AES Galois Counter Mode (GCM) Cipher Suites for TLS
RFC 5288

Network Working Group                                         J. Salowey
Request for Comments: 5288                                  A. Choudhury
Category: Standards Track                                      D. McGrew
                                                     Cisco Systems, Inc.
                                                             August 2008

          AES Galois Counter Mode (GCM) Cipher Suites for TLS

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 memo describes the use of the Advanced Encryption Standard (AES)
   in Galois/Counter Mode (GCM) as a Transport Layer Security (TLS)
   authenticated encryption operation.  GCM provides both
   confidentiality and data origin authentication, can be efficiently
   implemented in hardware for speeds of 10 gigabits per second and
   above, and is also well-suited to software implementations.  This
   memo defines TLS cipher suites that use AES-GCM with RSA, DSA, and
   Diffie-Hellman-based key exchange mechanisms.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Conventions Used in This Document . . . . . . . . . . . . . . . 2
   3.  AES-GCM Cipher Suites . . . . . . . . . . . . . . . . . . . . . 2
   4.  TLS Versions  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
     6.1.  Counter Reuse . . . . . . . . . . . . . . . . . . . . . . . 4
     6.2.  Recommendations for Multiple Encryption Processors  . . . . 4
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 6

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1.  Introduction

   This document describes the use of AES [AES] in Galois Counter Mode
   (GCM) [GCM] (AES-GCM) with various key exchange mechanisms as a
   cipher suite for TLS.  AES-GCM is an authenticated encryption with
   associated data (AEAD) cipher (as defined in TLS 1.2 [RFC5246])
   providing both confidentiality and data origin authentication.  The
   following sections define cipher suites based on RSA, DSA, and
   Diffie-Hellman key exchanges; ECC-based (Elliptic Curve Cryptography)
   cipher suites are defined in a separate document [RFC5289].

   AES-GCM is not only efficient and secure, but hardware
   implementations can achieve high speeds with low cost and low
   latency, because the mode can be pipelined.  Applications that
   require high data throughput can benefit from these high-speed
   implementations.  AES-GCM has been specified as a mode that can be
   used with IPsec ESP [RFC4106] and 802.1AE Media Access Control (MAC)
   Security [IEEE8021AE].

2.  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].

3.  AES-GCM Cipher Suites

   The following cipher suites use the new authenticated encryption
   modes defined in TLS 1.2 with AES in Galois Counter Mode (GCM) [GCM]:

      CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9C}
      CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9D}
      CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9E}
      CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9F}
      CipherSuite TLS_DH_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0xA0}
      CipherSuite TLS_DH_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0xA1}
      CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA2}
      CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA3}
      CipherSuite TLS_DH_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA4}
      CipherSuite TLS_DH_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA5}
      CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {0x00,0xA6}
      CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {0x00,0xA7}

   These cipher suites use the AES-GCM authenticated encryption with
   associated data (AEAD) algorithms AEAD_AES_128_GCM and
   AEAD_AES_256_GCM described in [RFC5116].  Note that each of these
   AEAD algorithms uses a 128-bit authentication tag with GCM (in
   particular, as described in Section 3.5 of [RFC4366], the

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   "truncated_hmac" extension does not have an effect on cipher suites
   that do not use HMAC).  The "nonce" SHALL be 12 bytes long consisting
   of two parts as follows: (this is an example of a "partially
   explicit" nonce; see Section 3.2.1 in [RFC5116]).

             struct {
                opaque salt[4];
                opaque nonce_explicit[8];
             } GCMNonce;

   The salt is the "implicit" part of the nonce and is not sent in the
   packet.  Instead, the salt is generated as part of the handshake
   process: it is either the client_write_IV (when the client is
   sending) or the server_write_IV (when the server is sending).  The
   salt length (SecurityParameters.fixed_iv_length) is 4 octets.

   The nonce_explicit is the "explicit" part of the nonce.  It is chosen
   by the sender and is carried in each TLS record in the
   GenericAEADCipher.nonce_explicit field.  The nonce_explicit length
   (SecurityParameters.record_iv_length) is 8 octets.

   Each value of the nonce_explicit MUST be distinct for each distinct
   invocation of the GCM encrypt function for any fixed key.  Failure to
   meet this uniqueness requirement can significantly degrade security.
   The nonce_explicit MAY be the 64-bit sequence number.

   The RSA, DHE_RSA, DH_RSA, DHE_DSS, DH_DSS, and DH_anon key exchanges
   are performed as defined in [RFC5246].

   The Pseudo Random Function (PRF) algorithms SHALL be as follows:

      For cipher suites ending with _SHA256, the PRF is the TLS PRF
      [RFC5246] with SHA-256 as the hash function.

      For cipher suites ending with _SHA384, the PRF is the TLS PRF
      [RFC5246] with SHA-384 as the hash function.

   Implementations MUST send TLS Alert bad_record_mac for all types of
   failures encountered in processing the AES-GCM algorithm.

