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RFC 4543 - The Use of Galois Message Authentication Code (GMAC)

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Network Working Group                                          D. McGrew
Request for Comments: 4543                           Cisco Systems, Inc.
Category: Standards Track                                       J. Viega
                                                            McAfee, Inc.
                                                                May 2006

        The Use of Galois Message Authentication Code (GMAC) in
                            IPsec ESP and AH

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.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This memo describes the use of the Advanced Encryption Standard (AES)
   Galois Message Authentication Code (GMAC) as a mechanism to provide
   data origin authentication, but not confidentiality, within the IPsec
   Encapsulating Security Payload (ESP) and Authentication Header (AH).
   GMAC is based on the Galois/Counter Mode (GCM) of operation, and can
   be efficiently implemented in hardware for speeds of 10 gigabits per
   second and above, and is also well-suited to software

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................3
   2. AES-GMAC ........................................................3
   3. The Use of AES-GMAC in ESP ......................................3
      3.1. Initialization Vector ......................................4
      3.2. Nonce Format ...............................................4
      3.3. AAD Construction ...........................................5
      3.4. Integrity Check Value (ICV) ................................6
      3.5. Differences with AES-GCM-ESP ...............................6
      3.6. Packet Expansion ...........................................7
   4. The Use of AES-GMAC in AH .......................................7
   5. IKE Conventions .................................................8
      5.1. Phase 1 Identifier .........................................8
      5.2. Phase 2 Identifier .........................................8
      5.3. Key Length Attribute .......................................9
      5.4. Keying Material and Salt Values ............................9
   6. Test Vectors ....................................................9
   7. Security Considerations ........................................10
   8. Design Rationale ...............................................11
   9. IANA Considerations ............................................11
   10. Acknowledgements ..............................................11
   11. References ....................................................12
      11.1. Normative References .....................................12
      11.2. Informative References ...................................12
1.  Introduction

   This document describes the use of AES-GMAC mode (AES-GMAC) as a
   mechanism for data origin authentication in ESP [RFC4303] and AH
   [RFC4302].  We refer to these methods as ENCR_NULL_AUTH_AES_GMAC and
   AUTH_AES_GMAC, respectively.  ENCR_NULL_AUTH_AES_GMAC is a companion
   to the AES Galois/Counter Mode ESP [RFC4106], which provides
   authentication as well as confidentiality.  ENCR_NULL_AUTH_AES_GMAC
   is intended for cases in which confidentiality is not desired.  Like
   GCM, GMAC is efficient and secure, and is amenable to high-speed
   implementations in hardware.  ENCR_NULL_AUTH_AES_GMAC and
   AUTH_AES_GMAC are designed so that the incremental cost of
   implementation, given an implementation is AES-GCM-ESP, is small.

   This document does not cover implementation details of GCM or GMAC.
   Those details can be found in [GCM], along with test vectors.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].


   GMAC is a block cipher mode of operation providing data origin
   authentication.  It is defined in terms of the GCM authenticated
   encryption operation as follows.  The GCM authenticated encryption
   operation has four inputs: a secret key, an initialization vector
   (IV), a plaintext, and an input for additional authenticated data
   (AAD).  It has two outputs, a ciphertext whose length is identical to
   the plaintext and an authentication tag.  GMAC is the special case of
   GCM in which the plaintext has a length of zero.  The (zero-length)
   ciphertext output is ignored, of course, so that the only output of
   the function is the Authentication Tag.  In the following, we
   describe how the GMAC IV and AAD are formed from the ESP and AH
   fields, and how the ESP and AH packets are formed from the
   Authentication Tag.

   Below we refer to the AES-GMAC IV input as a nonce, in order to
   distinguish it from the IV fields in the packets.  The same nonce and
   key combination MUST NOT be used more than once, since reusing a
   nonce/key combination destroys the security guarantees of AES-GMAC.

