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Versions: 00 01 02 03 04 05 06 07 08 RFC 4895

Network Working Group                                          M. Tuexen
Internet-Draft                        Muenster Univ. of Applied Sciences
Intended status: Standards Track                              R. Stewart
Expires: April 26, 2007                                           P. Lei
                                                     Cisco Systems, Inc.
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                        October 23, 2006


  Authenticated Chunks for Stream Control Transmission Protocol (SCTP)
                   draft-ietf-tsvwg-sctp-auth-05.txt

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   This Internet-Draft will expire on April 26, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes a new chunk type, several parameters and
   procedures for SCTP.  This new chunk type can be used to authenticate
   SCTP chunks by using shared keys between the sender and receiver.
   The new parameters are used to establish the shared keys.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  New Parameter Types  . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Random Parameter (RANDOM)  . . . . . . . . . . . . . . . .  4
     3.2.  Chunk List Parameter (CHUNKS)  . . . . . . . . . . . . . .  4
     3.3.  Requested HMAC Algorithm Parameter (HMAC-ALGO) . . . . . .  5
   4.  New Error Cause  . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Unsupported HMAC Identifier error cause  . . . . . . . . .  7
   5.  New Chunk Type . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Authentication Chunk (AUTH)  . . . . . . . . . . . . . . .  8
   6.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Establishment of an association shared key . . . . . . . .  9
     6.2.  Sending authenticated chunks . . . . . . . . . . . . . . . 10
     6.3.  Receiving authenticated chunks . . . . . . . . . . . . . . 11
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  A New Chunk Type . . . . . . . . . . . . . . . . . . . . . 13
     8.2.  Three New Parameter Types  . . . . . . . . . . . . . . . . 14
     8.3.  A New Error Cause  . . . . . . . . . . . . . . . . . . . . 14
     8.4.  A New Table For HMAC Identifiers . . . . . . . . . . . . . 14
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   11. Normative References . . . . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 18
























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1.  Introduction

   SCTP uses 32 bit verification tags to protect itself against blind
   attackers.  These values are not changed during the lifetime of an
   SCTP association.

   Looking at new SCTP extensions there is the need to have a method of
   proving that an SCTP chunk(s) was really sent by the original peer
   that started the association and not by a malicious attacker.

   Using TLS as defined in RFC3436 [5] does not help here because it
   only secures SCTP user data.

   Therefore an SCTP extension is presented which provides a mechanism
   for deriving shared keys for each association.  These association
   shared keys are derived from endpoint pair shared keys, which are
   configured and might be empty, and data which is exchanged during the
   SCTP association setup.

   The extension presented in this document allows an SCTP sender to
   sign chunks using shared keys between the sender and receiver.  The
   receiver can then verify that the chunks are sent from the sender and
   not from a malicious attacker as long as the attacker does not know
   an association shared key.


2.  Conventions

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL", when they appear in this document, are to be interpreted
   as described in RFC2119 [3].


3.  New Parameter Types

   This section defines the new parameter types that will be used to
   negotiate the authentication during association setup.  Table 1
   illustrates the new parameter types.

    +----------------+------------------------------------------------+
    | Parameter Type | Parameter Name                                 |
    +----------------+------------------------------------------------+
    | 0x8002         | Random Parameter (RANDOM)                      |
    | 0x8003         | Chunk List Parameter (CHUNKS)                  |
    | 0x8004         | Requested HMAC Algorithm Parameter (HMAC-ALGO) |
    +----------------+------------------------------------------------+




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                                  Table 1

   It should be noted that the parameter format requires the receiver to
   ignore the parameter and continue processing if it is not understood.
   This is accomplished as described in RFC2960 [4] section 3.2.1. by
   the use of the upper bits of the parameter type.

3.1.  Random Parameter (RANDOM)

   This parameter is used to carry an arbitrary length random number.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Parameter Type = 0x8002   |       Parameter Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   \                          Random Number                        /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 1

   Parameter Type: 2 bytes (unsigned integer)
      This value MUST be set to 0x8002.

   Parameter Length: 2 bytes (unsigned integer)
      This value is the length of the Random Number in bytes plus 4.

