Network Working Group                                           M. Badra
Request for Comments: 5487                         CNRS/LIMOS Laboratory
Category: Standards Track                                     March 2009


               Pre-Shared Key Cipher Suites for TLS with
                SHA-256/384 and AES Galois Counter Mode

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.

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Abstract

   RFC 4279 and RFC 4785 describe pre-shared key cipher suites for
   Transport Layer Security (TLS).  However, all those cipher suites use
   SHA-1 in their Message Authentication Code (MAC) algorithm.  This
   document describes a set of pre-shared key cipher suites for TLS that
   uses stronger digest algorithms (i.e., SHA-256 or SHA-384) and
   another set that uses the Advanced Encryption Standard (AES) in
   Galois Counter Mode (GCM).

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
     1.1.  Applicability Statement . . . . . . . . . . . . . . . . . . 3
     1.2.  Conventions Used in This Document . . . . . . . . . . . . . 3
   2.  PSK, DHE_PSK, and RSA_PSK Key Exchange Algorithms with
       AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  PSK, DHE_PSK, and RSA_PSK Key Exchange with SHA-256/384 . . . . 4
     3.1.  PSK Key Exchange Algorithm with SHA-256/384 . . . . . . . . 4
     3.2.  DHE_PSK Key Exchange Algorithm with SHA-256/384 . . . . . . 5
     3.3.  RSA_PSK Key Exchange Algorithm with SHA-256/384 . . . . . . 5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 7

1.  Introduction

   The benefits of pre-shared symmetric-key vs. public-/private-key pair
   based authentication for the key exchange in TLS have been explained
   in the Introduction of [RFC4279].  This document leverages the
   already defined algorithms for the application of newer, generally
   regarded stronger, cryptographic primitives and building blocks.

   TLS 1.2 [RFC5246] adds support for authenticated encryption with
   additional data (AEAD) cipher modes [RFC5116].  This document
   describes the use of Advanced Encryption Standard [AES] in Galois
   Counter Mode [GCM] (AES-GCM) with various pre-shared key (PSK)
   authenticated key exchange mechanisms ([RFC4279] and [RFC4785]) in
   cipher suites for TLS.

   This document also specifies PSK cipher suites for TLS that replace
   SHA-1 by SHA-256 or SHA-384 [SHS].  RFC 4279 [RFC4279] and RFC 4785
   [RFC4785] describe PSK cipher suites for TLS.  However, all of the
   RFC 4279 and the RFC 4785 cipher suites use HMAC-SHA1 as their MAC
   algorithm.  Due to recent analytic work on SHA-1 [Wang05], the IETF
   is gradually moving away from SHA-1 and towards stronger hash
   algorithms.

   Related TLS cipher suites with key exchange algorithms that are
   authenticated using public/private key pairs have recently been
   specified:

   o  RSA-, DSS-, and Diffie-Hellman-based cipher suites in [RFC5288],
      and

   o  ECC-based cipher suites with SHA-256/384 and AES-GCM in [RFC5289].

   The reader is expected to become familiar with these two memos prior
   to studying this document.

1.1.  Applicability Statement

   The cipher suites defined in Section 3 can be negotiated, whatever
   the negotiated TLS version is.

   The cipher suites defined in Section 2 can be negotiated in TLS
   version 1.2 or higher.

   The applicability statement in [RFC4279] applies to this document as
   well.

1.2.  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].

2.  PSK, DHE_PSK, and RSA_PSK Key Exchange Algorithms with AES-GCM

   The following six cipher suites use the new authenticated encryption
   modes defined in TLS 1.2 with AES in Galois Counter Mode [GCM].  The
   cipher suites with the DHE_PSK key exchange algorithm
   (TLS_DHE_PSK_WITH_AES_128_GCM_SHA256 and
   TLS_DHE_PSK_WITH_AES_256_GCM_SHA348) provide Perfect Forward Secrecy
   (PFS).

      CipherSuite TLS_PSK_WITH_AES_128_GCM_SHA256        = {0x00,0xA8};
      CipherSuite TLS_PSK_WITH_AES_256_GCM_SHA384        = {0x00,0xA9};
      CipherSuite TLS_DHE_PSK_WITH_AES_128_GCM_SHA256    = {0x00,0xAA};
      CipherSuite TLS_DHE_PSK_WITH_AES_256_GCM_SHA384    = {0x00,0xAB};
      CipherSuite TLS_RSA_PSK_WITH_AES_128_GCM_SHA256    = {0x00,0xAC};
      CipherSuite TLS_RSA_PSK_WITH_AES_256_GCM_SHA384    = {0x00,0xAD};

   These cipher suites use authenticated encryption with additional data
   (AEAD) algorithms, AEAD_AES_128_GCM and AEAD_AES_256_GCM, as
   described in RFC 5116.  GCM is used as described in [RFC5288].

   The PSK, DHE_PSK, and RSA_PSK key exchanges are performed as defined
   in [RFC4279].

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

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

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

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

3.  PSK, DHE_PSK, and RSA_PSK Key Exchange with SHA-256/384

   The first two cipher suites described in each of the following three
   sections use AES [AES] in Cipher Block Chaining (CBC) mode [MODES]
   for data confidentiality, whereas the other two cipher suites do not
   provide data confidentiality; all cipher suites provide integrity
   protection and authentication using HMAC-based MACs.

