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rfc:rfc5489

Network Working Group M. Badra Request for Comments: 5489 CNRS/LIMOS Laboratory Category: Informational I. Hajjeh

                                                            INEOVATION
                                                            March 2009
     ECDHE_PSK Cipher Suites for Transport Layer Security (TLS)

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (c) 2009 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents in effect on the date of
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 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Abstract

 This document extends RFC 4279, RFC 4492, and RFC 4785 and specifies
 a set of cipher suites that use a pre-shared key (PSK) to
 authenticate an Elliptic Curve Diffie-Hellman exchange with Ephemeral
 keys (ECDHE).  These cipher suites provide Perfect Forward Secrecy
 (PFS).

Badra & Hajjeh Informational [Page 1] RFC 5489 ECDHE_PSK Cipher Suites for TLS March 2009

Table of Contents

 1. Introduction ....................................................2
    1.1. Applicability Statement ....................................3
    1.2. Conventions Used in This Document ..........................3
 2. ECDHE_PSK Key Exchange Algorithm ................................3
 3. ECDHE_PSK-Based Cipher Suites ...................................4
    3.1. ECDHE_PSK Cipher Suites Using the SHA-1 Hash ...............4
    3.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes .................4
 4. ECDHE_PSK-Based Cipher Suites with NULL Encryption ..............5
    4.1. ECDHE_PSK Cipher Suite Using the SHA-1 Hash with
         NULL Encryption ............................................5
    4.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes with
         NULL Encryption ............................................5
 5. Security Considerations .........................................5
 6. IANA Considerations .............................................6
 7. Acknowledgments .................................................6
 8. Normative References ............................................6

1. Introduction

 RFC 4279 specifies cipher suites for supporting TLS using pre-shared
 symmetric keys that (a) use only symmetric key operations for
 authentication, (b) use a Diffie-Hellman exchange authenticated with
 a pre-shared key (PSK), or (c) combine public key authentication of
 the server with pre-shared key authentication of the client.
 RFC 4785 specifies authentication-only cipher suites (with no
 encryption).  These cipher suites are useful when authentication and
 integrity protection is desired, but confidentiality is not needed or
 not permitted.
 RFC 4492 defines a set of Elliptic Curve Cryptography (ECC)-based
 cipher suites for TLS and describes the use of ECC certificates for
 client authentication.  In particular, it specifies the use of
 Elliptic Curve Diffie-Hellman (ECDH) key agreement in a TLS handshake
 and the use of the Elliptic Curve Digital Signature Algorithm (ECDSA)
 as a new authentication mechanism.
 This document specifies a set of cipher suites that use a PSK to
 authenticate an ECDH exchange.  These cipher suites provide Perfect
 Forward Secrecy.  Some of these cipher suites provide authentication
 only.
 The reader is expected to become familiar with RFC 4279, RFC 4492,
 and RFC 4785 prior to studying this document.

Badra & Hajjeh Informational [Page 2] RFC 5489 ECDHE_PSK Cipher Suites for TLS March 2009

1.1. Applicability Statement

 The cipher suites defined in this document can be negotiated,
 whatever the negotiated TLS version is.
 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. ECDHE_PSK Key Exchange Algorithm

 The cipher suites described in this document make use of the elliptic
 curve (EC) parameter negotiation mechanism defined in RFC 4492.  When
 the cipher suites defined in this document are used, the
 'ec_diffie_hellman_psk' case inside the ServerKeyExchange and
 ClientKeyExchange structure MUST be used instead of the 'psk' case
 defined in [RFC4279] (i.e., the ServerKeyExchange and
 ClientKeyExchange messages include the EC Diffie-Hellman parameters
 in the form specified in Sections 5.4 and 5.7 of [RFC4492]).  The PSK
 identity and identity hint fields have the same meaning and encoding
 as specified in [RFC4279] (note that the ServerKeyExchange message is
 always sent, even if no PSK identity hint is provided).
 The format of the ServerKeyExchange and ClientKeyExchange messages is
 shown below.
    struct {
        select (KeyExchangeAlgorithm) {
            /* other cases for rsa, diffie_hellman, etc. */
            case ec_diffie_hellman_psk:  /* NEW */
                opaque psk_identity_hint<0..2^16-1>;
                ServerECDHParams params;
        };
    } ServerKeyExchange;
    struct {
        select (KeyExchangeAlgorithm) {
            /* other cases for rsa, diffie_hellman, etc. */
            case ec_diffie_hellman_psk:   /* NEW */
                opaque psk_identity<0..2^16-1>;
                ClientECDiffieHellmanPublic public;
        } exchange_keys;
    } ClientKeyExchange;

Badra & Hajjeh Informational [Page 3] RFC 5489 ECDHE_PSK Cipher Suites for TLS March 2009