   Random Number: n bytes (unsigned integer)
      This value represents an arbitrary Random Number in network byte
      order.

   The RANDOM parameter MUST be included once in the INIT or INIT-ACK
   chunk if the sender wants to send or receive authenticated chunks.

3.2.  Chunk List Parameter (CHUNKS)

   This parameter is used to specify which chunk types are required to
   be sent authenticated by the peer.











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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Parameter Type = 0x8003   |       Parameter Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Chunk Type 1  | Chunk Type 2  | Chunk Type 3  | Chunk Type 4  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               /
   \                              ...                              \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Chunk Type n  |                   Padding                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 2

   Parameter Type: 2 bytes (unsigned integer)
      This value MUST be set to 0x8003.

   Parameter Length: 2 bytes (unsigned integer)
      This value is the number of listed Chunk Types plus 4.

   Chunk Type n: 1 byte (unsigned integer)
      Each Chunk Type listed is required to be authenticated when sent
      by the peer.

   The CHUNKS parameter MUST be included once in the INIT or INIT-ACK
   chunk if the sender wants to receive authenticated chunks.  Its
   maximum length is 260 bytes.

   The chunk types for INIT, INIT-ACK, SHUTDOWN-COMPLETE and AUTH chunks
   MUST NOT be listed in the CHUNKS parameter.  However, if a CHUNKS
   parameter is received then the types for INIT, INIT-ACK, SHUTDOWN-
   COMPLETE and AUTH chunks MUST be ignored.

3.3.  Requested HMAC Algorithm Parameter (HMAC-ALGO)

   This parameter is used to list the HMAC identifiers the peer MUST
   use.












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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Parameter Type = 0x8004   |       Parameter Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          HMAC Identifier 1    |      HMAC Identifier 2        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               /
   \                              ...                              \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        HMAC Identifier n      |           Padding             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 3

   Parameter Type: 2 bytes (unsigned integer)
      This value MUST be set to 0x8004.

   Parameter Length: 2 bytes (unsigned integer)
      This value is the number of HMAC identifiers multiplied by 2 plus
      4.

   HMAC Identifier n: 2 bytes (unsigned integer)
      The values expressed are a list of HMAC identifiers that may be
      used by the peer.  The values are listed by preference, with
      respect to the sender, where the first HMAC identifier listed is
      the one most preferable to the sender.

   The HMAC-ALGO parameter MUST be included once in the INIT or INIT-ACK
   chunk if the sender wants to send or receive authenticated chunks.

   The following Table 2 shows the currently defined values for HMAC
   identifiers.

              +-----------------+--------------------------+
              | HMAC Identifier | Message Digest Algorithm |
              +-----------------+--------------------------+
              | 0               | Reserved                 |
              | 1               | SHA-1 defined in [6]     |
              | 3               | SHA-256 defined in [6]   |
              +-----------------+--------------------------+

                                  Table 2

   Every endpoint supporting SCTP chunk authentication MUST support the
   HMAC based on the SHA-1 algorithm.




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4.  New Error Cause

   This section defines a new error cause that will be sent if an AUTH
   chunk is received with an unsupported HMAC identifier.  Table 3
   illustrates the new error cause.

               +------------+-----------------------------+
               | Cause Code | Error Cause Name            |
               +------------+-----------------------------+
               | 0x0105     | Unsupported HMAC Identifier |
               +------------+-----------------------------+

                                  Table 3

4.1.  Unsupported HMAC Identifier error cause

   This error cause is used to indicate that an AUTH chunk was received
   with an unsupported HMAC Identifier.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Cause Code = 0x0105      |       Cause Length = 6        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         HMAC Identifier       |            Padding            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 4

   Cause Code: 2 bytes (unsigned integer)
      This value MUST be set to 0x0105.

   Cause Length: 2 bytes (unsigned integer)
      This value MUST be set to 6.

   HMAC Identifier: 2 bytes (unsigned integer)
      This value is the HMAC Identifier which is not supported.


5.  New Chunk Type

   This section defines the new chunk type that will be used to
   authenticate chunks.  Table 4 illustrates the new chunk type.








