


TLS Working Group                                             J. Salowey
Internet-Draft                                              A. Choudhury
Intended status: Standards Track                               D. McGrew
Expires: July 16, 2008                               Cisco Systems, Inc.
                                                        January 13, 2008


                RSA based AES-GCM Cipher Suites for TLS
                     draft-ietf-tls-rsa-aes-gcm-01

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

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 ciphersuites that use AES-GCM with RSA.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3

   2.  Conventions Used In This Document . . . . . . . . . . . . . . . 3

   3.  RSA Based AES-GCM Cipher Suites . . . . . . . . . . . . . . . . 3
     3.1.  Recommendations for Multiple Cryptographic Processors . . . 4

   4.  TLS Versions  . . . . . . . . . . . . . . . . . . . . . . . . . 5

   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6

   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
     6.1.  Perfect Forward Secrecy . . . . . . . . . . . . . . . . . . 6
     6.2.  Counter Reuse . . . . . . . . . . . . . . . . . . . . . . . 6

   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6

   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7

   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9


























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1.  Introduction

   This document describes the use of AES [AES]in Galois Counter Mode
   (GCM) [GCM] (AES-GCM) with RSA based key exchange as a ciphersuite
   for TLS.  This mechanism is not only efficient and secure, hardware
   implementations can achieve high speeds with low cost and low
   latency, because the mode can be pipelined.  Applications like
   CAPWAP, which uses DTLS, can benefit from the high-speed
   implementations when wireless termination points (WTPs) and
   controllers (ACs) have to meet requirements to support higher
   throughputs in the future.  AES-GCM has been specified as a mode that
   can be used with IPsec ESP [RFC4106] and 802.1AE MAC Security
   [IEEE8021AE].  It also is part of the NSA suite B ciphersuites for
   TLS [I-D.rescorla-tls-suiteb]; however in order to meet Suite B
   requirements these ciphersuites require ECC base key exchange and TLS
   1.2.  The ciphersuites defined in this document are based on RSA
   which allows the use of AES-GCM in environments that have not
   deployed ECC algorithms and do not need to meet NSA Suite B
   requirements.  AES-GCM is an authenticated encryption with associated
   data (AEAD) cipher, as defined in TLS 1.2[I-D.ietf-tls-rfc4346-bis].
   The ciphersuites defined in this draft may be used with Datagram TLS
   defined in [RFC4347].  This memo uses GCM in a way similar to
   [I-D.ietf-tls-ecc-new-mac].


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.  RSA Based AES-GCM Cipher Suites

   The following ciphersuites 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 = {TBD1,TBD1}
      CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {TBD2,TBD2}
      CipherSuite TLS_RSA_DHE_WITH_AES_128_GCM_SHA256 = {TBD3,TBD3}
      CipherSuite TLS_RSA_DHE_WITH_AES_256_GCM_SHA384 = {TBD4,TBD4}

   These ciphersuites use the AES-GCM authenticated encryption with
   associated data (AEAD) algorithms AEAD_AES_128_GCM and
   AEAD_AES_256_GCM described in [I-D.mcgrew-auth-enc].  Note that this
   specification uses a 128-bit authentication tag with GCM.  The
   "nonce" SHALL be 12 bytes long and it is "partially implicit" (see
   section 3.2.1 in [I-D.mcgrew-auth-enc]).  Part of the nonce is



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   generated as part of the handshake process and is static for the
   entire session and part is carried in each packet.

             Struct{
                opaque salt[4];
                opaque explicit_nonce_part[8];
             } GCMNonce

   The salt is the "implicit" part of the nonce and is not sent in the
   packet.  It is either the client_write_IV if the client is sending or
   the server_write_IV if the server is sending.  These IVs SHALL be 4
   bytes long.

   The explicit_nonce_part is chosen by the sender and included in the
   packet.  Each value of the explicit_nonce_part MUST be distinct for
   each distinct invocation of GCM encrypt function for any fixed key.
   Failure to meet this uniqueness requirement can significantly degrade
   security.  The explicit_nonce_part is carried in the IV field of the
   GenericAEADCipher structure.  Therefore, for all the algorithms
   defined in this section, SecurityParameters.iv_length=8.

   In the case of TLS the explicit_nonce_part MAY be the 64-bit sequence
   number.  In the case of Datagram TLS [RFC4347] the
   explicit_nonce_part MAY be formed from the concatenation of the 16-
   bit epoch with the 48-bit DTLS seq_num.

   The RSA and RSA-DHE key exchange is performed as defined in
   [I-D.ietf-tls-rfc4346-bis].

   The PRF algorithms SHALL be as follows:

   For TLS_RSA_WITH_AES_128_GCM_SHA256 and
   TLS_RSA_DHE_WITH_AES_128_GCM_SHA256 the hash function is SHA256.

   For TLS_RSA_WITH_AES_256_GCM_SHA384 and
   TLS_RSA_DHE_WITH_AES_256_GCM_SHA384 the hash function is SHA384.

3.1.  Recommendations for Multiple Cryptographic Processors

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

        explicit_nonce_part = 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 [I-D.mcgrew-auth-enc], 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, and the Variable field
   corresponds to the Counter field, and the explicit part exactly
   corresponds to the explicit_nonce_part.

   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 explicit_nonce_parts, are:

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

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

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



4.  TLS Versions

   These ciphersuites make use of the authenticated encryption with
   additional data defined in TLS 1.2 [I-D.ietf-tls-rfc4346-bis].  They
   MUST NOT be negotiated in older versions of TLS.  Clients MUST NOT



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   offer these cipher suites if they do not offer TLS 1.2 or later.
   Servers which 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 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 ciphersuites defined
   in this draft:

      CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {TBD1,TBD1}
      CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {TBD2,TBD2}
      CipherSuite TLS_RSA_DHE_WITH_AES_128_GCM_SHA256 = {TBD3,TBD3}
      CipherSuite TLS_RSA_DHE_WITH_AES_256_GCM_SHA384 = {TBD4,TBD4}


6.  Security Considerations

   The security considerations in [I-D.ietf-tls-rfc4346-bis] 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.  Perfect Forward Secrecy

   The perfect forward secrecy properties of RSA based TLS ciphersuites
   are discussed in the security analysis of [I-D.ietf-tls-rfc4346-bis].
   It should be noted that only the ephemeral Diffie-Hellman based
   ciphersuites (RSA_DHE) are capable of providing perfect forward
   secrecy.

6.2.  Counter Reuse

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


7.  Acknowledgements

   This draft borrows heavily from [I-D.ietf-tls-ecc-new-mac].


8.  References



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8.1.  Normative References

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

   [GCM]      National Institute of Standards and Technology,
              "Recommendation for Block Cipher Modes of Operation:
              Galois Counter Mode (GCM) for Confidentiality and
              Authentication", SP 800-38D, April 2006.

   [I-D.ietf-tls-rfc4346-bis]
              Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", draft-ietf-tls-rfc4346-bis-07
              (work in progress), November 2007.

   [I-D.mcgrew-auth-enc]
              McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", draft-mcgrew-auth-enc-05 (work in progress),
              November 2007.

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

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

8.2.  Informative References

   [I-D.ietf-tls-ecc-new-mac]
              Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
              256/384 and AES Galois Counter  Mode",
              draft-ietf-tls-ecc-new-mac-02 (work in progress),
              December 2007.

   [I-D.rescorla-tls-suiteb]
              Salter, M. and E. Rescorla, "Suite B Cipher Suites for
              TLS", draft-rescorla-tls-suiteb-01 (work in progress),
              June 2007.

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




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


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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