently discarded.

  1. After unmasking, when invalid Padding is detected, the variable

length Attribute-Choices do not match the UDP Length, or an

    attribute was not in the Offered-Attributes, the message is
    silently discarded.
  1. When an invalid Identification is detected, or the message

verification fails, a Verification_Failure message is sent.

  1. Whenever such a problem is detected, the Security Association is

not established; the implementation SHOULD log the occurance, and

    notify an operator as appropriate.
  1. Once a valid message has been received, later Identity_Responses

with both matching cookies are also silently discarded, until a

    new Cookie_Request is sent.
 When the message is valid, the Initiator sets its Exchange timer to
 the Exchange LifeTime (if this has not already been done for a
 previous exchange).
 The Initiator calculates the SPI session-keys in both directions.
 At this time, the Initiator begins the authentication and/or
 encryption of user datagrams.

Karn & Simpson Experimental [Page 30] RFC 2522 Photuris Protocol March 1999

5.1. Identity_Messages

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Initiator-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Responder-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Message    |                    LifeTime                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   Security-Parameters-Index                   |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |        Identity-Choice        |                               |
 + + + + + + + + + + + + + + + + +                               +
 |                                                               |
 ~                        Identification                         ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                         Verification                          ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Attribute-Choices ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                    ... Padding  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Initiator-Cookie  16 bytes.  Copied from the Value_Request.
 Responder-Cookie  16 bytes.  Copied from the Value_Request.
 Message          4 (Request) or 7 (Response)
 LifeTime         3 bytes.  The number of seconds remaining before the
                  indicated SPI expires.
                  When the SPI is zero, this field MUST be filled with
                  a random non-zero value.
 Security-Parameters-Index (SPI)
                  4 bytes.  The SPI to be used for incoming
                  communications.
                  When zero, indicates that no SPI is created in this

Karn & Simpson Experimental [Page 31] RFC 2522 Photuris Protocol March 1999

                  direction.
 Identity-Choice  2 or more bytes.  An identity attribute is selected
                  from the list of Offered-Attributes sent by the
                  peer, and is used to calculate the Verification.
                  The field may be any integral number of bytes in
                  length, as indicated by its Length field.  It does
                  not require any particular alignment.  The 16-bit
                  alignment shown is for convenience in the
                  illustration.
 Identification   Variable Precision Integer, or alternative format
                  indicated by the Identity-Choice.  See the "Basic
                  Attributes" for details.
                  The field may be any integral number of bytes in
                  length.  It does not require any particular
                  alignment.  The 32-bit alignment shown is for
                  convenience in the illustration.
 Verification     Variable Precision Integer, or alternative format
                  indicated by the Identity-Choice.  The calculation
                  of the value is described in "Identity
                  Verification".
                  The field may be any integral number of bytes in
                  length.  It does not require any particular
                  alignment.  The 32-bit alignment shown is for
                  convenience in the illustration.
 Attribute-Choices
                  0 or more bytes.  When the SPI is non-zero, a list
                  of attributes selected from the list of Offered-
                  Attributes supported by the peer.
                  The contents and usage of this list are further
                  described in "Attribute Choices List".  The end of
                  the list is indicated by the UDP Length after
                  removing the Padding (UDP Length - last Padding
                  value).
 Padding          8 to 255 bytes.  This field is filled up to at least
                  a 128 byte boundary, measured from the beginning of
                  the message.  The number of pad bytes are chosen
                  randomly.
                  In addition, when a Privacy-Method indicated by the

Karn & Simpson Experimental [Page 32] RFC 2522 Photuris Protocol March 1999

                  current Scheme-Choice requires the plaintext to be a
                  multiple of some number of bytes (the block size of
                  a block cipher), this field is adjusted as necessary
                  to the size required by the algorithm.
                  Self-Describing-Padding begins with the value 1.
                  Each byte contains the index of that byte.  Thus,
                  the final pad byte indicates the number of pad bytes
                  to remove.  For example, when the unpadded message
                  length is 120 bytes, the padding values might be 1,
                  2, 3, 4, 5, 6, 7, and 8.
 The portion of the message after the SPI field is masked using the
 Privacy-Method indicated by the current Scheme-Choice.
 The fields following the SPI are opaque.  That is, the values are set
 prior to masking (and optional encryption), and examined only after
 unmasking (and optional decryption).

5.2. Attribute Choices List

 This list specifies the attributes of the SPI.  The attribute formats
 are specified in the "Basic Attributes".
 The list is composed of one or two sections: Authentication-
 Attributes, and/or Encapsulation-Attributes.
 When sending from the SPI User to the SPI Owner, the attributes are
 processed in the order listed.  For example,
    "ESP-Attributes",
    "Deflate" (Compression),
    "XOR" (Encryption),
    "DES-CBC" (Encryption),
    "XOR" (Encryption),
    "AH-Attributes",
    "AH-Sequence",
    "MD5-IPMAC" (Authentication),
 would result in ESP with compression and triple encryption (inside),
 and then AH authentication with sequence numbers (outside) of the ESP
 payload.
 The SPI Owner will naturally process the datagram in the reverse
 order.
 This ordering also affects the order of key generation.  Both SPI

Karn & Simpson Experimental [Page 33] RFC 2522 Photuris Protocol March 1999

 Owner and SPI User generate the keys in the order listed.
 Implementation Notes:
    When choices are made from the list of Offered-Attributes, it is
    not required that any Security Association include every kind of
    offered attribute in any single SPI, or that a separate SPI be
    created for every offered attribute.
    Some kinds of attributes may be included more than once in a
    single SPI.  The set of allowable combinations of attributes are
    dependent on implementation and operational policy.  Such
    considerations are outside the scope of this document.
    The list may be divided into additional sections.  This can occur
    only when both parties recognize the affected attributes.
    The authentication, compression, encryption and identification
    mechanisms chosen, as well as the encapsulation modes (if any),
    need not be the same in both directions.

5.3. Shared-Secret

 A shared-secret is used in a number of calculations.  Regardless of
 the internal representation of the shared-secret, when used in
 calculations it is in the same form as the Value part of a Variable
 Precision Integer:
  1. most significant byte first.
  2. bits used are right justified within byte boundaries.
  3. any unused bits are in the most significant byte.
  4. unused bits are zero filled.
 The shared-secret does not include a Size field.

5.4. Identity Verification

 These messages are authenticated using the Identity-Choice.  The
 Verification value is calculated prior to masking (and optional
 encryption), and verified after unmasking (and optional decryption).
 The Identity-Choice authentication function is supplied with two
 input values:
  1. the sender (SPI Owner) verification-key,
  2. the data to be verified (as a concatenated sequence of bytes).

Karn & Simpson Experimental [Page 34] RFC 2522 Photuris Protocol March 1999

 The resulting output value is stored in the Verification field.
 The Identity-Choice verification data consists of the following
 concatenated values:
  + the Initiator Cookie,
  + the Responder Cookie,
  + the Message, LifeTime and SPI fields,
  + the Identity-Choice and Identification,
  + the SPI User Identity Verification (response only),
  + the Attribute-Choices following the Verification field,
  + the Padding,
  + the SPI Owner TBV,
  + the SPI Owner Exchange-Value,
  + the SPI Owner Offered-Attributes,
  + the SPI User TBV,
  + the SPI User Exchange-Value,
  + the SPI User Offered-Attributes,
  + the Responder Offered-Schemes.
 The TBV (Three Byte Value) consists of the Counter and Scheme-Choice
 fields from the Value_Request, or the Reserved field from the
 Value_Response, immediately preceding the associated Exchange-Value.
 Note that the order of the Exchange-Value and Offered-Attributes
 fields is different in each direction, and the Identification and SPI
 fields are also likely to be different in each direction.  Note also
 that the SPI User Identity Verification (from the Identity_Request)
 is present only in the Identity_Response.
 If the verification fails, the users are notified, and a
 Verification_Failure message is sent, without adding any SPI.  On
 success, normal operation begins with the authentication and/or
 encryption of user datagrams.
 Implementation Notes:
    This is distinct from any authentication method specified for the
    SPI.
    The exact details of the Identification and verification-key
    included in the Verification calculation are dependent on the
    Identity-Choice, as described in the "Basic Attributes".
    Each party may wish to keep their own trusted databases, such as
    the Pretty Good Privacy (PGP) web of trust, and accept only those
    identities found there.  Failure to find the Identification in
    either an internal or external database results in the same

Karn & Simpson Experimental [Page 35] RFC 2522 Photuris Protocol March 1999

    Verification_Failure message as failure of the verification
    computation.
    The Exchange-Value data includes both the Size and Value fields.
    The Offered-Attributes and Attribute-Choices data includes the
    Attribute, Length and Value fields.

