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rfc:rfc4895

Network Working Group M. Tuexen Request for Comments: 4895 Muenster Univ. of Applied Sciences Category: Standards Track R. Stewart

                                                                P. Lei
                                                   Cisco Systems, Inc.
                                                           E. Rescorla
                                                            RTFM, Inc.
                                                           August 2007
                     Authenticated Chunks for
          the Stream Control Transmission Protocol (SCTP)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document describes a new chunk type, several parameters, and
 procedures for the Stream Control Transmission Protocol (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.

Tuexen, et al. Standards Track [Page 1] RFC 4895 SCTP Authentication Chunk August 2007

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
 2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
 3.  New Parameter Types  . . . . . . . . . . . . . . . . . . . . .  4
   3.1.  Random Parameter (RANDOM)  . . . . . . . . . . . . . . . .  4
   3.2.  Chunk List Parameter (CHUNKS)  . . . . . . . . . . . . . .  5
   3.3.  Requested HMAC Algorithm Parameter (HMAC-ALGO) . . . . . .  6
 4.  New Error Cause  . . . . . . . . . . . . . . . . . . . . . . .  7
   4.1.  Unsupported HMAC Identifier Error Cause  . . . . . . . . .  7
 5.  New Chunk Type . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.1.  Authentication Chunk (AUTH)  . . . . . . . . . . . . . . .  8
 6.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   6.1.  Establishment of an Association Shared Key . . . . . . . . 10
   6.2.  Sending Authenticated Chunks . . . . . . . . . . . . . . . 11
   6.3.  Receiving Authenticated Chunks . . . . . . . . . . . . . . 12
 7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
 8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   8.1.  A New Chunk Type . . . . . . . . . . . . . . . . . . . . . 15
   8.2.  Three New Parameter Types  . . . . . . . . . . . . . . . . 15
   8.3.  A New Error Cause  . . . . . . . . . . . . . . . . . . . . 15
   8.4.  A New Table for HMAC Identifiers . . . . . . . . . . . . . 16
 9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
 10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
 11. Normative References . . . . . . . . . . . . . . . . . . . . . 17

Tuexen, et al. Standards Track [Page 2] RFC 4895 SCTP Authentication Chunk August 2007

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 Transport Layer Security (TLS), as defined in RFC 3436 [6],
 does not help because it only secures SCTP user data.
 Therefore, an SCTP extension that provides a mechanism for deriving
 shared keys for each association is presented.  These association
 shared keys are derived from endpoint pair shared keys, which are
 configured and might be empty, and data that is exchanged during the
 SCTP association setup.
 The extension presented in this document allows an SCTP sender to
 authenticate 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).
 The extension described in this document places the result of a
 Hashed Message Authentication Code (HMAC) computation before the data
 covered by that computation.  Placing it at the end of the packet
 would have required placing a control chunk after DATA chunks in case
 of authenticating DATA chunks.  This would break the rule that
 control chunks occur before DATA chunks in SCTP packets.  It should
 also be noted that putting the result of the HMAC computation after
 the data being covered would not allow sending the packet during the
 computation of the HMAC because the result of the HMAC computation is
 needed to compute the CRC32C checksum of the SCTP packet, which is
 placed in the common header of the SCTP packet.
 The SCTP extension for Dynamic Address Reconfiguration (ADD-IP)
 requires the usage of the extension described in this document.  The
 SCTP Partial Reliability Extension (PR-SCTP) can be used in
 conjunction with the extension described in this document.

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 RFC 2119 [3].

Tuexen, et al. Standards Track [Page 3] RFC 4895 SCTP Authentication Chunk August 2007

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) |
  +----------------+------------------------------------------------+
                                Table 1
 Note that the parameter format requires the receiver to ignore the
 parameter and continue processing if the parameter is not understood.
 This is accomplished (as described in RFC 2960 [5], 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 a random number of an arbitrary
 length.
  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                        /
 /                               +-------------------------------\
 |                               |           Padding             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                               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.

