AES-GCM in ESP: The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating Security Payload (ESP)

Network Working Group                                           J. Viega
Request for Comments: 4106                         Secure Software, Inc.
Category: Standards Track                                      D. McGrew
                                                     Cisco Systems, Inc.
                                                               June 2005


                 The Use of Galois/Counter Mode (GCM)
             in IPsec Encapsulating Security Payload (ESP)

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 (2005).

Abstract

   This memo describes the use of the Advanced Encryption Standard (AES)
   in Galois/Counter Mode (GCM) as an IPsec Encapsulating Security
   Payload (ESP) mechanism to provide confidentiality and data origin
   authentication.  This method can be efficiently implemented in
   hardware for speeds of 10 gigabits per second and above, and is also
   well-suited to software implementations.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................2
   2. AES-GCM .........................................................3
   3. ESP Payload Data ................................................3
      3.1. Initialization Vector (IV) .................................3
      3.2. Ciphertext .................................................4
   4. Nonce Format ....................................................4
   5. AAD Construction ................................................5
   6. Integrity Check Value (ICV) .....................................5
   7. Packet Expansion ................................................6
   8. IKE Conventions .................................................6
      8.1. Keying Material and Salt Values ............................6
      8.2. Phase 1 Identifier .........................................6
      8.3. Phase 2 Identifier .........................................7
      8.4. Key Length Attribute .......................................7
   9. Test Vectors ....................................................7
   10. Security Considerations ........................................7
   11. Design Rationale ...............................................8
   12. IANA Considerations ............................................8
   13. Acknowledgements ...............................................9
   14. Normative References ...........................................9
   15. Informative References .........................................9

1.  Introduction

   This document describes the use of AES in GCM mode (AES-GCM) as an
   IPsec ESP mechanism for confidentiality and data origin
   authentication.  We refer to this method as AES-GCM-ESP.  This
   mechanism is not only efficient and secure, but it also enables
   high-speed implementations in hardware.  Thus, AES-GCM-ESP allows
   IPsec connections that can make effective use of emerging 10-gigabit
   and 40-gigabit network devices.

   Counter mode (CTR) has emerged as the preferred encryption method for
   high-speed implementations.  Unlike conventional encryption modes
   such as Cipher Block Chaining (CBC) and Cipher Block Chaining Message
   Authentication Code (CBC-MAC), CTR can be efficiently implemented at
   high data rates because it can be pipelined.  The ESP CTR protocol
   describes how this mode can be used with IPsec ESP [RFC3686].

   Unfortunately, CTR provides no data origin authentication, and thus
   the ESP CTR standard requires the use of a data origin authentication
   algorithm in conjunction with CTR.  This requirement is problematic,
   because none of the standard data origin authentication algorithms
   can be efficiently implemented for high data rates.  GCM solves this
   problem, because under the hood, it combines CTR mode with a secure,
   parallelizable, and efficient authentication mechanism.

   This document does not cover implementation details of GCM.  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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  AES-GCM

   GCM is a block cipher mode of operation providing both
   confidentiality and data origin authentication.  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.  In
   the following, we describe how the IV, plaintext, and AAD are formed
   from the ESP fields, and how the ESP packet is formed from the
   ciphertext and authentication tag.

   ESP also defines an IV.  For clarity, we refer to the AES-GCM IV as a
   nonce in the context of AES-GCM-ESP.  The same nonce and key
   combination MUST NOT be used more than once.

   Because reusing an nonce/key combination destroys the security
   guarantees of AES-GCM mode, it can be difficult to use this mode
   securely when using statically configured keys.  For safety's sake,
   implementations MUST use an automated key management system, such as
   the Internet Key Exchange (IKE) [RFC2409], to ensure that this
   requirement is met.

3.  ESP Payload Data

   The ESP Payload Data is comprised of an eight-octet initialization
   vector (IV), followed by the ciphertext.  The payload field, as
   defined in [RFC2406], is structured as shown in Figure 1, along with
   the ICV associated with the payload.

    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                    |
   |                            (8 octets)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Ciphertext (variable)                   ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 1: ESP Payload Encrypted with AES-GCM.

3.1.  Initialization Vector (IV)

   The AES-GCM-ESP IV field MUST be eight octets.  For a given key, the
   IV MUST NOT repeat.  The most natural way to implement this is with a
   counter, but anything that guarantees uniqueness can be used, such as
   a linear feedback shift register (LFSR).  Note that the encrypter can
   use any IV generation method that meets the uniqueness requirement,
   without coordinating with the decrypter.

3.2.  Ciphertext

   The plaintext input to AES-GCM is formed by concatenating the
   plaintext data described by the Next Header field with the Padding,
   the Pad Length, and the Next Header field.  The Ciphertext field
   consists of the ciphertext output from the AES-GCM algorithm.  The
   length of the ciphertext is identical to that of the plaintext.

   Implementations that do not seek to hide the length of the plaintext
   SHOULD use the minimum amount of padding required, which will be less
   than four octets.

4.  Nonce Format

   The nonce passed to the GCM-AES encryption 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 2: Nonce Format

   The components of the nonce are as follows:

   Salt
      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 8.1.

   Initialization Vector
      The IV field is described in Section 3.1.

5.  AAD Construction

   The authentication of data integrity and data origin for the SPI and
   (Extended) Sequence Number fields is provided without encryption.
   This is done by including those fields in the AES-GCM 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
   3, and the format with 64-bit extended sequence numbers is shown in
   Figure 4.