5.5. Privacy-Key Computation

 Identification Exchange messages are masked using the Privacy-Method
 indicated by the current Scheme-Choice.  Masking begins with the next
 field after the SPI, and continues to the end of the data indicated
 by the UDP Length, including the Padding.
 The Scheme-Choice specified Key-Generation-Function is used to create
 a special privacy-key for each message.  This function is calculated
 over the following concatenated values:
  + the SPI Owner Exchange-Value,
  + the SPI User Exchange-Value,
  + the Initiator Cookie,
  + the Responder Cookie,
  + the Message, LifeTime and SPI (or Reserved) fields,
  + the computed shared-secret.
 Since the order of the Exchange-Value fields is different in each
 direction, and the Message, LifeTime and SPI fields are also
 different in each direction, the resulting privacy-key will usually
 be different in each direction.
 When a larger number of keying-bits are needed than are available
 from one iteration of the specified Key-Generation-Function, more
 keying-bits are generated by duplicating the trailing shared-secret,
 and recalculating the function.  That is, the first iteration will
 have one trailing copy of the shared-secret, the second iteration
 will have two trailing copies of the shared-secret, and so forth.
 Implementation Notes:
    This is distinct from any encryption method specified for the SPI.
    The length of the Padding, and other details, are dependent on the
    Privacy-Method.  See the "Basic Privacy-Method" list for details.
    To avoid keeping the Exchange-Values in memory after the initial
    verification, it is often possible to pre-compute the function
    over the initial bytes of the concatenated data values for each

Karn & Simpson Experimental [Page 36] RFC 2522 Photuris Protocol March 1999

    direction, and append the trailing copies of the shared-secret.
    The Exchange-Value data includes both the Size and Value fields.

5.6. Session-Key Computation

 Each SPI has one or more session-keys.  These keys are generated
 based on the attributes of the SPI.  See the "Basic Attributes" for
 details.
 The Scheme-Choice specified Key-Generation-Function is used to create
 the SPI session-key for that particular attribute.  This function is
 calculated over the following concatenated values:
  + the Initiator Cookie,
  + the Responder Cookie,
  + the SPI Owner generation-key,
  + the SPI User generation-key,
  + the message Verification field,
  + the computed shared-secret.
 Since the order of the generation-keys is different in each
 direction, and the Verification field is also likely to be different
 in each direction, the resulting session-key will usually be
 different in each direction.
 When a larger number of keying-bits are needed than are available
 from one iteration of the specified Key-Generation-Function, more
 keying-bits are generated by duplicating the trailing shared-secret,
 and recalculating the function.  That is, the first iteration will
 have one trailing copy of the shared-secret, the second iteration
 will have two trailing copies of the shared-secret, and so forth.
 Implementation Notes:
    This is distinct from any privacy-key generated for the Photuris
    exchange.  Different initialization data is used, and iterations
    are maintained separately.
    The exact details of the Verification field and generation-keys
    that are included in the session-key calculation are dependent on
    the Identity-Choices, as described in the "Basic Attributes".
    To avoid keeping the generation-keys in memory after the initial
    verification, it is often possible to pre-compute the function
    over the initial bytes of the concatenated data values for each
    direction, and append the trailing copies of the shared-secret.

Karn & Simpson Experimental [Page 37] RFC 2522 Photuris Protocol March 1999

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

6. SPI Messages

 SPI User                             SPI Owner
 ========                             =========
 SPI_Needed                     ->
    list SPI attribute(s)
    make validity key
    authenticate
    make privacy key(s)
    mask/encrypt message
                                 <-   SPI_Update
                                         make SPI
                                         pick SPI attribute(s)
                                         make SPI session-key(s)
                                         make validity key
                                         authenticate
                                         make privacy key(s)
                                         mask/encrypt message
 The exchange of messages is not related to the Initiator and
 Responder.  Instead, either party may send one of these messages at
 any time.  The messages are easily distinguished by the parties.

6.0.1. Send SPI_Needed

 At any time after completion of the Identification Exchange, either
 party can send SPI_Needed.  This message is sent when a prospective
 SPI User needs particular attributes for a datagram (such as
 confidentiality), and no current SPI has those attributes.
 The prospective SPI User selects from the intersection of attributes
 that both parties have previously offered, calculates the
 Verification, and masks the message using the Privacy-Method
 indicated by the current Scheme-Choice.

Karn & Simpson Experimental [Page 38] RFC 2522 Photuris Protocol March 1999

6.0.2. Receive SPI_Needed

 The potential SPI Owner validates the pair of Cookies, the Padding,
 the Verification, and the Attributes-Needed.
  1. When an invalid/expired cookie is detected, a Bad_Cookie message

is sent.

  1. When too many SPI values are already in use for this particular

peer, or some other resource limit is reached, a Resource_Limit

    message is sent.
  1. After unmasking, when invalid Padding is detected, the variable

length Attributes-Needed do not match the UDP Length, or an

    attribute was not in the Offered-Attributes, the message is
    silently discarded.
  1. When the message verification fails, a Verification_Failure

message is sent.

  1. Whenever such a problem is detected, the SPI is not established;

the implementation SHOULD log the occurance, and notify an

    operator as appropriate.
 When the message is valid, the party SHOULD send SPI_Update with the
 necessary attributes.
 If an existing SPI has those attributes, that SPI is returned in the
 SPI_Update with the remaining SPILT.

6.0.3. Send SPI_Update

 At any time after completion of the Identification Exchange, either
 party can send SPI_Update.  This message has effect in only one
 direction, from the SPI Owner to the SPI User.
 The SPI Owner chooses the SPI and SPILT, a set of Attributes for the
 SPI, calculates the Verification, and masks the message using the
 Privacy-Method indicated by the current Scheme-Choice.

6.0.4. Receive SPI_Update

 The prospective SPI User validates the pair of Cookies, the Padding,
 the Verification, and the Attributes-Needed.
  1. When an invalid/expired cookie is detected, a Bad_Cookie message

Karn & Simpson Experimental [Page 39] RFC 2522 Photuris Protocol March 1999

    is sent.
  1. After unmasking, when invalid Padding is detected, the variable

length Attribute-Choices do not match the UDP Length, an attribute

    was not in the Offered-Attributes, or the message modifies an
    existing SPI, the message is silently discarded.
  1. When the message verification fails, a Verification_Failure

message is sent.

