tech-invite   World Map     

IETF     RFCs     Groups     SIP     ABNFs    |    3GPP     Specs     Glossaries     Architecture     IMS     UICC    |    search

RFC 2522

Experimental
Pages: 80
Top     in Index     Prev     Next
in Group Index     Prev in Group     Next in Group     Group: ~photuris

Photuris: Session-Key Management Protocol

Part 1 of 3, p. 1 to 25
None       Next RFC Part

 


Top       ToC       Page 1 
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.

Top       Page 2 
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

Top      ToC       Page 3 
        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

Top      ToC       Page 4 
           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

Top      ToC       Page 5 
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

Top      ToC       Page 6 
                    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.

Top      ToC       Page 7 
   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.

Top      ToC       Page 8 
      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]

Top      ToC       Page 9 
   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.

Top      ToC       Page 10 
   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

Top      ToC       Page 11 
   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

Top      ToC       Page 12 
   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].

Top      ToC       Page 13 
2.  Protocol Details

   The Initiator begins a Photuris exchange under several circumstances:

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

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

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

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

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

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


2.1.  UDP

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

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

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

Top      ToC       Page 14 
   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:

Top      ToC       Page 15 
                        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

Top      ToC       Page 16 
                    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.

Top      ToC       Page 17 
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

Top      ToC       Page 18 
   Attribute.  Multiple kinds of the same Attribute with varying Lengths
   MAY be present in any list of attributes.


3.  Cookie Exchange

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



3.0.1.  Send Cookie_Request

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

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

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

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

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

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

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

Top      ToC       Page 19 
      By default, the Initiator operates in the same manner as when all
      of its previous exchange state has expired.  The Responder will
      send a Resource_Limit when its own exchange state has not expired.

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

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


3.0.2.  Receive Cookie_Request

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

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

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

   Otherwise, the Responder returns a Cookie_Response.

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


3.0.3.  Send Cookie_Response

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

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

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

Top      ToC       Page 20 
   the Counter is incremented again, until a unique Counter value is
   reached.

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

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

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


3.0.4.  Receive Cookie_Response

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

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

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

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

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

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

   Implementation Notes:

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

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

Top      ToC       Page 21 
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.

Top      ToC       Page 22 
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.

Top      ToC       Page 23 
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.

Top      ToC       Page 24 
   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

Top      ToC       Page 25 
      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.




(page 25 continued on part 2)

Next RFC Part