Tuexen, et al. Standards Track [Page 4] RFC 4895 SCTP Authentication Chunk August 2007

 Random Number: n bytes (unsigned integer)
    This value represents an arbitrary Random Number in network byte
    order.
 Padding: 0, 1, 2, or 3 bytes (unsigned integer)
    If the length of the Random Number is not a multiple of 4 bytes,
    the sender MUST pad the parameter with all zero bytes to make the
    parameter 32-bit aligned.  The Padding MUST NOT be longer than 3
    bytes and it MUST be ignored by the receiver.
 The RANDOM parameter MUST be included once in the INIT or INIT-ACK
 chunk, if the sender wants to send or receive authenticated chunks,
 to provide a 32-byte Random Number.  For 32-byte Random Numbers, the
 Padding is empty.

3.2. Chunk List Parameter (CHUNKS)

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

Tuexen, et al. Standards Track [Page 5] RFC 4895 SCTP Authentication Chunk August 2007

 Padding: 0, 1, 2, or 3 bytes (unsigned integer)
    If the number of Chunk Types is not a multiple of 4, the sender
    MUST pad the parameter with all zero bytes to make the parameter
    32-bit aligned.  The Padding MUST NOT be longer than 3 bytes and
    it MUST be ignored by the receiver.
 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.
  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.

Tuexen, et al. Standards Track [Page 6] RFC 4895 SCTP Authentication Chunk August 2007

 Padding: 0 or 2 bytes (unsigned integer)
    If the number of HMAC Identifiers is not even, the sender MUST pad
    the parameter with all zero bytes to make the parameter 32-bit
    aligned.  The Padding MUST be 0 or 2 bytes long and it MUST be
    ignored by the receiver.
 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.
 Table 2 shows the currently defined values for HMAC Identifiers.
            +-----------------+--------------------------+
            | HMAC Identifier | Message Digest Algorithm |
            +-----------------+--------------------------+
            | 0               | Reserved                 |
            | 1               | SHA-1 defined in [8]     |
            | 2               | Reserved                 |
            | 3               | SHA-256 defined in [8]   |
            +-----------------+--------------------------+
                                Table 2
 Every endpoint supporting SCTP chunk authentication MUST support the
 HMAC based on the SHA-1 algorithm.

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 has been
 received with an unsupported HMAC Identifier.

Tuexen, et al. Standards Track [Page 7] RFC 4895 SCTP Authentication Chunk August 2007

  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.
 Padding: 2 bytes (unsigned integer)
    The sender MUST pad the error cause with all zero bytes to make
    the cause 32-bit aligned.  The Padding MUST be 2 bytes long and it
    MUST be ignored by the receiver.

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.
             +------------+-----------------------------+
             | 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 RFC 2960
 [5], 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.

Tuexen, et al. Standards Track [Page 8] RFC 4895 SCTP Authentication Chunk August 2007

  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                              /
 /                                                               \
 /                               +-------------------------------\
 |                               |           Padding             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                               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.
 HMAC: n bytes (unsigned integer)
    This holds the result of the HMAC calculation.
 Padding: 0, 1, 2, or 3 bytes (unsigned integer)
    If the length of the HMAC is not a multiple of 4 bytes, the sender
    MUST pad the chunk with all zero bytes to make the chunk 32-bit
    aligned.  The Padding MUST NOT be longer than 3 bytes and it MUST
    be ignored by the receiver.
 The control chunk AUTH MUST NOT appear more than once in an SCTP
 packet.  All control and data chunks that 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.

Tuexen, et al. Standards Track [Page 9] RFC 4895 SCTP Authentication Chunk August 2007

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.  The Random Number
 should be generated in accordance with RFC 4086 [7].  If the Random
 Number is not 32 bytes, 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 RFC 2960 [5].  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 the SCTP endpoint 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 an 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
 that are byte vectors and pre-configured or established by another
 mechanism.  They are identified by the Shared Key Identifier.  For
 each endpoint pair shared key, an association shared key is computed.
 If there is no endpoint pair shared key, only one association shared
 key is computed by using an empty byte vector as the endpoint pair
 shared key.
 The RANDOM parameter, the CHUNKS parameter, and the HMAC-ALGO
 parameter sent by each endpoint are concatenated as byte vectors.
 These parameters include the parameter type, parameter length, and
 the parameter value, but padding is omitted; all padding MUST be
 removed from this concatenation before proceeding with further
 computation of keys.  Parameters that were not sent are simply
 omitted from the concatenation process.  The resulting two vectors
 are called the two key vectors.