4.  TLS Versions

   These cipher suites make use of the authenticated encryption with
   additional data defined in TLS 1.2 [RFC5246].  They MUST NOT be
   negotiated in older versions of TLS.  Clients MUST NOT offer these
   cipher suites if they do not offer TLS 1.2 or later.  Servers that
   select an earlier version of TLS MUST NOT select one of these cipher
   suites.  Because TLS has no way for the client to indicate that it

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   supports TLS 1.2 but not earlier, a non-compliant server might
   potentially negotiate TLS 1.1 or earlier and select one of the cipher
   suites in this document.  Clients MUST check the TLS version and
   generate a fatal "illegal_parameter" alert if they detect an
   incorrect version.

5.  IANA Considerations

   IANA has assigned the following values for the cipher suites defined
   in this document:

      CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9C}
      CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9D}
      CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9E}
      CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9F}
      CipherSuite TLS_DH_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0xA0}
      CipherSuite TLS_DH_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0xA1}
      CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA2}
      CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA3}
      CipherSuite TLS_DH_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA4}
      CipherSuite TLS_DH_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA5}
      CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {0x00,0xA6}
      CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {0x00,0xA7}

6.  Security Considerations

   The security considerations in [RFC5246] apply to this document as
   well.  The remainder of this section describes security
   considerations specific to the cipher suites described in this
   document.

6.1.  Counter Reuse

   AES-GCM security requires that the counter is never reused.  The IV
   construction in Section 3 is designed to prevent counter reuse.

   Implementers should also understand the practical considerations of
   IV handling outlined in Section 9 of [GCM].

6.2.  Recommendations for Multiple Encryption Processors

   If multiple cryptographic processors are in use by the sender, then
   the sender MUST ensure that, for a particular key, each value of the
   nonce_explicit used with that key is distinct.  In this case, each
   encryption processor SHOULD include, in the nonce_explicit, a fixed
   value that is distinct for each processor.  The recommended format is

        nonce_explicit = FixedDistinct || Variable

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   where the FixedDistinct field is distinct for each encryption
   processor, but is fixed for a given processor, and the Variable field
   is distinct for each distinct nonce used by a particular encryption
   processor.  When this method is used, the FixedDistinct fields used
   by the different processors MUST have the same length.

   In the terms of Figure 2 in [RFC5116], the Salt is the Fixed-Common
   part of the nonce (it is fixed, and it is common across all
   encryption processors), the FixedDistinct field exactly corresponds
   to the Fixed-Distinct field, the Variable field corresponds to the
   Counter field, and the explicit part exactly corresponds to the
   nonce_explicit.

   For clarity, we provide an example for TLS in which there are two
   distinct encryption processors, each of which uses a one-byte
   FixedDistinct field:

          Salt          = eedc68dc
          FixedDistinct = 01       (for the first encryption processor)
          FixedDistinct = 02       (for the second encryption processor)

   The GCMnonces generated by the first encryption processor, and their
   corresponding nonce_explicit, are:

          GCMNonce                    nonce_explicit
          ------------------------    ----------------------------
          eedc68dc0100000000000000    0100000000000000
          eedc68dc0100000000000001    0100000000000001
          eedc68dc0100000000000002    0100000000000002
          ...

   The GCMnonces generated by the second encryption processor, and their
   corresponding nonce_explicit, are

          GCMNonce                    nonce_explicit
          ------------------------    ----------------------------
          eedc68dc0200000000000000    0200000000000000
          eedc68dc0200000000000001    0200000000000001
          eedc68dc0200000000000002    0200000000000002
          ...

7.  Acknowledgements

   This document borrows heavily from [RFC5289].  The authors would like
   to thank Alex Lam, Simon Josefsson, and Pasi Eronen for providing
   useful comments during the review of this document.

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8.  References

8.1.  Normative References

   [AES]         National Institute of Standards and Technology,
                 "Advanced Encryption Standard (AES)", FIPS 197,
                 November 2001.

   [GCM]         Dworkin, M., "Recommendation for Block Cipher Modes of
                 Operation: Galois/Counter Mode (GCM) and GMAC",
                 National Institute of Standards and Technology SP 800-
                 38D, November 2007.

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

   [RFC5116]     McGrew, D., "An Interface and Algorithms for
                 Authenticated Encryption", RFC 5116, January 2008.

   [RFC5246]     Dierks, T. and E. Rescorla, "The Transport Layer
                 Security (TLS) Protocol Version 1.2", RFC 5246,
                 August 2008.

8.2.  Informative References

   [IEEE8021AE]  Institute of Electrical and Electronics Engineers,
                 "Media Access Control Security", IEEE Standard 802.1AE,
                 August 2006.

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

   [RFC4366]     Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
                 J., and T. Wright, "Transport Layer Security (TLS)
                 Extensions", RFC 4366, April 2006.

   [RFC5289]     Rescorla, E., "TLS Elliptic Curve Cipher Suites with
                 SHA-256/384 and AES Galois Counter Mode", RFC 5289,
                 August 2008.

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Authors' Addresses

   Joseph Salowey
   Cisco Systems, Inc.
   2901 3rd. Ave
   Seattle, WA  98121
   USA

   EMail: jsalowey@cisco.com

   Abhijit Choudhury
   Cisco Systems, Inc.
   3625 Cisco Way
   San Jose, CA  95134
   USA

   EMail: abhijitc@cisco.com

   David McGrew
   Cisco Systems, Inc.
   170 W Tasman Drive
   San Jose, CA  95134
   USA

   EMail: mcgrew@cisco.com

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