   Because of this restriction, it can be difficult to use this mode
   securely when using statically configured keys.  For the sake of good
   security, implementations MUST use an automated key management
   system, such as the Internet Key Exchange (IKE) (either version two
   [RFC4306] or version one [RFC2409]), to ensure that this requirement
   is met.

3.  The Use of AES-GMAC in ESP

   The AES-GMAC algorithm for ESP is defined as an ESP "combined mode"
   algorithm (see Section 3.2.3 of [RFC4303]), rather than an ESP
   integrity algorithm.  It is called ENCR_NULL_AUTH_AES_GMAC to
   highlight the fact that it performs no encryption and provides no

      Rationale: ESP makes no provision for integrity transforms to
      place an initialization vector within the Payload field; only
      encryption transforms are expected to use IVs.  Defining GMAC as
      an encryption transform avoids this issue, and allows GMAC to
      benefit from the same pipelining as does GCM.

   Like all ESP combined modes, it is registered in IKEv2 as an
   encryption transform, or "Type 1" transform.  It MUST NOT be used in
   conjunction with any other ESP encryption transform (within a
   particular ESP encapsulation).  If confidentiality is desired, then
   GCM ESP [RFC4106] SHOULD be used instead.

3.1.  Initialization Vector

   With ENCR_NULL_AUTH_AES_GMAC, an explicit Initialization Vector (IV)
   is included in the ESP Payload, at the outset of that field.  The IV
   MUST be eight octets long.  For a given key, the IV MUST NOT repeat.
   The most natural way to meet this requirement is to set the IV using
   a counter, but implementations are free to set the IV field in any
   way that guarantees uniqueness, such as a linear feedback shift
   register (LFSR).  Note that the sender can use any IV generation
   method that meets the uniqueness requirement without coordinating
   with the receiver.

3.2.  Nonce Format

   The nonce passed to the AES-GMAC authentication algorithm has the
   following layout:

    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
   |                             Salt                              |
   |                     Initialization Vector                     |
   |                                                               |

                        Figure 1: Nonce Format

   The components of the nonce are as follows:

      The salt field is a four-octet value that is assigned at the
      beginning of the security association, and then remains constant
      for the life of the security association.  The salt SHOULD be
      unpredictable (i.e., chosen at random) before it is selected, but
      need not be secret.  We describe how to set the salt for a
      Security Association established via the Internet Key Exchange in
      Section 5.4.

   Initialization Vector
      The IV field is described in Section 3.1.

3.3.  AAD Construction

   Data integrity and data origin authentication are provided for the
   SPI, (Extended) Sequence Number, Authenticated Payload, Padding, Pad
   Length, and Next Header fields.  This is done by including those
   fields in the AES-GMAC Additional Authenticated Data (AAD) field.
   Two formats of the AAD are defined: one for 32-bit sequence numbers,
   and one for 64-bit extended sequence numbers.  The format with 32-bit
   sequence numbers is shown in Figure 2, and the format with 64-bit
   extended sequence numbers is shown in Figure 3.

    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
   |                               SPI                             |
   |                     32-bit Sequence Number                    |
   |                                                               |
   ~                Authenticated Payload (variable)               ~
   |                                                               |
   |                    Padding (0-255 bytes)                      |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |  Pad Length   | Next Header   |

            Figure 2: AAD Format with 32-bit Sequence 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
   |                               SPI                             |
   |                 64-bit Extended Sequence Number               |
   |                                                               |
   |                                                               |
   ~                Authenticated Payload (variable)               ~
   |                                                               |
   |                    Padding (0-255 bytes)                      |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |  Pad Length   | Next Header   |

        Figure 3: AAD Format with 64-bit Extended Sequence Number

   The use of 32-bit sequence numbers vs. 64-bit extended sequence
   numbers is determined by the security association (SA) management
   protocol that is used to create the SA.  For IKEv2 [RFC4306] this is
   negotiated via Transform Type 5, and the default for ESP is to use
   64-bit extended sequence numbers in the absence of negotiation (e.g.,
   see Section 2.2.1 of [RFC4303]).

