Tech-invite3GPPspecsSIPRFCs
898887868584838281807978777675747372717069686766656463626160595857565554535251504948474645444342414039383736353433323130292827262524232221201918171615141312111009080706050403020100

in Index   Prev   Next

RFC 2522

Photuris: Session-Key Management Protocol

Pages: 80
Experimental
Part 1 of 3 – Pages 1 to 25
None   None   Next

Top   ToC   RFC2522 - 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   ToC   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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   RFC2522 - 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 Section