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               +------------+-----------------------------+
               | Chunk Type | Chunk Name                  |
               +------------+-----------------------------+
               | 0x0F       | Authentication Chunk (AUTH) |
               +------------+-----------------------------+

                                  Table 4

   It should be noted that the AUTH-chunk format requires the receiver
   to ignore the chunk if it is not understood and silently discard all
   chunks that follow.  This is accomplished as described in RFC2960 [4]
   section 3.2. by the use of the upper bits of the chunk type.

5.1.  Authentication Chunk (AUTH)

   This chunk is used to hold the result of the HMAC calculation.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type = 0x0F   |   Flags=0     |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Shared Key Identifier      |        HMAC Identifier        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   \                             HMAC                              /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 5

   Type: 1 byte (unsigned integer)
      This value MUST be set to 0x0F for all AUTH-chunks.

   Flags: 1 byte (unsigned integer)
      SHOULD be set to zero on transmit and MUST be ignored on receipt.

   Length: 2 bytes (unsigned integer)
      This value holds the length of the HMAC in bytes plus 8.

   Shared Key Identifier: 2 bytes (unsigned integer)
      This value describes which endpoint pair shared key is used.

   HMAC Identifier: 2 bytes (unsigned integer)
      This value describes which message digest is being used.  Table 2
      shows the currently defined values.





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   HMAC: n bytes (unsigned integer)
      This hold the result of the HMAC calculation.

   The control chunk AUTH MUST NOT appear more than once in an SCTP
   packet.  All control and data chunks which are placed after the AUTH
   chunk in the packet are sent in an authenticated way.  Those chunks
   placed in a packet before the AUTH chunk are not authenticated.
   Please note that DATA chunks can not appear before control chunks in
   an SCTP packet.


6.  Procedures

6.1.  Establishment of an association shared key

   An SCTP endpoint willing to receive or send authenticated chunks MUST
   send one RANDOM parameter in its INIT or INIT-ACK chunk.  The RANDOM
   parameter MUST contain a 32 byte random number.  If the random number
   is not 32 byte long the association MUST be aborted.  The ABORT chunk
   SHOULD contain the error cause 'Protocol Violation'.  In case of INIT
   collision, the rules governing the handling of this random number
   follow the same pattern as those for the Verification Tag, as
   explained in section 5.2.4 of RFC2960 [4].  Therefore each endpoint
   knows its own random number and the peer's random number after the
   association has been established.

   An SCTP endpoint has a list of chunks it only accepts if they are
   received in an authenticated way.  This list is included in the INIT
   and INIT-ACK and MAY be omitted if it is empty.  Since this list does
   not change during the lifetime of there is no problem in case of INIT
   collision.

   Each SCTP endpoint MUST include in the INIT and INIT-ACK a HMAC-ALGO
   parameter containing a list of HMAC Identifiers it requests the peer
   to use.  The receiver of a HMAC-ALGO parameter SHOULD use the first
   listed algorithm it supports.  The HMAC algorithm based on SHA-1 MUST
   be supported and included in the HMAC-ALGO parameter.  An SCTP
   endpoint MUST NOT change the parameters listed in the HMAC-ALGO
   parameter during the lifetime of the endpoint.

   Both endpoints of an association MAY have endpoint pair shared keys
   which are byte vectors and pre-configured or established by another
   mechanism.  They are identified by the shared key identifier.  If no
   endpoint pair shared keys are preconfigured or established by another
   mechanism an empty byte vector is used.

   The random number value, the list of chunks and the list of HMAC
   identifiers in network byte order sent by each endpoint are



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   concatenated as byte vectors.  The resulting two vectors are called
   the two key numbers.

   From the endpoint pair shared keys and the key numbers the
   association shared keys are computed.  This is performed by selecting
   the smaller key number and concatenating it to the endpoint pair
   shared key, and then concatenating the larger of the key numbers to
   that.  If both key numbers are equal, then the concatenation order is
   the endpoint shared key, followed by the key number with the shorter
   length, followed by the key number with the longer length.  If the
   key number lengths are the same, then they may be concatenated to the
   endpoint pair key in any order.  The concatenation is performed on
   byte vectors representing all numbers in network byte order.  The
   result is the association shared key.