  1. Whenever such a problem is detected, the SPI is not established;

the implementation SHOULD log the occurance, and notify an

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

6.0.5. Automated SPI_Updates

 Each SPI requires replacement under several circumstances:
  1. the volume of data processed (inhibiting probability

cryptanalysis),

  1. exhaustion of available anti-replay Sequence Numbers,
  1. and expiration of the LifeTime.
 In general, a determination is made upon receipt of a datagram.  If
 the transform specific processing finds that refreshment is needed,
 an automated SPI_Update is triggered.
 In addition, automated SPI_Updates allow rapid SPI refreshment for
 high bandwidth applications in a high delay environment.  The update
 messages flow in the opposite direction from the primary traffic,
 conserving bandwidth and avoiding service interruption.
 When creating each SPI, the implementation MAY optionally set an
 Update TimeOut (UTO); by default, to half the value of the LifeTime
 (SPILT/2).  This time is highly dynamic, and adjustable to provide an
 automated SPI_Update long before transform specific processing.  If
 no new Photuris exchange occurs within the time limit, and the
 current exchange state has not expired, an automated SPI_Update is
 sent.

Karn & Simpson Experimental [Page 40] RFC 2522 Photuris Protocol March 1999

6.1. SPI_Needed

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Initiator-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Responder-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Message    |                  Reserved-LT                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Reserved-SPI                          |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                                                               |
 ~                         Verification                          ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Attributes-Needed ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                    ... Padding  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Initiator-Cookie  16 bytes.  Copied from the Value_Request.
 Responder-Cookie  16 bytes.  Copied from the Value_Request.
 Message          8
 Reserved-LT      3 bytes.  For future use; MUST be filled with a
                  random non-zero value when transmitted, and MUST be
                  ignored when received.
 Reserved-SPI     4 bytes.  For future use; MUST be set to zero when
                  transmitted, and MUST be ignored when received.
 Verification     Variable Precision Integer, or other format
                  indicated by the current Scheme-Choice.  The
                  calculation of the value is described in "Validity
                  Verification".
                  The field may be any integral number of bytes in
                  length.  It does not require any particular
                  alignment.  The 32-bit alignment shown is for
                  convenience in the illustration.

Karn & Simpson Experimental [Page 41] RFC 2522 Photuris Protocol March 1999

 Attributes-Needed
                  4 or more bytes.  A list of two or more attributes,
                  selected from the list of Offered-Attributes
                  supported by the peer.
                  The contents and usage of this list are as
                  previously described in "Attribute Choices List".
                  The end of the list is indicated by the UDP Length
                  after removing the Padding (UDP Length - last
                  Padding value).
 Padding          8 or more bytes.  The message is padded in the same
                  fashion specified for Identification Exchange
                  messages.
 The portion of the message after the SPI field is masked using the
 Privacy-Method indicated by the current Scheme-Choice.
 The fields following the SPI are opaque.  That is, the values are set
 prior to masking (and optional encryption), and examined only after
 unmasking (and optional decryption).

Karn & Simpson Experimental [Page 42] RFC 2522 Photuris Protocol March 1999

6.2. SPI_Update

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Initiator-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Responder-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Message    |                    LifeTime                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   Security-Parameters-Index                   |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                                                               |
 ~                         Verification                          ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Attribute-Choices ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                    ... Padding  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Initiator-Cookie  16 bytes.  Copied from the Value_Request.
 Responder-Cookie  16 bytes.  Copied from the Value_Request.
 Message          9
 LifeTime         3 bytes.  The number of seconds remaining before the
                  indicated SPI expires.  The value zero indicates
                  deletion of the indicated SPI.
 Security-Parameters-Index (SPI)
                  4 bytes.  The SPI to be used for incoming
                  communications.
                  This may be a new SPI value (for creation), or an
                  existing SPI value (for deletion).  The value zero
                  indicates special processing.
 Verification     Variable Precision Integer, or other format
                  indicated by the current Scheme-Choice.  The
                  calculation of the value is described in "Validity
                  Verification".

Karn & Simpson Experimental [Page 43] RFC 2522 Photuris Protocol March 1999

                  The field may be any integral number of bytes in
                  length.  It does not require any particular
                  alignment.  The 32-bit alignment shown is for
                  convenience in the illustration.
 Attribute-Choices
                  0 or more bytes.  When the SPI and SPILT are non-
                  zero, a list of attributes selected from the list of
                  Offered-Attributes supported by the peer.
                  The contents and usage of this list are as
                  previously described in "Attribute Choices List".
                  The end of the list is indicated by the UDP Length
                  after removing the Padding (UDP Length - last
                  Padding value).
 Padding          8 or more bytes.  The message is padded in the same
                  fashion specified for Identification Exchange
                  messages.
 The portion of the message after the SPI field is masked using the
 Privacy-Method indicated by the current Scheme-Choice.
 The fields following the SPI are opaque.  That is, the values are set
 prior to masking (and optional encryption), and examined only after
 unmasking (and optional decryption).

6.2.1. Creation

 When the LifeTime is non-zero, and the SPI is also non-zero, the
 SPI_Update can be used to create a new SPI.  When the SPI is zero,
 the SPI_Update is silently discarded.
 The new session-keys are calculated in the same fashion as the
 Identity_Messages.  Since the SPI value is always different than any
 previous SPI during the Exchange LifeTime of the shared-secret, the
 resulting session-keys will necessarily be different from all others
 used in the same direction.
 No retransmission timer is necessary.  Success is indicated by the
 peer use of the new SPI.
 Should all creation attempts fail, eventually the peer will find that
 all existing SPIs have expired, and will begin the Photuris exchange
 again by sending a new Cookie_Request.  When appropriate, this
 Cookie_Request MAY include a Responder-Cookie to retain previous
 party pairings.

Karn & Simpson Experimental [Page 44] RFC 2522 Photuris Protocol March 1999

6.2.2. Deletion

 When the LifeTime is zero, the SPI_Update can be used to delete a
 single existing SPI.  When the SPI is also zero, the SPI_Update will
 delete all existing SPIs related to this Security Association, and
 mark the Photuris exchange state as expired.  This is especially
 useful when the application that needed them terminates.
 No retransmission timer is necessary.  This message is advisory, to
 reduce the number of ICMP Security Failures messages.
 Should any deletion attempts fail, the peer will learn that the
 deleted SPIs are invalid through the normal ICMP Security Failures
 messages, and will initiate a Photuris exchange by sending a new
 Cookie_Request.

6.2.3. Modification

 The SPI_Update cannot be used to modify existing SPIs, such as
 lengthen an existing SPI LifeTime, resurrect an expired SPI, or
 add/remove an Attribute-Choice.
 On receipt, such an otherwise valid message is silently discarded.

6.3. Validity Verification

 These messages are authenticated using the Validity-Method indicated
 by the current Scheme-Choice.  The Verification value is calculated
 prior to masking (and optional encryption), and verified after
 unmasking (and optional decryption).
 The Validity-Method authentication function is supplied with two
 input values:
  1. the sender (SPI Owner) verification-key,
  2. the data to be verified (as a concatenated sequence of bytes).
 The resulting output value is stored in the Verification field.
 The Validity-Method verification data consists of the following
 concatenated values:

Karn & Simpson Experimental [Page 45] RFC 2522 Photuris Protocol March 1999

  + the Initiator Cookie,
  + the Responder Cookie,
  + the Message, LifeTime and SPI (or Reserved) fields,
  + the SPI Owner Identity Verification,
  + the SPI User Identity Verification,
  + the Attribute-Choices following the Verification field,
  + the Padding.
 Note that the order of the Identity Verification fields (from the
 Identity_Messages) is different in each direction, and the Message,
 LifeTime and SPI fields are also likely to be different in each
 direction.
 If the verification fails, the users are notified, and a
 Verification_Failure message is sent, without adding or deleting any
 SPIs.  On success, normal operation begins with the authentication
 and/or encryption of user datagrams.
 Implementation Notes:
    This is distinct from any authentication method specified for the
    SPI.
    The Identity Verification data includes both the Size and Value
    fields.  The Attribute-Choices data includes the Attribute, Length
    and Value fields.