3.1.  PSK Key Exchange Algorithm with SHA-256/384

      CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA256        = {0x00,0xAE};
      CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA384        = {0x00,0xAF};
      CipherSuite TLS_PSK_WITH_NULL_SHA256               = {0x00,0xB0};
      CipherSuite TLS_PSK_WITH_NULL_SHA384               = {0x00,0xB1};

   The above four cipher suites are the same as the corresponding cipher
   suites in RFC 4279 and RFC 4785 (with names ending in "_SHA" in place
   of "_SHA256" or "_SHA384"), except for the hash and PRF algorithms,
   as explained below.

   o  For cipher suites with names ending in "_SHA256":

      *  The MAC is HMAC [RFC2104] with SHA-256 as the hash function.

      *  When negotiated in a version of TLS prior to 1.2, the PRF from
         that version is used; otherwise, the PRF is the TLS PRF
         [RFC5246] with SHA-256 as the hash function.

   o  For cipher suites with names ending in "_SHA384":

      *  The MAC is HMAC [RFC2104] with SHA-384 as the hash function.

      *  When negotiated in a version of TLS prior to 1.2, the PRF from
         that version is used; otherwise, the PRF is the TLS PRF
         [RFC5246] with SHA-384 as the hash function.

3.2.  DHE_PSK Key Exchange Algorithm with SHA-256/384

      CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA256    = {0x00,0xB2};
      CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA384    = {0x00,0xB3};
      CipherSuite TLS_DHE_PSK_WITH_NULL_SHA256           = {0x00,0xB4};
      CipherSuite TLS_DHE_PSK_WITH_NULL_SHA384           = {0x00,0xB5};

   The above four cipher suites are the same as the corresponding cipher
   suites in RFC 4279 and RFC 4785 (with names ending in "_SHA" in place
   of "_SHA256" or "_SHA384"), except for the hash and PRF algorithms,
   as explained in Section 3.1.

3.3.  RSA_PSK Key Exchange Algorithm with SHA-256/384

      CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA256    = {0x00,0xB6};
      CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA384    = {0x00,0xB7};
      CipherSuite TLS_RSA_PSK_WITH_NULL_SHA256           = {0x00,0xB8};
      CipherSuite TLS_RSA_PSK_WITH_NULL_SHA384           = {0x00,0xB9};

   The above four cipher suites are the same as the corresponding cipher
   suites in RFC 4279 and RFC 4785 (with names ending in "_SHA" in place
   of "_SHA256" or "_SHA384"), except for the hash and PRF algorithms,
   as explained in Section 3.1.

4.  Security Considerations

   The security considerations in [RFC4279], [RFC4785], and [RFC5288]
   apply to this document as well.  In particular, as authentication-
   only cipher suites (with no encryption) defined here do not support
   confidentiality, care should be taken not to send sensitive
   information (such as passwords) over connections protected with one
   of the cipher suites with NULL encryption defined in this document.

5.  IANA Considerations

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

      CipherSuite TLS_PSK_WITH_AES_128_GCM_SHA256        = {0x00,0xA8};
      CipherSuite TLS_PSK_WITH_AES_256_GCM_SHA384        = {0x00,0xA9};
      CipherSuite TLS_DHE_PSK_WITH_AES_128_GCM_SHA256    = {0x00,0xAA};
      CipherSuite TLS_DHE_PSK_WITH_AES_256_GCM_SHA384    = {0x00,0xAB};
      CipherSuite TLS_RSA_PSK_WITH_AES_128_GCM_SHA256    = {0x00,0xAC};
      CipherSuite TLS_RSA_PSK_WITH_AES_256_GCM_SHA384    = {0x00,0xAD};
      CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA256        = {0x00,0xAE};
      CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA384        = {0x00,0xAF};
      CipherSuite TLS_PSK_WITH_NULL_SHA256               = {0x00,0xB0};
      CipherSuite TLS_PSK_WITH_NULL_SHA384               = {0x00,0xB1};

      CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA256    = {0x00,0xB2};
      CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA384    = {0x00,0xB3};
      CipherSuite TLS_DHE_PSK_WITH_NULL_SHA256           = {0x00,0xB4};
      CipherSuite TLS_DHE_PSK_WITH_NULL_SHA384           = {0x00,0xB5};
      CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA256    = {0x00,0xB6};
      CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA384    = {0x00,0xB7};
      CipherSuite TLS_RSA_PSK_WITH_NULL_SHA256           = {0x00,0xB8};
      CipherSuite TLS_RSA_PSK_WITH_NULL_SHA384           = {0x00,0xB9};

6.  Acknowledgments

   This document borrows from [RFC5289].  The author appreciates Alfred
   Hoenes for his detailed review and effort on resolving issues in
   discussion.  The author would like also to acknowledge Ibrahim
   Hajjeh, Simon Josefsson, Hassnaa Moustafa, Joseph Salowey, and Pascal
   Urien for their reviews of the content of the document.

7.  References

7.1.  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, November 2007.

   [MODES]    National Institute of Standards and Technology,
              "Recommendation for Block Cipher Modes of Operation -
              Methods and Techniques", SP 800-38A, December 2001.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

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

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279,
              December 2005.

   [RFC4785]  Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK)
              Ciphersuites with NULL Encryption for Transport Layer
              Security (TLS)", RFC 4785, January 2007.

   [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.

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              August 2008.

   [SHS]      National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS 180-2, August 2002.

7.2.  Informative References

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

   [Wang05]   Wang, X., Yin, Y., and H. Yu, "Finding Collisions in the
              Full SHA-1", CRYPTO 2005, August 2005.

Author's Address

   Mohamad Badra
   CNRS/LIMOS Laboratory
   Campus de cezeaux, Bat. ISIMA
   Aubiere  63170
   France

   EMail: badra@isima.fr