6.2.  Sending authenticated chunks

   Endpoints MUST send all requested chunks authenticated where this has
   been requested by the peer.  The other chunks MAY be sent
   authenticated or not.  If endpoint pair shared keys are used, one of
   them MUST be selected for authentication.

   To send chunks in an authenticated way, the sender MUST include these
   chunks after an AUTH chunk.  This means that a sender MUST bundle
   chunks in order to authenticate them.

   If the endpoint has no endpoint shared key for the peer, it MUST use
   Shared Key Identifier 0 with an empty endpoint pair shared key.

   The sender MUST calculate the MAC as described in RFC2104 [2] using
   the hash function H as described by the MAC Identifier and the shared
   association key K based on the endpoint pair shared key described by
   the shared key identifier.  The 'data' used for the computation of
   the AUTH-chunk is given by Figure 6 and all chunks that are placed
   after the AUTH chunk in the SCTP packet.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type = 0x0F   |   Flags=0     |         Chunk Length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Shared Key Identifier      |        HMAC Identifier        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   \                               0                               /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 6




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   Please note that all fields are in network byte order and that the
   field which will contain the complete HMAC is filled with zeroes.
   The length of the field shown as 0 is the length of the HMAC
   described by the HMAC Identifier.  The padding of all chunks being
   authenticated MUST be included in the HMAC computation.

   The sender fills the HMAC into the HMAC field and sends the packet.

6.3.  Receiving authenticated chunks

   The receiver has a list of chunk types which it expects to be
   received only after an AUTH-chunk.  This list has been sent to the
   peer during the association setup.  It MUST silently discard these
   chunks if they are not placed after an AUTH chunk in the packet.

   The receiver MUST use the HMAC algorithm indicated in the HMAC
   Identifier field.  If this algorithm was not specified by the
   receiver in the HMAC-ALGO parameter in the INIT or INIT-ACK chunk
   during association setup, the AUTH chunk and all chunks after it MUST
   be discarded and an ERROR chunk SHOULD be sent with the error cause
   defined in Section 4.1.

   If an endpoint with no shared key receives a Shared Key Identifier
   other than 0, it MUST silently discard all authenticated chunks.  If
   the endpoint has at least one endpoint pair shared key for the peer,
   it MUST use the key specified by the Shared Key Identifier if a key
   has been configured for that Shared Key Identifier.  If no endpoint
   pair shared key has been configured for that Shared Key Identifier,
   all authenticated chunks MUST be silently discarded.

   The receiver now performs the same calculation as described for the
   sender based on Figure 6.  If the result of the calculation is the
   same as given in the HMAC field, all chunks following the AUTH chunk
   are processed.  If the field does not match the result of the
   calculation, all the chunks following the AUTH chunk MUST be silently
   discarded.

   It should be noted that if the receiver wants to tear down an
   association in an authenticated way only, the handling of malformed
   packets should be in tune with this.

   An SCTP implementation has to maintain state for each SCTP
   association.  In the following we call this data structure the SCTP
   transmission control block (STCB).

   If the receiver does not find a STCB for a packet containing an AUTH
   chunk as a first chunk and a COOKIE-ECHO chunk as the second chunk
   and possibly more chunks after them, the receiver MUST authenticate



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   the chunks by using the random numbers included in the COOKIE-ECHO,
   and possibly the local shared secret.  If authentication fails then
   the packet discarded.  If the authentication is successful the
   COOKIE-ECHO and all chunks after the COOKIE-ECHO MUST be processed.
   If the receiver has a STCB, it MUST process the AUTH chunk as
   described above using the STCB from the existing association to
   authenticate the COOKIE-ECHO chunk and all chunks after it.

   If the receiver does not find a STCB for a packet containing an AUTH
   chunk as the first chunk and not a COOKIE-ECHO chunk as the second
   chunk, it MUST use the chunks after the AUTH chunk to look up an
   existing association.  If no association is found, the packet MUST be
   considered as out of the blue.  The out of the blue handling MUST be
   based on the packet without taking the AUTH chunk into account.  If
   an association is found, it MUST process the AUTH chunk using the
   STCB from the existing association as described earlier.