Tuexen, et al. Standards Track [Page 10] RFC 4895 SCTP Authentication Chunk August 2007

 From the endpoint pair shared keys and the key vectors, the
 association shared keys are computed.  This is performed by selecting
 the numerically smaller key vector and concatenating it to the
 endpoint pair shared key, and then concatenating the numerically
 larger key vector to that.  If the key vectors are equal as numbers
 but differ in length, then the concatenation order is the endpoint
 shared key, followed by the shorter key vector, followed by the
 longer key vector.  Otherwise, the key vectors are identical, and may
 be concatenated to the endpoint pair key in any order.  The
 concatenation is performed on byte vectors, and all numerical
 comparisons use network byte order to convert the key vectors to a
 number.  The result of the concatenation is the association shared
 key.

6.2. Sending Authenticated Chunks

 Endpoints MUST send all requested chunks that have been authenticated
 where this has been requested by the peer.  The other chunks MAY be
 sent whether or not they have been authenticated.  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 pair shared key for the peer, it MUST
 use Shared Key Identifier zero with an empty endpoint pair shared
 key.  If there are multiple endpoint shared keys the sender selects
 one and uses the corresponding Shared Key Identifier.
 The sender MUST calculate the Message Authentication Code (MAC) (as
 described in RFC 2104 [2]) using the hash function H as described by
 the HMAC 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 the
 AUTH chunk with its HMAC field set to zero (as shown in Figure 6)
 followed by all the chunks that are placed after the AUTH chunk in
 the SCTP packet.

Tuexen, et al. Standards Track [Page 11] RFC 4895 SCTP Authentication Chunk August 2007

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type = 0x0F   |   Flags=0     |         Chunk Length          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Shared Key Identifier      |        HMAC Identifier        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 \                               0                               /
 /                               +-------------------------------\
 |                               |           Padding             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                               Figure 6
 Please note that all fields are in network byte order and that the
 field that will contain the complete HMAC is filled with zeroes.  The
 length of the field shown as zero 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 that 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 the 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

Tuexen, et al. Standards Track [Page 12] RFC 4895 SCTP Authentication Chunk August 2007

 same as given in the HMAC field, all the 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 not result in tearing down the association.
 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).
 When an endpoint requires COOKIE-ECHO chunks to be authenticated,
 some special procedures have to be followed because the reception of
 a COOKIE-ECHO chunk might result in the creation of an SCTP
 association.  If a packet arrives containing an AUTH chunk as a first
 chunk, a COOKIE-ECHO chunk as the second chunk, and possibly more
 chunks after them, and the receiver does not have an STCB for that
 packet, then authentication is based on the contents of the COOKIE-
 ECHO chunk.  In this situation, the receiver MUST authenticate the
 chunks in the packet by using the RANDOM parameters, CHUNKS
 parameters and HMAC_ALGO parameters obtained from the COOKIE-ECHO
 chunk, and possibly a local shared secret as inputs to the
 authentication procedure specified in Section 6.3.  If authentication
 fails, then the packet is discarded.  If the authentication is
 successful, the COOKIE-ECHO and all the chunks after the COOKIE-ECHO
 MUST be processed.  If the receiver has an 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 the chunks
 after it.
 If the receiver does not find an STCB for a packet containing an AUTH
 chunk as the first chunk and does not find 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.
 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 the 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

Tuexen, et al. Standards Track [Page 13] RFC 4895 SCTP Authentication Chunk August 2007

 procedure does not work, because the restarting endpoint most likely
 does not know the association shared key of the old association to be
 restarted.  However, if the restarting endpoint does know the old
 association shared key, he can successfully send the COOKIE-ECHO
 chunk in a way that it is accepted by the peer by using this old
 association shared key for the packet containing the AUTH chunk.
 After this operation, both endpoints have to use the new association
 shared key.
 If a server has an endpoint pair shared key with some clients, it can
 request the COOKIE_ECHO chunk to be authenticated and can ensure that
 only associations from clients with a correct endpoint pair shared
 key are accepted.
 Furthermore, it is important that the cookie contained in an INIT-ACK
 chunk and in a COOKIE-ECHO chunk MUST NOT contain any endpoint pair
 shared keys.