7. Error Messages

 These messages are issued in response to Photuris state loss or other
 problems.  A message has effect in only one direction.  No
 retransmission timer is necessary.
 These messages are not masked.
 The receiver checks the Cookies for validity.  Special care MUST be
 taken that the Cookie pair in the Error Message actually match a pair
 currently in use, and that the protocol is currently in a state where
 such an Error Message might be expected.  Otherwise, these messages
 could provide an opportunity for a denial of service attack.  Invalid
 messages are silently discarded.

Karn & Simpson Experimental [Page 46] RFC 2522 Photuris Protocol March 1999

7.1. Bad_Cookie

 For the format of the 33 byte message, see "Header Format".  There
 are no additional fields.
 Initiator-Cookie  16 bytes.  Copied from the offending message.
 Responder-Cookie  16 bytes.  Copied from the offending message.
 Message          10
 This error message is sent when a Value_Request, Identity_Request,
 SPI_Needed, or SPI_Update is received, and the receiver specific
 Cookie is invalid or the associated exchange state has expired.
 During the Photuris exchange, when this error message is received, it
 has no immediate effect on the operation of the protocol phases.
 Later, when Retransmissions have been exceeded, and this error
 message has been received, the Initiator SHOULD begin the Photuris
 exchange again by sending a new Cookie_Request with the Responder-
 Cookie and Counter updated appropriately.
 When this error message is received in response to SPI_Needed, the
 exchange state SHOULD NOT be marked as expired, but the party SHOULD
 initiate a Photuris exchange by sending a new Cookie_Request.
 When this error message is received in response to SPI_Update, the
 exchange state SHOULD NOT be marked as expired, and no further action
 is taken.  A new exchange will be initiated later when needed by the
 peer to send authenticated and/or encrypted data.
 Existing SPIs are not deleted.  They expire normally, and are purged
 sometime later.

7.2. Resource_Limit

 For the format of the 34 byte message, see "Cookie_Request".  There
 are no additional fields.
 Initiator-Cookie  16 bytes.  Copied from the offending message.
 Responder-Cookie  16 bytes.  Copied from the offending message.
                  Special processing is applied to a Cookie_Request.
                  When the offending message Responder-Cookie and
                  Counter were both zero, and an existing exchange has
                  not yet been purged, this field is replaced with the

Karn & Simpson Experimental [Page 47] RFC 2522 Photuris Protocol March 1999

                  Responder-Cookie from the existing exchange.
 Message          11
 Counter          1 byte.  Copied from the offending message.
                  When zero, the Responder-Cookie indicates the
                  Initiator of a previous exchange, or no previous
                  exchange is specified.
                  When non-zero, the Responder-Cookie indicates the
                  Responder to a previous exchange.  This value is set
                  to the Counter from the corresponding
                  Cookie_Response.
 This error message is sent when a Cookie_Request, Value_Request or
 SPI_Needed is received, and too many SPI values are already in use
 for that peer, or some other Photuris resource is unavailable.
 During the Photuris exchange, when this error message is received in
 response to a Cookie_Request or Value_Request, the implementation
 SHOULD double the retransmission timeout (as usual) for sending
 another Cookie_Request or Value_Request.  Otherwise, it has no
 immediate effect on the operation of the protocol phases.  Later,
 when Retransmissions have been exceeded, and this error message has
 been received, the Initiator SHOULD begin the Photuris exchange again
 by sending a new Cookie_Request with the Responder-Cookie and Counter
 updated appropriately.
 When this error message is received in response to SPI_Needed, the
 implementation SHOULD NOT send another SPI_Needed until one of the
 existing SPIs associated with this exchange is deleted or has
 expired.

7.3. Verification_Failure

 For the format of the 33 byte message, see "Header Format".  There
 are no additional fields.
 Initiator-Cookie  16 bytes.  Copied from the offending message.
 Responder-Cookie  16 bytes.  Copied from the offending message.
 Message          12
 This error message is sent when an Identity_Message, SPI_Needed or
 SPI_Update is received, and verification fails.

Karn & Simpson Experimental [Page 48] RFC 2522 Photuris Protocol March 1999

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

7.4. Message_Reject

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Initiator-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                       Responder-Cookie                        ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Message    |  Bad-Message  |             Offset            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Initiator-Cookie  16 bytes.  Copied from the offending message.
 Responder-Cookie  16 bytes.  Copied from the offending message.
 Message          13
 Bad-Message      1 byte.  Indicates the Message number of the
                  offending message.
 Offset           2 bytes.  The number of bytes from the beginning of
                  the offending message where the unrecognized field
                  starts.  The minimum value is 32.
 This error message is sent when an optional Message type is received
 that is not supported, or an optional format of a supported Message
 is not recognized.
 When this error message is received, the implementation SHOULD log
 the occurance, and notify an operator as appropriate.  However,
 receipt has no effect on the operation of the protocol.

Karn & Simpson Experimental [Page 49] RFC 2522 Photuris Protocol March 1999

8. Public Value Exchanges

 Photuris is based in principle on public-key cryptography,
 specifically Diffie-Hellman key exchange.  Exchange of public D-H
 Exchange-Values based on private-secret values results in a mutual
 shared-secret between the parties.  This shared-secret can be used on
 its own, or to generate a series of session-keys for authentication
 and encryption of subsequent traffic.
 This document assumes familiarity with the Diffie-Hellman public-key
 algorithm.  A good description can be found in [Schneier95].

8.1. Modular Exponentiation Groups

 The original Diffie-Hellman technique [DH76] specified modular
 exponentiation.  A public-value is generated using a generator (g),
 raised to a private-secret exponent (x), modulo a prime (p):
    (g**x) mod p.
 When these public-values are exchanged between parties, the parties
 can calculate a shared-secret value between themselves:
    (g**xy) mod p.
 The generator (g) and modulus (p) are established by the Scheme-
 Choice (see the "Basic Exchange-Schemes" for details).  They are
 offered in the Cookie_Response, and one pair is chosen in the
 Value_Request.
 The private exponents (x) and (y) are kept secret by the parties.
 Only the public-value result of the modular exponentiation with (x)
 or (y) is sent as the Initiator and Responder Exchange-Value.
 These public-values are represented in single Variable Precision
 Integers.  The Size of these Exchange-Values will match the Size of
 the modulus (p).

8.2. Moduli Selection

 Each implementation proposes one or more moduli in its Offered-
 Schemes.  Every implementation MUST support up to 1024-bit moduli.
 For any particular Photuris node, these moduli need not change for
 significant periods of time; likely days or weeks.  A background
 process can periodically generate new moduli.

Karn & Simpson Experimental [Page 50] RFC 2522 Photuris Protocol March 1999

    For 512-bit moduli, current estimates would provide 64
    (pessimistic) bit-equivalents of cryptographic strength.
    For 1024-bit moduli, current estimates would range from 80
    (pessimistic) through 98 (optimistic) bit-equivalents of
    cryptographic strength.
 These estimates are used when choosing moduli that are appropriate
 for the desired Security Parameter attributes.

8.2.1. Bootstrap Moduli

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

8.2.2. Learning Moduli

 As Photuris exchanges are initiated, new moduli will be learned from
 the Responder Offered-Schemes.  The Initiator MAY cache these moduli
 for its own use.
 Before offering any learned modulus, the implementation MUST perform
 at least one iteration of probable primality verification.  In this
 fashion, many processors will perform verification in parallel as
 moduli are passed around.
 When primality verification failures are found, the failed moduli
 SHOULD be retained for some (implementation dependent) period of
 time, to avoid re-learning and re-testing after subsequent exchanges.

8.3. Generator Selection

 The generator (g) should be chosen such that the private-secret
 exponents will generate all possible public-values, evenly
 distributed throughout the range of the modulus (p), without cycling
 through a smaller subset.  Such a generator is called a "primitive
 root" (which is trivial to find when p is "safe").
 Only one generator (2) is required to be supported.