   If the receiver of the packet does not have a STCB when it needs to
   process the AUTH chunk, it MUST ignore the AUTH chunk.  This applies
   to a packet containing an AUTH chunk as a first chunk and an COOKIE-
   ECHO chunk as the second chunk received in the CLOSED state.  If the
   receiver has a STCB, it MUST process the AUTH chunk as described
   above.

   Requiring ABORT chunks and COOKIE-ECHO chunks to be authenticated
   makes it impossible for an attacker to bring down or restart an
   association as long as the attacker does not know an association
   shared key.  But it should also be noted that if an endpoint accepts
   ABORT chunks only in an authenticated way, it may take longer to
   detect that the peer is no longer available.  If an endpoint accepts
   COOKIE-ECHO chunks only in an authenticated way, the restart
   procedure does not work.

   Furthermore it is important that the cookie contained in an INIT-ACK
   chunk and in a COOKIE-ECHO chunk MUST NOT contain the end-point pair
   shared key.


7.  Examples

   This section gives examples of message exchanges for association
   setup.

   The simplest way of using the extension described in this document is
   given by the following message exchange.






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       ---------- INIT[RANDOM; CHUNKS; HMAC-ALGO] ---------->
       <------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] ---------
       -------------------- COOKIE-ECHO -------------------->
       <-------------------- COOKIE-ACK ---------------------

   Please note that the CHUNKS parameter is optional in the INIT and
   INIT-ACK.

   If the server wants to receive DATA chunks in an authenticated way,
   the following message exchange is possible:

       ---------- INIT[RANDOM; CHUNKS; HMAC-ALGO] ---------->
       <------- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] ---------
       --------------- COOKIE-ECHO; AUTH; DATA ------------->
       <----------------- COOKIE-ACK; SACK ------------------

   Please note that if the endpoint pair shared key depends on the
   client and the server and that it is only known by the upper layer
   this message exchange requires an upper layer intervention between
   the processing of the COOKIE-ECHO chunk (COMMUNICATION-UP
   notification followed by the presentation of the endpoint pair shared
   key by the upper layer to the SCTP stack) and the processing of the
   AUTH and DATA chunk at the server side.  If this intervention is not
   possible due to limitations of the API (for example the socket API)
   the server might discard the AUTH and DATA chunk making a
   retransmission of the DATA chunk necessary.  If the same endpoint
   pair shared key is used for multiple endpoints and does not depend on
   the client this intervention might not be necessary.


8.  IANA Considerations

   [NOTE to RFC-Editor:

      "RFCXXXX" is to be replaced by the RFC number you assign this
      document.

   ]

   This document (RFCXXX) is the reference for all registrations
   described in this section.  All registrations need to be listed in
   the document available at sctp-parameters [7].  The suggested changes
   are described below.

8.1.  A New Chunk Type

   A chunk type for the AUTH chunk has to be assigned by IANA.  It is
   suggested to use the value given in Table 4.  This requires an



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   additional line in the "CHUNK TYPES" table of sctp-parameters [7]:

   CHUNK TYPES

   ID Value    Chunk Type                                     Reference
   -----       ----------                                     ---------
   15          Authentication Chunk (AUTH)                    [RFCXXXX]

8.2.  Three New Parameter Types

   Parameter types have to be assigned for the RANDOM, CHUNKS, and HMAC-
   ALGO parameter by IANA.  It is suggested to use the values given in
   Table 1.  This requires two modifications of the "CHUNK PARAMETER
   TPYES" tables in sctp-parameters [7]: The first change is the
   addition of three new lines to the "INIT Chunk Parameter Types"
   table:

   Chunk Parameter Type                       Value
   --------------------                       -----
   Random                             32770 (0x8002)
   Chunk List                         32771 (0x8003)
   Requested HMAC Algorithm Parameter 32772 (0x8004)

   The second required change is the addition of the same three lines to
   the to the "INIT ACK Chunk Parameter Types" table.