    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                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 3: 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               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 4: AAD Format with 64-bit Extended Sequence Number

6.  Integrity Check Value (ICV)

   The ICV consists solely of the AES-GCM Authentication Tag.
   Implementations MUST support a full-length 16-octet ICV, and MAY
   support 8 or 12 octet ICVs, and MUST NOT support other ICV lengths.
   Although ESP does not require that an ICV be present, AES-GCM-ESP
   intentionally does not allow a zero-length ICV.  This is because GCM
   provides no integrity protection whatsoever when used with a zero-
   length Authentication Tag.

7.  Packet Expansion

   The IV adds an additional eight octets to the packet, and the ICV
   adds an additional 8, 12, or 16 octets.  These are the only sources
   of packet expansion, other than the 10-13 octets taken up by the ESP
   SPI, Sequence Number, Padding, Pad Length, and Next Header fields (if
   the minimal amount of padding is used).

8.  IKE Conventions

   This section describes the conventions used to generate keying
   material and salt values, for use with AES-GCM-ESP, using the
   Internet Key Exchange (IKE) [RFC2409] protocol.  The identifiers and
   attributes needed to negotiate a security association using AES-GCM-
   ESP are also defined.

8.1.  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 AES-GCM-ESP MUST be four octets longer
   than is needed for the associated AES key.  The keying material is
   used as follows:

   AES-GCM-ESP with a 128 bit key
      The KEYMAT requested for each AES-GCM 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.

   AES-GCM-ESP with a 192 bit key
      The KEYMAT requested for each AES-GCM 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.

   AES-GCM-ESP with a 256 bit key
      The KEYMAT requested for each AES GCM 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.

8.2.  Phase 1 Identifier

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

8.3.  Phase 2 Identifier

   For IKE Phase 2 negotiations, IANA has assigned three ESP Transform
   Identifiers for AES-GCM with an eight-byte explicit IV:

      18 for AES-GCM with an 8 octet ICV;
      19 for AES-GCM with a 12 octet ICV; and
      20 for AES-GCM with a 16 octet ICV.

8.4.  Key Length Attribute

   Because the AES supports three key lengths, the Key Length attribute
   MUST be specified in the IKE Phase 2 exchange [RFC2407].  The Key
   Length attribute MUST have a value of 128, 192, or 256.

9.  Test Vectors

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

10.  Security Considerations

   GCM is provably secure against adversaries that can adaptively choose
   plaintexts, ciphertexts, 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 GCM implies a break of the
   underlying block cipher.  The proof of security for GCM is available
   in [GCM].

   The most important security consideration is that the IV never repeat
   for a given key.  In part, this is handled by disallowing the use of
   AES-GCM 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 encrypt 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
   implementations that permit use of the same key for encrypting and
   decrypting 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 encryption mode, 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 encrypting 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, GCM calls the block cipher once for each
   full 16-octet block in the payload, once for any remaining octets in
   the payload, and one additional time for computing the ICV.

   Clearly, smaller ICV values are more likely to be subject to forgery
   attacks.  Implementations SHOULD use as large a size as reasonable.

11.  Design Rationale

   This specification was designed to be as similar to the AES-CCM ESP
   [CCM-ESP] and AES-CTR ESP [RFC3686] mechanisms as reasonable, while
   promoting simple, efficient implementations in both hardware and
   software.  We re-use the design and implementation experience from
   those standards.

   The major difference with CCM is that the CCM ESP mechanism requires
   an 11-octet nonce, whereas the GCM ESP mechanism requires using a
   12-octet nonce.  GCM is specially optimized to handle the 12-octet
   nonce case efficiently.  Nonces of other lengths would cause
   unnecessary, additional complexity and delays, particularly in
   hardware implementations.  The additional octet of nonce is used to
   increase the size of the salt.

12.  IANA Considerations

   IANA has assigned three ESP Transform Identifiers for AES-GCM with an
   eight-byte explicit IV:

      18 for AES-GCM with an 8 octet ICV;
      19 for AES-GCM with a 12 octet ICV; and
      20 for AES-GCM with a 16 octet ICV.

13.  Acknowledgements

   This work is closely modeled after Russ Housley's AES-CCM transform
   [CCM-ESP].  Portions of this document are directly copied from that
   work in progress.  We thank Russ for his support of this work.

   Additionally, the GCM mode of operation was originally conceived as
   an improvement to Carter-Wegman Counter (CWC) mode [CWC], the first
   unencumbered block cipher mode capable of supporting high-speed
   authenticated encryption.

14.  Normative References

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

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

   [RFC2406]  Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, November 1998.

   [RFC2407]  Piper, D., "The Internet IP Security Domain of
              Interpretation for ISAKMP", RFC 2407, November 1998.

   [RFC3602]  Frankel, S., Glenn, R. and S. Kelly, "The AES-CBC Cipher
              Algorithm and Its Use with IPsec", RFC 3602, September
              2003.

15.  Informative References

   [CCM-ESP]  Housley, R., "Using AES CCM Mode With IPsec ESP", Work In
              Progress.

   [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.

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

   [RFC3686]  Housley, R., "Using Advanced Encryption Standard (AES)
              Counter Mode With IPsec Encapsulating Security Payload
              (ESP)", RFC 3686, January 2004.

Authors' Addresses

   John Viega
   Secure Software, Inc.
   4100 Lafayette Center Dr., Suite 100
   Chantilly, VA  20151
   US

   Phone: (703) 814 4402
   EMail: viega@securesoftware.com


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

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

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