 The premaster secret is formed as follows.  First, perform the ECDH
 computation as described in Section 5.10 of [RFC4492].  Let Z be the
 octet string produced by this computation.  Next, concatenate a
 uint16 containing the length of Z (in octets), Z itself, a uint16
 containing the length of the PSK (in octets), and the PSK itself.
 This corresponds to the general structure for the premaster secrets
 (see Note 1 in Section 2 of [RFC4279]), with "other_secret"
 containing Z.
    struct {
        opaque other_secret<0..2^16-1>;
        opaque psk<0..2^16-1>;
    };

3. ECDHE_PSK-Based Cipher Suites

3.1. ECDHE_PSK Cipher Suites Using the SHA-1 Hash

    CipherSuite TLS_ECDHE_PSK_WITH_RC4_128_SHA          = {0xC0,0x33};
    CipherSuite TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA     = {0xC0,0x34};
    CipherSuite TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA      = {0xC0,0x35};
    CipherSuite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA      = {0xC0,0x36};
 The above four cipher suites match the cipher suites defined in
 [RFC4279], except that they use an Elliptic Curve Diffie-Hellman
 exchange [RFC4492] authenticated with a PSK, and:
 o  The Message Authentication Code (MAC) is the Hashed Message
    Authentication Code (HMAC) [RFC2104] with SHA-1 as the hash
    function.
 o  When negotiated in a version of TLS prior to 1.2, the Pseudo-
    Random Function (PRF) from that version is used; otherwise, the
    PRF is the TLS PRF [RFC5246] with SHA-256 as the hash function.

3.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes

    CipherSuite TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256   = {0xC0,0x37};
    CipherSuite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384   = {0xC0,0x38};
 The above two cipher suites are the same as the corresponding
 Advanced Encryption Standard (AES) cipher suites in Section 3.1
 above, except for the hash and PRF algorithms, which SHALL be as
 follows:

Badra & Hajjeh Informational [Page 4] RFC 5489 ECDHE_PSK Cipher Suites for TLS March 2009

 o  For the cipher suite TLS_ECDHE_PSK_WITH_AES_128_CBC_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 the cipher suite TLS_ECDHE_PSK_WITH_AES_256_CBC_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.

4. ECDHE_PSK-Based Cipher Suites with NULL Encryption

4.1. ECDHE_PSK Cipher Suite Using the SHA-1 Hash with NULL Encryption

 The following cipher suite matches the cipher suites defined in
 Section 3.1, except that we define a suite with NULL encryption.
    CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA             = {0xC0,0x39};

4.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes with NULL Encryption

 The following two cipher suites are the same as the corresponding
 cipher suites in Section 3.2, but with NULL encryption (instead of
 AES).
    CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA256          = {0xC0,0x3A};
    CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA384          = {0xC0,0x3B};

5. Security Considerations

 The security considerations described throughout [RFC5246],
 [RFC4785], [RFC4492], and [RFC4279] apply here as well.  In
 particular, as the 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 Section 4 of this document.

Badra & Hajjeh Informational [Page 5] RFC 5489 ECDHE_PSK Cipher Suites for TLS March 2009

 Implementers and administrators should monitor the general statements
 on recommended cryptographic algorithms (e.g., SHA-1 hash function)
 that are published from time to time by various forums, including the
 IETF, as a base for the portfolio they support and the policies for
 strength of function acceptable for the cipher suites they set.

6. IANA Considerations

 This document defines the following new cipher suites, whose values
 have been assigned from the TLS Cipher Suite registry defined in
 [RFC5246].
    CipherSuite TLS_ECDHE_PSK_WITH_RC4_128_SHA          = {0xC0,0x33};
    CipherSuite TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA     = {0xC0,0x34};
    CipherSuite TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA      = {0xC0,0x35};
    CipherSuite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA      = {0xC0,0x36};
    CipherSuite TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256   = {0xC0,0x37};
    CipherSuite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384   = {0xC0,0x38};
    CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA             = {0xC0,0x39};
    CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA256          = {0xC0,0x3A};
    CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA384          = {0xC0,0x3B};

7. Acknowledgments

 The author appreciates Alfred Hoenes for his detailed review and
 effort on resolving issues in discussion.  The author would like to
 acknowledge Bodo Moeller, Simon Josefsson, Uri Blumenthal, Pasi
 Eronen, Paul Hoffman, Joseph Salowey, Mark Tillinghast, and the TLS
 mailing list members for their comments on the document.

8. Normative References

 [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.
 [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
            Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
            for Transport Layer Security (TLS)", RFC 4492, May 2006.

Badra & Hajjeh Informational [Page 6] RFC 5489 ECDHE_PSK Cipher Suites for TLS March 2009

 [RFC4785]  Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK)
            Ciphersuites with NULL Encryption for Transport Layer
            Security (TLS)", RFC 4785, January 2007.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008.

Authors' Addresses

 Mohamad Badra
 CNRS/LIMOS Laboratory
 Campus de cezeaux, Bat. ISIMA
 Aubiere  63170
 France
 EMail: badra@isima.fr
 Ibrahim Hajjeh
 INEOVATION
 France
 EMail: ibrahim.hajjeh@ineovation.fr

Badra & Hajjeh Informational [Page 7]

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