8.3.  A New Error Cause

   An error cause for the Unsupported HMAC Identifier error cause has to
   be assigned.  It is suggested to use the value given in Table 3.
   This requires an additional line of the "CAUSE CODES" table in sctp-
   parameters [7]:

   VALUE            CAUSE CODE                               REFERENCE
   -----            ----------------                         ---------
   261 (0x0105)     Unsupported HMAC Identifier              RFCXXXX

8.4.  A New Table For HMAC Identifiers

   HMAC Identifiers have to be maintained by IANA.  Three initial values
   should be assigned by IANA as described in Table 2.  This requires a
   new table "HMAC IDENTIFIERS" in sctp-parameters [7]:

   HMAC Identifier      Message Digest Algorithm             REFERENCE
   ---------------      ------------------------             ---------
   0                    Reserved                             RFCXXXX
   1                    SHA-1                                RFCXXXX
   3                    SHA-256                              RFCXXXX



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   For registering at IANA a new HMAC Identifier in this table a request
   has to be made to assign such a number.  This number must be unique
   and a message digest algorithm usable with the HMAC defined in
   RFC2104 [2] MUST be specified.


9.  Security Considerations

   Without using endpoint shared keys this extension only protects
   against modification or injection of authenticated chunks by
   attackers who did not capture the initial handshake setting up the
   SCTP association.

   If an endpoint pair shared key is used even a true man in the middle
   cannot inject chunks which are required to be authenticated even if
   he intercepts the initial message exchange.  The endpoint also knows
   that it is accepting authenticated chunks from a peer who knows the
   endpoint pair shared key.

   The establishment of endpoint pair shared keys is out of scope of
   this document.  Other mechanisms can be used like using TLS or manual
   configuration.

   Because SCTP has already a mechanism built-in that handles the
   reception of duplicated chunks, the presented solution makes use of
   this functionality and does not provide a method to avoid replay
   attacks by itself.  Of course, this only works within each SCTP
   association.  Therefore a separate shared key is used for each SCTP
   association to handle replay attacks covering multiple SCTP
   associations.

   Each endpoint presenting a list of more than one element in the HMAC-
   ALGO parameter must be prepared that the peer uses the weakest
   algorithm listed.

   If an endpoint requests the authentication of some chunks using the
   CHUNKS parameter and an attacker intercepts the handshake used to
   setup the association and modifies or removes this CHUNKS parameter
   this endpoint will not accept chunks which are authenticated or needs
   to be authenticated and are not.  This might result in the failure of
   the association.

   When an endpoint pair uses non-NULL endpoint pair shared keys and one
   of the endpoints still accepts a NULL key an attacker who captured
   the initial handshake can still inject or modify authenticated chunks
   by using the NULL key.





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10.  Acknowledgments

   The authors wish to thank Sascha Grau, Ivan Arias Rodriguez, Irene
   Ruengeler, and Magnus Westerlund for their invaluable comments.


11.  Normative References

   [1]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
        April 1992.

   [2]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
        for Message Authentication", RFC 2104, February 1997.

   [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [4]  Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
        H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V. Paxson,
        "Stream Control Transmission Protocol", RFC 2960, October 2000.

   [5]  Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport Layer
        Security over Stream Control Transmission Protocol", RFC 3436,
        December 2002.

   [6]  National Institute of Standards and Technology, "Secure Hash
        Standard", FIPS PUB 180-2, August 2002,
        <http://csrc.nist.gov/publications/fips/fips180-2/
        fips180-2.pdf>.

   [7]  <http://www.iana.org/assignments/sctp-parameters>


Authors' Addresses

   Michael Tuexen
   Muenster Univ. of Applied Sciences
   Stegerwaldstr. 39
   48565 Steinfurt
   Germany

   Email: tuexen@fh-muenster.de









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   Randall R. Stewart
   Cisco Systems, Inc.
   4875 Forest Drive
   Suite 200
   Columbia, SC  29206
   USA

   Email: rrs@cisco.com


   Peter Lei
   Cisco Systems, Inc.
   8735 West Higgins Road
   Suite 300
   Chicago, IL  60631
   USA

   Phone:
   Email: peterlei@cisco.com


   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA 94303
   USA

   Phone: +1 650-320-8549
   Email: ekr@rtfm.com






















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