3.4.  Integrity Check Value (ICV)

   The ICV consists solely of the AES-GMAC Authentication Tag.  The
   Authentication Tag MUST NOT be truncated, so the length of the ICV is
   16 octets.

3.5.  Differences with AES-GCM-ESP

   In this section, we highlight the differences between this
   specification and AES-GCM-ESP [RFC4106].  The essential difference is
   that in this document, the AAD consists of the SPI, Sequence Number,
   and ESP Payload, and the AES-GCM plaintext is zero-length, while in
   AES-GCM-ESP, the AAD consists only of the SPI and Sequence Number,
   and the AES-GCM plaintext consists of the ESP Payload.  These
   differences are illustrated in Figure 4.  This figure shows the case
   in which the Extended Sequence Number option is not used.  When that
   option is exercised, the Sequence Number field in the figure would be
   replaced with the Extended Sequence Number.

   Importantly, ENCR_NULL_AUTH_AES_GMAC is *not* equivalent to AES-GCM-
   ESP with encryption "turned off".  However, the ICV computations
   performed in both cases are similar because of the structure of the
   GHASH function [GCM].

                     +-> +-----------------------+ <-+
      AES-GCM-ESP    |   |          SPI          |   |
          AAD -------+   +-----------------------+   |
                     |   |    Sequence Number    |   |
                     +-> +-----------------------+   |
                         |    Authentication     |   |
                         |          IV           |   |
                  +->+-> +-----------------------+   +
      AES-GCM-ESP |      |                       |   |
       Plaintext -+      ~       ESP Payload     ~   |
                  |      |                       |   |
                  |      +-----------+-----+-----+   |
                  |      | Padding   |  PL | NH  |   |
                  +----> +-----------+-----+-----+ <-+
                       ENCR_NULL_AUTH_AES_GMAC AAD --+

   Figure 4: Differences between ENCR_NULL_AUTH_AES_GMAC and AES-GCM-ESP

3.6.  Packet Expansion

   The IV adds an additional eight octets to the packet and the ICV adds
   an additional 16 octets.  These are the only sources of packet
   expansion, other than the 10-13 bytes taken up by the ESP SPI,
   Sequence Number, Padding, Pad Length, and Next Header fields (if the
   minimal amount of padding is used).

4.  The Use of AES-GMAC in AH

   In AUTH_AES_GMAC, the AH Authentication Data field consists of the IV
   and the Authentication Tag, as shown in Figure 5.  Unlike the usual
   AH case, the Authentication Data field contains both an input to the
   authentication algorithm (the IV) and the output of the
   authentication algorithm (the tag).  No padding is required in the
   Authentication Data field, because its length is a multiple of 64

    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
   |                    Initialization Vector (IV)                 |
   |                            (8 octets)                         |
   |                                                               |
   |             Integrity Check Value (ICV) (16 octets)           |
   |                                                               |
   |                                                               |

       Figure 5: The AUTH_AES_GMAC Authentication Data Format

   The IV is as described in Section 3.1.  The Integrity Check Value
   (ICV) is as described in Section 3.4.

   The GMAC Nonce input is formed as described in Section 3.2.  The GMAC
   AAD input consists of the authenticated data as defined in Section
   3.1 of [RFC4302].  These values are provided as to that algorithm,
   along with the secret key, and the resulting authentication tag given
   as output is used to form the ICV.

5.  IKE Conventions

   This section describes the conventions used to generate keying
   material and salt values for use with ENCR_NULL_AUTH_AES_GMAC and
   AUTH_AES_GMAC using the Internet Key Exchange (IKE) versions one
   [RFC2409] and two [RFC4306].

