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: [email protected]


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

  EMail: [email protected]


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

  EMail: [email protected]
























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Full Copyright Statement

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