Karn & Simpson Experimental [Page 51] RFC 2522 Photuris Protocol March 1999

 Implementation Notes:
    One useful technique is to select the generator, and then limit
    the modulus selection sieve to primes with that generator:
       2   when p (mod 24) = 11.
       3   when p (mod 12) = 5.
       5   when p (mod 10) = 3 or 7.
    The required generator (2) improves efficiency in multiplication
    performance.  It is usable even when it is not a primitive root,
    as it still covers half of the space of possible residues.

8.4. Exponent Selection

 Each implementation generates a separate random private-secret
 exponent for each different modulus.  Then, a D-H Exchange-Value is
 calculated for the given modulus, generator, and exponent.
 This specification recommends that the exponent length be at least
 twice the desired cryptographic strength of the longest session-key
 needed by the strongest offered-attribute.
 Based on the estimates in "Moduli Selection" (above):
    For 512-bit moduli, exponent lengths of 128 bits (or more) are
    recommended.
    For 1024-bit moduli, exponent lengths of 160 to 256 bits (or more)
    are recommended.
 Although the same exponent and Exchange-Value may be used with
 several parties whenever the same modulus and generator are used, the
 exponent SHOULD be changed at random intervals.  A background process
 can periodically destroy the old values, generate a new random
 private-secret exponent, and recalculate the Exchange-Value.
 Implementation Notes:
    The size of the exponent is entirely implementation dependent, is
    unknown to the other party, and can be easily changed.
    Since these operations involve several time-consuming modular
    exponentiations, moving them to the "background" substantially
    improves the apparent execution speed of the Photuris protocol.
    It also reduces CPU loading sufficiently to allow a single
    public/private key-pair to be used in several closely spaced

Karn & Simpson Experimental [Page 52] RFC 2522 Photuris Protocol March 1999

    Photuris executions, when creating Security Associations with
    several different nodes over a short period of time.
    Other pre-computation suggestions are described in [BGMW93, LL94,
    Rooij94].

8.5. Defective Exchange Values

 Some exponents do not qualify as secret.  The exponent 0 will
 generate the Exchange-Value 1, and the exponent 1 will generate the
 Exchange-Value g.  Small exponents will be easily visible and SHOULD
 be avoided where:
    g**x < p.
 Depending on the structure of the moduli, certain exponents can be
 used for sub-group confinement attacks.  For "safe" primes (p =
 2q+1), these exponents are p-1 and (p-1)/2, which will generate the
 Exchange-Values 1 and p-1 respectively.
 When an implementation chooses a random exponent, the resulting
 Exchange-Value is examined.  If the Exchange-Value is represented in
 less than half the number of significant bits in the modulus, then a
 new random exponent MUST be chosen.
    For 512-bit moduli, Exchange-Values of 2**256 or greater are
    required.
    For 1024-bit moduli, Exchange-Values of 2**512 or greater are
    required.
 In addition, if the resulting Exchange-Value is p-1, then a new
 random exponent MUST be chosen.
 Upon receipt of an Exchange-Value that fails to meet the
 requirements, the Value Exchange message is silently discarded.
 Implementation Notes:
    Avoidance of small exponents can be assured by setting at least
    one bit in the most significant half of the exponent.

Karn & Simpson Experimental [Page 53] RFC 2522 Photuris Protocol March 1999

9. Basic Exchange-Schemes

 Initial values are assigned as follows:
 (0)   Reserved.
 (1)   Reserved.
 (2)   Implementation Required.  Any modulus (p) with a recommended
       generator (g) of 2.  When the Exchange-Scheme Size is non-zero,
       the modulus is contained in the Exchange-Scheme Value field in
       the list of Offered-Schemes.
       An Exchange-Scheme Size of zero is invalid.
       Key-Generation-Function     "MD5 Hash"
       Privacy-Method              "Simple Masking"
       Validity-Method             "MD5-IPMAC Check"
       This combination of features requires a modulus with at least
       64-bits of cryptographic strength.
 (3)   Exchange-Schemes 3 to 255 are intended for future well-known
       published schemes.
 (256)  Exchange-Schemes 256 to 32767 are intended for vendor-specific
       unpublished schemes.  Implementors wishing a number MUST
       request the number from the authors.
 (32768)
       Exchange-Schemes 32768 to 65535 are available for cooperating
       parties to indicate private schemes, regardless of vendor
       implementation.  These numbers are not reserved, and are
       subject to duplication.  Other criteria, such as the IP Source
       and Destination of the Cookie_Request, are used to
       differentiate the particular Exchange-Schemes available.

Karn & Simpson Experimental [Page 54] RFC 2522 Photuris Protocol March 1999

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

 MD5 [RFC-1321] is used as a pseudo-random-function for generating the
 key(s).  The key(s) begin with the most significant bits of the hash.
 MD5 is iterated as needed to generate the requisite length of key
 material.
 When an individual key does not use all 128-bits of the last hash,
 any remaining unused (least significant) bits of the last hash are
 discarded.  When combined with other uses of key generation for the
 same purpose, the next key will begin with a new hash iteration.

11. Basic Privacy-Method 11.1. Simple Masking

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

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

 As described in "Validity Verification", the Verification field value
 is the MD5 [RFC-1321] hash over the concatenation of
    MD5( key, keyfill, data, datafill, key, md5fill )
 where the key is the computed verification-key.
 The keyfill and datafill use the same pad-with-length technique
 defined for md5fill.  This padding and length is implicit, and does
 not appear in the datagram.
 The resulting Verification field is a 128-bit Variable Precision
 Integer (18 bytes including Size).  When used in calculations, the
 Verification data includes both the Size and Value fields.

Karn & Simpson Experimental [Page 55] RFC 2522 Photuris Protocol March 1999

13. Basic Attributes

 Implementors wishing a number MUST request the number from the
 authors.  Initial values are assigned as follows:
   Use    Type
    -       0* padding
    -       1* AH-Attributes
    -       2+ ESP-Attributes
   AEI      5* MD5-IPMAC
   AEIX   255+ Organizational
   A      AH Attribute-Choice
    E     ESP Attribute-Choice
     I    Identity-Choice
      X   dependent on list location
       +  feature must be recognized even when not supported
       *  feature must be supported (mandatory)
 Other attributes are specified in companion documents.

13.1. Padding

 +-+-+-+-+-+-+-+-+
 |   Attribute   |
 +-+-+-+-+-+-+-+-+
 Attribute        0
 Each attribute may have value fields that are multiple bytes.  To
 facilitate processing efficiency, these fields are aligned on
 integral modulo 8 byte (64-bit) boundaries.
 Padding is accomplished by insertion of 1 to 7 Attribute 0 padding
 bytes before the attribute that needs alignment.
 No padding is used after the final attribute in a list.

Karn & Simpson Experimental [Page 56] RFC 2522 Photuris Protocol March 1999

13.2. AH-Attributes

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Attribute   |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Attribute        1
 Length           0
 When a list of Attributes is specified, this Attribute begins the
 section of the list which applies to the Authentication Header (AH).

13.3. ESP-Attributes

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Attribute   |    Length     |  PayloadType  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Attribute        2
 Length           1
 PayloadType      1 byte.  Indicates the contents of the ESP Transform
                  Data field, using the IP Next Header (Protocol)
                  value.  Up-to-date values of the IP Next Header
                  (Protocol) are specified in the most recent
                  "Assigned Numbers" [RFC-1700].
                  For example, when encrypting an entire IP datagram,
                  this field will contain the value 4, indicating IP-
                  in-IP encapsulation.
 When a list of Attributes is specified, this Attribute begins the
 section of the list which applies to the Encapsulating Security
 Payload (ESP).
 When listed as an Offered-Attribute, the PayloadType is set to 255.
 When selected as an Attribute-Choice, the PayloadType is set to the
 actual value to be used.