5.1.  Phase 1 Identifier

   This document does not specify the conventions for using AES-GMAC for
   IKE Phase 1 negotiations.  For AES-GMAC to be used in this manner, a
   separate specification would be needed, and an Encryption Algorithm
   Identifier would need to be assigned.  Implementations SHOULD use an
   IKE Phase 1 cipher that is at least as strong as AES-GMAC.  The use
   of AES-CBC [RFC3602] with the same AES key size as used by

5.2.  Phase 2 Identifier

   For IKE Phase 2 negotiations, IANA has assigned identifiers as
   described in Section 9.

5.3.  Key Length Attribute

   AES-GMAC can be used with any of the three AES key lengths.  The way
   that the key length is indicated is different for AH and ESP.

   For AH, each key length has its own separate integrity transform
   identifier and algorithm name (Section 9).  The IKE Key Length
   attribute MUST NOT be used with these identifiers.  This transform
   MUST NOT be used with ESP.

   For ESP, there is a single encryption transform identifier (which
   represents the combined transform) (Section 9).  The IKE Key Length
   attribute MUST be used with each use of this identifier to indicate
   the key length.  The Key Length attribute MUST have a value of 128,
   192, or 256.

5.4.  Keying Material and Salt Values

   IKE makes use of a pseudo-random function (PRF) to derive keying
   material.  The PRF is used iteratively to derive keying material of
   arbitrary size, called KEYMAT.  Keying material is extracted from the
   output string without regard to boundaries.

   The size of the KEYMAT for the ENCR_NULL_AUTH_AES_GMAC and
   AUTH_AES_GMAC MUST be four octets longer than is needed for the
   associated AES key.  The keying material is used as follows:

   ENCR_NULL_AUTH_AES_GMAC with a 128-bit key and AUTH_AES_128_GMAC
      The KEYMAT requested for each AES-GMAC key is 20 octets.  The
      first 16 octets are the 128-bit AES key, and the remaining four
      octets are used as the salt value in the nonce.

   ENCR_NULL_AUTH_AES_GMAC with a 192-bit key and AUTH_AES_192_GMAC
      The KEYMAT requested for each AES-GMAC key is 28 octets.  The
      first 24 octets are the 192-bit AES key, and the remaining four
      octets are used as the salt value in the nonce.

   ENCR_NULL_AUTH_AES_GMAC with a 256-bit key and AUTH_AES_256_GMAC
      The KEYMAT requested for each AES-GMAC key is 36 octets.  The
      first 32 octets are the 256-bit AES key, and the remaining four
      octets are used as the salt value in the nonce.

6.  Test Vectors

   Appendix B of [GCM] provides test vectors that will assist
   implementers with AES-GMAC.

7.  Security Considerations

   Since the authentication coverage is different between AES-GCM-ESP
   and this specification (see Figure 4), it is worth pointing out that
   both specifications are secure.  In ENCR_NULL_AUTH_AES_GMAC, the IV
   is not included in either the plaintext or the additional
   authenticated data.  This does not adversely affect security, because
   the IV field only provides an input to the GMAC IV, which is not
   required to be authenticated (see [GCM]).  In AUTH_AES_GMAC, the IV
   is included in the additional authenticated data.  This fact has no
   adverse effect on security; it follows from the property that GMAC is
   secure even against attacks in which the adversary can manipulate
   both the IV and the message.  Even an adversary with these powerful
   capabilities cannot forge an authentication tag for any message
   (other than one that was submitted to the chosen-message oracle).
   Since such an adversary could easily choose messages that contain the
   IVs with which they correspond, there are no security problems with
   the inclusion of the IV in the AAD.

   GMAC is provably secure against adversaries that can adaptively
   choose plaintexts, ICVs and the AAD field, under standard
   cryptographic assumptions (roughly, that the output of the underlying
   cipher under a randomly chosen key is indistinguishable from a
   randomly selected output).  Essentially, this means that, if used
   within its intended parameters, a break of GMAC implies a break of
   the underlying block cipher.  The proof of security is available in

   The most important security consideration is that the IV never
   repeats for a given key.  In part, this is handled by disallowing the
   use of AES-GMAC when using statically configured keys, as discussed
   in Section 2.

