Network Working Group
Request for Comments: 5487
Category: Standards Track

M. Badra
CNRS/LIMOS Laboratory
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).

   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

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:

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 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.

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.

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};

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};

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