Karn & Simpson Experimental [Page 57] RFC 2522 Photuris Protocol March 1999

13.4. MD5-IPMAC

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Attribute   |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Attribute        5
 Length           0

13.4.1. Symmetric Identification

 When selected as an Identity-Choice, the immediately following
 Identification field contains an unstructured Variable Precision
 Integer.  Valid Identifications and symmetric secret-keys are
 preconfigured by the parties.
 There is no required format or content for the Identification value.
 The value may be a number or string of any kind.  See "Use of
 Identification and Secrets" for details.
 The symmetric secret-key (as specified) is selected based on the
 contents of the Identification field.  All implementations MUST
 support at least 62 bytes.  The selected symmetric secret-key SHOULD
 provide at least 64-bits of cryptographic strength.
 As described in "Identity Verification", the Verification field value
 is the MD5 [RFC-1321] hash over the concatenation of:
    MD5( key, keyfill, data, datafill, key, md5fill )
 where the key is the computed verification-key.
 The keyfill and datafill use the same pad-with-length technique
 defined for md5fill.  This padding and length is implicit, and does
 not appear in the datagram.
 The resulting Verification field is a 128-bit Variable Precision
 Integer (18 bytes including Size).  When used in calculations, the
 Verification data includes both the Size and Value fields.
 For both "Identity Verification" and "Validity Verification", the
 verification-key is the MD5 [RFC-1321] hash of the following
 concatenated values:

Karn & Simpson Experimental [Page 58] RFC 2522 Photuris Protocol March 1999

  + the symmetric secret-key,
  + the computed shared-secret.
 For "Session-Key Computation", the symmetric secret-key is used
 directly as the generation-key.
 Regardless of the internal representation of the symmetric secret-
 key, when used in calculations it is in the same form as the Value
 part of a Variable Precision Integer:
  1. most significant byte first.
  2. bits used are right justified within byte boundaries.
  3. any unused bits are in the most significant byte.
  4. unused bits are zero filled.
 The symmetric secret-key does not include a Size field.

13.4.2. Authentication

 May be selected as an AH or ESP Attribute-Choice, pursuant to [RFC-
 1828] et sequitur.  The selected Exchange-Scheme SHOULD provide at
 least 64-bits of cryptographic strength.
 As described in "Session-Key Computation", the most significant 384-
 bits (48 bytes) of the Key-Generation-Function iterations are used
 for the key.
 Profile:
    When negotiated with Photuris, the transform differs slightly from
    [RFC-1828].
    The form of the authenticated message is:
       MD5( key, keyfill, datagram, datafill, key, md5fill )
    where the key is the SPI session-key.
    The additional datafill protects against the (impractical) attack
    described in [PO96].  The keyfill and datafill use the same pad-
    with-length technique defined for md5fill.  This padding and
    length is implicit, and does not appear in the datagram.

Karn & Simpson Experimental [Page 59] RFC 2522 Photuris Protocol March 1999

13.5. Organizational

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Attribute   |    Length     |              OUI
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ...      |     Kind      |  Value(s) ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Attribute        255
 Length           >= 4
                  When the Length is four, no Value(s) field is
                  present.
 OUI              3 bytes.  The vendor's Organizationally Unique
                  Identifier, assigned by IEEE 802 or IANA (see [RFC-
                  1700] for contact details).  The bits within the
                  byte are in canonical order, and the most
                  significant byte is transmitted first.
 Kind             1 byte.  Indicates a sub-type for the OUI.  There is
                  no standardization for this field.  Each OUI
                  implements its own values.
 Value(s)         0 or more bytes.  The details are implementation
                  specific.
 Some implementors might not need nor want to publish their
 proprietary algorithms and attributes.  This OUI mechanism is
 available to specify these without encumbering the authors with
 proprietary number requests.

Karn & Simpson Experimental [Page 60] RFC 2522 Photuris Protocol March 1999

A. Automaton

 An example automaton is provided to illustrate the operation of the
 protocol.  It is incomplete and non-deterministic; many of the
 Good/Bad semantic decisions are policy-based or too difficult to
 represent in tabular form.  Where conflicts appear between this
 example and the text, the text takes precedence.
 The finite-state automaton is defined by events, actions and state
 transitions.  Events include reception of external commands such as
 expiration of a timer, and reception of datagrams from a peer.
 Actions include the starting of timers and transmission of datagrams
 to the peer.
 Events
 DU13 = Communication Administratively Prohibited
 SF0  = Bad SPI
 SF4  = Need Authentication
 SF5  = Need Authorization
 WC   = Want Confidentiality
 RCQ+ = Receive Cookie_Request (Good)
 RCQ- = Receive Cookie_Request (Bad)
 RCR+ = Receive Cookie_Response (Good)
 RCR- = Receive Cookie_Response (Bad)
 RVQ+ = Receive Value_Request (Good)
 RVQ- = Receive Value_Request (Bad)
 RVR+ = Receive Value_Response (Good)
 RVR- = Receive Value_Response (Bad)
 RIQ+ = Receive Identity_Request (Good)
 RIQ- = Receive Identity_Request (Bad)
 RIR+ = Receive Identity_Response (Good)
 RIR- = Receive Identity_Response (Bad)
 RUN+ = Receive SPI_Needed (Good)
 RUN- = Receive SPI_Needed (Bad)
 RUM+ = Receive SPI_Update (Good)
 RUM- = Receive SPI_Update (Bad)
 RBC  = Receive Bad Cookie
 RRL  = Receive Resource Limit
 RVF  = Receive Verification Failure
 RMR  = Receive Message Reject
 TO+  = Timeout with counter > 0

Karn & Simpson Experimental [Page 61] RFC 2522 Photuris Protocol March 1999

 TO-  = Timeout with counter expired
 UTO  = Update TimeOut
 XTO  = Exchange TimeOut
 Actions
 scq  = Send Cookie_Request
 scr  = Send Cookie_Response
 svq  = Send Value_Request
 svr  = Send Value_Response
 siq  = Send Identity_Request
 sir  = Send Identity_Response
 sum  = Send SPI_Update
 se*  = Send error message (see text)
 sbc  = Send Bad Cookie
 srl  = Send Resource Limit
 svf  = Send Verification Failure
 brto = Backoff Retransmission TimeOut
 buto = Backoff Update TimeOut
 rto  = Set Retransmission TimeOut
 uto  = Set Update TimeOut
 xto  = Set Exchange TimeOut
 log  = log operator message

A.1. State Transition Table

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

Karn & Simpson Experimental [Page 62] RFC 2522 Photuris Protocol March 1999

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

Karn & Simpson Experimental [Page 63] RFC 2522 Photuris Protocol March 1999

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

Karn & Simpson Experimental [Page 64] RFC 2522 Photuris Protocol March 1999

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

A.2. States

 Following is a more detailed description of each automaton state.
 The "Bad" version of a state is to indicate that the Bad_Cookie or
 Resource_Limit message has been received.

A.2.1. Initial

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

Karn & Simpson Experimental [Page 65] RFC 2522 Photuris Protocol March 1999

A.2.2. Cookie

 In the Cookie state, the Initiator has sent a Cookie_Request, and is
 waiting for a Cookie_Response.  Both the Restart and Exchange timers
 are running.
 Note that the Responder has no Cookie state.

A.2.3. Value

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

A.2.4. Identity

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

A.2.5. Ready

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

A.2.6. Update

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

Karn & Simpson Experimental [Page 66] RFC 2522 Photuris Protocol March 1999

B. Use of Identification and Secrets

 Implementation of the base protocol requires support for operator
 configuration of participant identities and associated symmetric
 secret-keys.
 The form of the Identification and Secret fields is not constrained
 to be a readable string.  In addition to a simpler quoted string
 configuration, an implementation MUST allow configuration of an
 arbitrary stream of bytes.