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.
  1. ——— INIT[RANDOM; CHUNKS; HMAC-ALGO] ———→

←—— INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] ———

  1. ——————- 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:
  1. ——— INIT[RANDOM; CHUNKS; HMAC-ALGO] ———→

←—— INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO] ———

  1. ————– COOKIE-ECHO; AUTH; DATA ————→

←—————- COOKIE-ACK; SACK ——————

 Please note that if the endpoint pair shared key depends on the
 client and the server, and is only known by the upper layer, this
 message exchange requires an upper layer intervention between the
 processing of the COOKIE-ECHO chunk and the processing of the AUTH
 and DATA chunk at the server side.  This intervention may be realized
 by a COMMUNICATION-UP notification followed by the presentation of

Tuexen, et al. Standards Track [Page 14] RFC 4895 SCTP Authentication Chunk August 2007

 the endpoint pair shared key by the upper layer to the SCTP stack,
 see for example Section 10 of RFC 2960 [5].  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

 This document (RFC 4895) is the reference for all registrations
 described in this section.  All registrations need to be listed in
 the document available at SCTP-parameters [9].  The changes are
 described below.

8.1. A New Chunk Type

 A chunk type for the AUTH chunk has been assigned by IANA.  IANA has
 assigned the value (15), as given in Table 4.  An additional line has
 been added in the "CHUNK TYPES" table of SCTP-parameters [9]:
 CHUNK TYPES
 ID Value    Chunk Type                                     Reference
 -----       ----------                                     ---------
 15          Authentication Chunk (AUTH)                    [RFC4895]

8.2. Three New Parameter Types

 Parameter types have been assigned for the RANDOM, CHUNKS, and HMAC-
 ALGO parameter by IANA.  The values are as given in Table 1.  This
 required two modifications to the "CHUNK PARAMETER TYPES" tables in
 SCTP-parameters [9]: the first 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
 been assigned.  The value (261) has been assigned as in Table 3.

Tuexen, et al. Standards Track [Page 15] RFC 4895 SCTP Authentication Chunk August 2007

 This requires an additional line of the "CAUSE CODES" table in SCTP-
 parameters [9]:
 VALUE            CAUSE CODE                               REFERENCE
 -----            ----------------                         ---------
 261 (0x0105)     Unsupported HMAC Identifier              [RFC4895]

8.4. A New Table for HMAC Identifiers

 HMAC Identifiers have to be maintained by IANA.  Four initial values
 have been assigned by IANA as described in Table 2.  This required a
 new table "HMAC IDENTIFIERS" in SCTP-parameters [9]:
 HMAC Identifier      Message Digest Algorithm             REFERENCE
 ---------------      ------------------------             ---------
 0                    Reserved                             [RFC4895]
 1                    SHA-1                                [RFC4895]
 2                    Reserved                             [RFC4895]
 3                    SHA-256                              [RFC4895]
 For registering a new HMAC Identifier with IANA, 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
 RFC 2104 [2] MUST be specified.  The "Specification Required" policy
 of RFC 2434 [4] MUST be applied.

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 the scope of
 this document.  Other mechanisms can be used, like using TLS or
 manual configuration.
 When an endpoint accepts COOKIE-ECHO chunks only in an authenticated
 way the restart procedure does not work.  Neither an attacker nor a
 restarted endpoint not knowing the association shared key can perform
 an restart.  However, if the association shared key is known, it is
 possible to restart the association.

Tuexen, et al. Standards Track [Page 16] RFC 4895 SCTP Authentication Chunk August 2007

 Because SCTP already has a built-in mechanism 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 for the peer using the weakest
 algorithm listed.
 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.

10. Acknowledgments

 The authors wish to thank David Black, Sascha Grau, Russ Housley,
 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]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
      Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
 [5]  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.
 [6]  Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport Layer
      Security over Stream Control Transmission Protocol", RFC 3436,
      December 2002.
 [7]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
      Requirements for Security", BCP 106, RFC 4086, June 2005.

Tuexen, et al. Standards Track [Page 17] RFC 4895 SCTP Authentication Chunk August 2007

 [8]  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>.
 [9]  <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
 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

Tuexen, et al. Standards Track [Page 18] RFC 4895 SCTP Authentication Chunk August 2007

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Tuexen, et al. Standards Track [Page 19]

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