   When IKE is used to establish fresh keys between two peer entities,
   separate keys are established for the two traffic flows.  If a
   different mechanism is used to establish fresh keys, one that
   establishes only a single key to protect packets, then there is a
   high probability that the peers will select the same IV values for
   some packets.  Thus, to avoid counter block collisions, ESP or AH
   implementations that permit use of the same key for protecting
   packets with the same peer MUST ensure that the two peers assign
   different salt values to the security association (SA).

   The other consideration is that, as with any block cipher mode of
   operation, the security of all data protected under a given security
   association decreases slightly with each message.

   To protect against this problem, implementations MUST generate a
   fresh key before processing 2^64 blocks of data with a given key.
   Note that it is impossible to reach this limit when using 32-bit
   Sequence Numbers.

   Note that, for each message, GMAC calls the block cipher only once.

8.  Design Rationale

   This specification was designed to be as similar to AES-GCM-ESP
   [RFC4106] as possible.  We re-use the design and implementation
   experience from that specification.  We include all three AES key
   sizes since AES-GCM-ESP supports all of those sizes, and the larger
   key sizes provide future users with more high-security options.

9.  IANA Considerations

   IANA has assigned the following IKEv2 parameters.  For the use of AES
   GMAC in AH, the following integrity (type 3) transform identifiers
   have been assigned:

       "9" for AUTH_AES_128_GMAC

      "10" for AUTH_AES_192_GMAC

      "11" for AUTH_AES_256_GMAC

   For the use of AES-GMAC in ESP, the following encryption (type 1)
   transform identifier has been assigned:

      "21" for ENCR_NULL_AUTH_AES_GMAC

10.  Acknowledgements

   Our discussions with Fabio Maino and David Black significantly
   improved this specification, and Tero Kivinen provided us with useful
   comments.  Steve Kent provided guidance on ESP interactions.  This
   work is closely modeled after AES-GCM, which itself is closely
   modeled after Russ Housley's AES-CCM transform [RFC4309].
   Additionally, the GCM mode of operation was originally conceived as
   an improvement to the CWC mode [CWC] in which Doug Whiting and Yoshi
   Kohno participated.  We express our thanks to Fabio, David, Tero,
   Steve, Russ, Doug, and Yoshi.

11.  References

11.1.  Normative References

   [GCM]      McGrew, D. and J. Viega, "The Galois/Counter Mode of
              Operation (GCM)", Submission to NIST. http://
              gcm-spec.pdf, January 2004.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3602]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
              Algorithm and Its Use with IPsec", RFC 3602, September

11.2.  Informative References

   [CWC]      Kohno, T., Viega, J., and D. Whiting, "CWC: A high-
              performance conventional authenticated encryption mode",
              Fast Software Encryption.
              http://eprint.iacr.org/2003/106.pdf, February 2004.

   [GCMP]     McGrew, D. and J. Viega, "The Security and Performance of
              the Galois/Counter Mode (GCM)", Proceedings of INDOCRYPT
              '04, http://eprint.iacr.org/2004/193, December 2004.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

   [RFC4106]  Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
              (GCM) in IPsec Encapsulating Security Payload (ESP)", RFC
              4106, June 2005.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302, December

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
              4303, December 2005.

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
              4306, December 2005.

   [RFC4309]  Housley, R., "Using Advanced Encryption Standard (AES) CCM
              Mode with IPsec Encapsulating Security Payload (ESP)", RFC
              4309, December 2005.

Authors' Addresses

   David A. McGrew
   Cisco Systems, Inc.
   510 McCarthy Blvd.
   Milpitas, CA  95035

   Phone: (408) 525 8651
   EMail: mcgrew@cisco.com
   URI:   http://www.mindspring.com/~dmcgrew/dam.htm

   John Viega
   McAfee, Inc.
   1145 Herndon Parkway, Suite 500
   Herndon, VA 20170

   EMail: viega@list.org

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