B.1. Identification

 Typically, the Identification is a user name, a site name, a Fully
 Qualified Domain Name, or an email address which contains a user name
 and a domain name.  Examples include:
    user
    node.site.
    user@node.site.
    rcmd@node.site.
    "Mundane Name" <user@node.site>
 There is no requirement that the domain name match any of the
 particular IP addresses in use by the parties.

B.2. Group Identity With Group Secret

 A simple configuration approach could use a single Identity and
 Secret, distributed to all the participants in the trusted group.
 This might be appropriate between routers under a single
 administration comprising a Virtual Private Network over the
 Internet.
 Nota Bene:
    The passwords used in these examples do not meet the "MD5-IPMAC
    Symmetric Identification" recommendation for at least 64-bits of
    cryptographic strength.
 The administrator configures each router with the same username and
 password:
    identity local "Tiny VPN 1995 November" "abracadabra"
    identity remote "Tiny VPN 1995 November" "abracadabra"
 When the Initiator sends its Identity_Request, the SPI Owner

Karn & Simpson Experimental [Page 67] RFC 2522 Photuris Protocol March 1999

 Identification field is "Tiny VPN 1995 November" and the SPI Owner
 secret-key is "abracadabra".
 When the Responder sends its Identity_Response, the SPI Owner
 Identification field is "Tiny VPN 1995 November" and the SPI Owner
 secret-key is "abracadabra".  The SPI User Identification is "Tiny
 VPN 1995 November" (taken from the request), and the SPI User
 secret-key is "abracadabra".
 Note that even in the face of implementations with very poor random
 number generation yielding the same random numbers for both parties
 at every step, and with this completely identical configuration, the
 addition of the SPI User Verification field in the response
 calculation is highly likely to produce a different Verification
 value (see "Identity Verification").  In turn, the different
 Verification values affect the calculation of SPI session-keys that
 are highly likely to be different in each direction (see "Session-Key
 Computation").

B.3. Multiple Identities With Group Secrets

 A more robust configuration approach could use a separate Identity
 and Secret for each party, distributed to the participants in the
 trusted group.  This might be appropriate for authenticated firewall
 traversal.
 An administrator has one or more networks, and a number of mobile
 users.  It is desirable to restrict access to authorized external
 users.  The example boundary router is 10.0.0.1.
 The administrator gives each user a different username and password,
 together with a group username and password for the router.
 The administrator configures (in part):
    identity local "199511@router.site" "FalDaRah"
    identity remote "Happy_Wanderer@router.site" "FalDaRee"
 Each mobile user adds commands to tunnel and authenticate.
    route addprivate 10.0.0.0/8 tunnel 10.0.0.1
    secure 10.0.0.1 authenticate-only
    identity local "Happy_Wanderer@router.site" "FalDaRee"
    identity remote "199511@router.site" "FalDaRah"
    identity remote "199512@router.site" "FalDaHaHaHaHaHaHa"
 When the mobile Initiator sends its Identity_Request, the SPI Owner

Karn & Simpson Experimental [Page 68] RFC 2522 Photuris Protocol March 1999

 Identification field is "Happy_Wanderer@router.site" and the SPI
 Owner secret-key is "FalDaRee".
 When the firewall Responder sends its Identity_Response, the SPI
 Owner Identification field is "199511@router.site" and the SPI Owner
 secret-key is "FalDaRah".  The SPI User Identification field is
 "Happy_Wanderer@router.site" (taken from the request), and the SPI
 User secret-key is "FalDaRee".
 In this example, the mobile user is already prepared for a monthly
 password changeover, where the router might identify itself as
 "199512@router.site".

B.4. Multiple Identities With Multiple Secrets

 Greater security might be achieved through configuration of a pair of
 secrets between each party.  As before, one secret is used for
 initial contact to any member of the group, but another secret is
 used between specific parties.  Compromise of one secret or pair of
 secrets does not affect any other member of the group.  This might be
 appropriate between the routers forming a boundary between
 cooperating Virtual Private Networks that establish local policy for
 each VPN member access.
 One administrator configures:
    identity local "Apple" "all for one"
    identity local "Apple-Baker" "Apple to Baker" "Baker"
    identity remote "Baker" "one for all"
    identity remote "Baker-Apple" "Baker to Apple"
 Another configures:
    identity local "Baker" "one for all"
    identity local "Baker-Apple" "Baker to Apple" "Apple"
    identity remote "Apple" "all for one"
    identity remote "Apple-Baker" "Apple to Baker"
 When the Initiator sends its Identity_Request, the SPI Owner
 Identification field is "Apple" and the SPI Owner secret-key is "all
 for one".
 When the Responder sends its Identity_Response, finding that the
 special pairing exists for "Apple" (in this example, indicated by a
 third field), the SPI Owner Identification field is "Baker-Apple" and
 the SPI Owner secret-key is "Baker to Apple".  The SPI User
 Identification is "Apple" (taken from the request), and the SPI User

Karn & Simpson Experimental [Page 69] RFC 2522 Photuris Protocol March 1999

 secret-key is "all for one".

Operational Considerations

 The specification provides only a few configurable parameters, with
 defaults that should satisfy most situations.
 Retransmissions
    Default: 3.
 Initial Retransmission TimeOut (IRTO)
    Default: 5 seconds.
 Exchange TimeOut (ETO)
    Default: 30 seconds.  Minimum: Retransmissions * IRTO.
 Exchange LifeTime (ELT)
    Default: 30 minutes.  Minimum: 2 * ETO.
 SPI LifeTime (SPILT)
    Default: 5 minutes.  Minimum: 3 * ETO.
 Each party configures a list of known identities and symmetric
 secret-keys.
 In addition, each party configures local policy that determines what
 access (if any) is granted to the holder of a particular identity.
 For example, the party might allow anonymous FTP, but prohibit
 Telnet.  Such considerations are outside the scope of this document.

Security Considerations

 Photuris was based on currently available tools, by experienced
 network protocol designers with an interest in cryptography, rather
 than by cryptographers with an interest in network protocols.  This
 specification is intended to be readily implementable without
 requiring an extensive background in cryptology.
 Therefore, only minimal background cryptologic discussion and
 rationale is included in this document.  Although some review has
 been provided by the general cryptologic community, it is anticipated
 that design decisions and tradeoffs will be thoroughly analysed in
 subsequent dissertations and debated for many years to come.
 Cryptologic details are reserved for separate documents that may be
 more readily and timely updated with new analysis.

Karn & Simpson Experimental [Page 70] RFC 2522 Photuris Protocol March 1999

History

 The initial specification of Photuris, now called version 1 (December
 1994 to March 1995), was based on a short list of design
 requirements, and simple experimental code by Phil Karn.  Only one
 modular exponentiation form was used, with a single byte index of
 pre-specified group parameters.  The transform attributes were
 selected during the public value exchange.  Party privacy was
 protected in the identification signature exchange with standard ESP
 transforms.
 Upon submission for review by the IP Security Working Group, a large
 number of features were demanded.  A mere 254 future group choices
 were not deemed enough; it was expanded to two bytes (and renamed
 schemes), and was expanded again to carry variable parameters.  The
 transform attributes were made variable length to accomodate optional
 parameters.  Every other possible parameter was made negotiable.
 Some participants were unable to switch modes on the UDP sockets to
 use standard ESP transforms for only some messages, and party privacy
 was integrated into the protocol.  The message headers were
 reorganized, and selection of transform attributes was delayed until
 the identification exchange.  An additional update message phase was
 added.
 Version 2 (July 1995 to December 1995) specification stability was
 achieved in November 1995 by moving most parameters into separate
 documents for later discussion, and leaving only a few mandatory
 features in the base specification.  Within a month, multiple
 interoperable implementations were produced.
 Unfortunately, in a fit of demagoguery, the IP Security Working Group
 decided in a straw poll to remove party privacy protection, and the
 Working Group chair terminated the meeting without allowing further
 discussion.  Because the identification exchange messages required
 privacy to function correctly, the messages were reorganized again.
 Party privacy and other optional schemes were split into a separate
 document.
 The implementors established a separate discussion group.  Version 3
 (April 1996 to June 1997) enjoyed a long period of specification
 stability and multiple implementations on half a dozen platforms.
 Meanwhile, the IP Security Working Group has developed a competing
 specification with large numbers of negotiable parameters.  Also, the
 PPP Extensions Working Group has deployed link security transforms.
 Version 4 (July 1997 onward) attempts to maintain a semblance of
 interface compatibility with these other efforts.  Minor changes are

Karn & Simpson Experimental [Page 71] RFC 2522 Photuris Protocol March 1999

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

Acknowledgements

    Thou shalt make no law restricting the size of integers that may
    be multiplied together, nor the number of times that an integer
    may be multiplied by itself, nor the modulus by which an integer
    may be reduced.  [Prime Commandment]
 Phil Karn was principally responsible for the design of the protocol
 phases, particularly the "cookie" anti-clogging defense, developed
 the initial testing implementation, and provided much of the design
 rationale text (now removed to a separate document).
 William Simpson was responsible for the packet formats and
 attributes, additional message types, editing and formatting.  All
 such mistakes are his responsibility.
 This protocol was later discovered to have many elements in common
 with the Station-To-Station authentication protocol [DOW92].
 Angelos Keromytis developed the first completely independent
 implementation (circa October 1995).  Also, he suggested the cookie
 exchange rate limitation counter.
 Paul C van Oorschot suggested signing both the public exponents and
 the shared-secret, to provide an authentication-only version of
 identity verification.  Also, he provided text regarding moduli,
 generator, and exponent selection (now removed to a separate
 document).
 Hilarie Orman suggested adding secret "nonces" to session-key
 generation for asymmetric public/private-key identity methods (now
 removed to a separate document), and provided extensive review of the
 protocol details.
 Bart Preneel and Paul C van Oorschot in [PO96] recommended padding
 between the data and trailing key when hashing for authentication.
 Niels Provos developed another independent implementation (circa May
 1997), ported to AIX, Linux, OpenBSD, and Solaris.  Also, he made

Karn & Simpson Experimental [Page 72] RFC 2522 Photuris Protocol March 1999

 suggestions regarding automated update, and listing multiple moduli
 per scheme.
 Bill Sommerfeld suggested including the authentication symmetric
 secret-keys in the session-key generation, and using the Cookie
 values on successive exchanges to provide bi-directional user-
 oriented keying (now removed to a separate document).
 Oliver Spatscheck developed the second independent implementation
 (circa December 1995) for the Xkernel.
 International interoperability testing between early implementors
 provided the impetus for many of the implementation notes herein, and
 numerous refinements in the semantics of the protocol messages.
 Randall Atkinson, Steven Bellovin, Wataru Hamada, James Hughes, Brian
 LaMacchia, Cheryl Madson, Lewis McCarthy, Perry Metzger, Bob Quinn,
 Ron Rivest, Rich Schroeppel, and Norman Shulman provided useful
 critiques of earlier versions of this document.
 Special thanks to the Center for Information Technology Integration
 (CITI) for providing computing resources.

References

 [BGMW93]    E. Brickell, D. Gordon, K. McCurley, and D. Wilson, "Fast
             Exponentiation with Precomputation (Extended Abstract)",
             Advances in Cryptology -- Eurocrypt '92, Lecture Notes in
             Computer Science 658 (1993), Springer-Verlag, 200-207.
             Also U.S. Patent #5,299,262, E.F. Brickell, D.M. Gordon,
             K.S. McCurley, "Method for exponentiating in
             cryptographic systems", 29 Mar 1994.
 [DH76]      Diffie, W., and Hellman, H.E., "New Directions in
             Cryptography", IEEE Transactions on Information Theory, v
             IT-22 n 6 pp 644-654, November 1976.
 [DOW92]     Whitfield Diffie, Paul C van Oorshot, and Michael J
             Wiener, "Authentication and Authenticated Key Exchanges",
             Designs, Codes and Cryptography, v 2 pp 107-125, Kluwer
             Academic Publishers, 1992.
 [Firefly]   "Photuris" is the latin name for the firefly.  "Firefly"
             is in turn the name for the USA National Security
             Administration's (classified) key exchange protocol for
             the STU-III secure telephone.  Informed speculation has

Karn & Simpson Experimental [Page 73] RFC 2522 Photuris Protocol March 1999

             it that Firefly is based on very similar design
             principles.
 [LL94]      Lim, C.H., Lee, P.J., "More flexible exponentiation with
             precomputation", Advances in Cryptology -- Crypto '94,
             Lecture Notes in Computer Science 839 (1994), Springer-
             Verlag, pages 95-107.
 [Prime Commandment]
             A derivation of an apocryphal quote from the usenet list
             sci.crypt.
 [PO96]      Bart Preneel, and Paul C van Oorshot, "On the security of
             two MAC algorithms", Advances in Cryptology -- Eurocrypt
             '96, Lecture Notes in Computer Science 1070 (May 1996),
             Springer-Verlag, pages 19-32.
 [RFC-768]   Postel, J., "User Datagram Protocol", STD 6,
             USC/Information Sciences Institute, August 1980.
 [RFC-791]   Postel, J., "Internet Protocol", STD 5, USC/Information
             Sciences Institute, September 1981.
 [RFC-1321]  Rivest, R., "The MD5 Message-Digest Algorithm", MIT
             Laboratory for Computer Science, April 1992.
 [RFC-1700]  Reynolds, J., and Postel, J., "Assigned Numbers", STD 2,
             USC/Information Sciences Institute, October 1994.
 [RFC-1812]  Baker, F., Editor, "Requirements for IP Version 4
             Routers", Cisco Systems, June 1995.
 [RFC-1828]  Metzger, P., Simpson, W., "IP Authentication using Keyed
             MD5", July 1995.
 [RFC-1829]  Karn, P., Metzger, P., Simpson, W., "The ESP DES-CBC
             Transform", July 1995.
 [RFC-2119]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, Harvard University, March
             1997.
 [RFC-2521]  Karn, P., and Simpson, W., "ICMP Security Failures
             Messages", March 1999.
 [Rooij94]   P. de Rooij, "Efficient exponentiation using
             precomputation and vector addition chains", Advances in
             Cryptology -- Eurocrypt '94, Lecture Notes in Computer

Karn & Simpson Experimental [Page 74] RFC 2522 Photuris Protocol March 1999

             Science, Springer-Verlag, pages 403-415.
 [Schneier95]
             Schneier, B., "Applied Cryptography Second Edition", John
             Wiley & Sons, New York, NY, 1995.  ISBN 0-471-12845-7.

Contacts

 Comments about this document should be discussed on the
 photuris@adk.gr mailing list.
 Questions about this document can also be directed to:
    Phil Karn
    Qualcomm, Inc.
    6455 Lusk Blvd.
    San Diego, California  92121-2779
        karn@qualcomm.com
        karn@unix.ka9q.ampr.org (preferred)
    William Allen Simpson
    DayDreamer
    Computer Systems Consulting Services
    1384 Fontaine
    Madison Heights, Michigan  48071
        wsimpson@UMich.edu
        wsimpson@GreenDragon.com (preferred)

Karn & Simpson Experimental [Page 75] RFC 2522 Photuris Protocol March 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  Copyright (C) Philip Karn
 and William Allen Simpson (1994-1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards (in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed), or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 (BUT NOT LIMITED TO) ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Karn & Simpson Experimental [Page 76]

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