TLS Working Group Mohamad Badra
Internet Draft LIMOS Laboratory
Intended status: Informational July 28, 2008
Expires: December 2008
ECDHE_PSK Ciphersuites for Transport Layer Security (TLS)
draft-ietf-tls-ecdhe-psk-02.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This document extends RFC 4279, RFC 4492 and RFC 4785, and specifies
a set of ciphersuites that use a pre-shared key (PSK) to authenticate
an Elliptic Curve Diffie-Hellman exchange (ECDH). These ciphersuites
provide Perfect Forward Secrecy (PFS).
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Table of Contents
1. Introduction...................................................3
1.1. Conventions used in this document.........................3
2. ECDHE_PSK Key Exchange Algorithm...............................3
3. ECDHE_PSK Key Exchange Algorithm with NULL Encryption..........5
4. Security Considerations........................................5
5. IANA Considerations............................................5
6. Acknowledgments................................................5
7. References.....................................................5
7.1. Normative References......................................5
Author's Addresses................................................6
Intellectual Property and Copyright Statements....................6
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1. Introduction
RFC 4279 specifies cipher suites for supporting TLS using pre-shared
symmetric keys and they (a) use only symmetric key operations for
authentication, (b) use a Diffie-Hellman exchange authenticated with
a pre-shared key, 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 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 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. One of these ciphersuites provides authentication-
only.
The reader is expected to become familiar with RFC 4279, RFC 4492,
and RFC 4785 prior to studying this document.
1.1. Applicability Statement
The ciphersuites defined in this document can be negotiated, whatever
the negotiated TLS version
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 in this section match the cipher suites defined in
[RFC4279], except that they use an Elliptic Curve Diffie-Hellman
exchange [RFC4492] authenticated with a PSK. They are defined as
follow:
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CipherSuite Key Exchange Cipher Hash
TLS_ECDHE_PSK_WITH_RC4_128_SHA ECDHE_PSK RC4_128 SHA
TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA ECDHE_PSK 3DES_EDE_CBC SHA
TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA ECDHE_PSK AES_128_CBC SHA
TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA ECDHE_PSK AES_256_CBC SHA
These ciphersuites make use of the EC parameter negotiation mechanism
defined in RFC 4492. When the ciphersuites 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 Diffie-
Hellman parameters). 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;
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.
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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 Key Exchange Algorithm with NULL Encryption
The cipher suite in this section matches the cipher suites defined in
section 2, except that we define a suite with null encryption.
CipherSuite Key Exchange Cipher Hash
TLS_ECDHE_PSK_WITH_NULL_SHA ECDHE_PSK NULL SHA
4. Security Considerations
The security considerations described throughout [RFC4279], [ID-ietf-
tls-rfc4346-bis], [RFC4492], and [RFC4785] apply here as well.
5. IANA Considerations
This document defines the following new cipher suites, whose values
are to be assigned from the TLS Cipher Suite registry defined in [ID-
ietf-tls-rfc4346-bis].
CipherSuite TLS_ECDHE_PSK_WITH_RC4_128_SHA = { 0xXX, 0xXX };
CipherSuite TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA = { 0xXX, 0xXX };
CipherSuite TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA = { 0xXX, 0xXX };
CipherSuite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA = { 0xXX, 0xXX };
CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA = { 0xXX, 0xXX };
6. Acknowledgments
The author would like to thank Bodo Moeller, Simon Josefsson, Uri
Blumenthal, Pasi Eronen, Alfred Hoenes, Paul Hoffman, Joseph Salowey,
and the TLS mailing list members for their comments on the document.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279, December
2005.
[ID-ietf-tls-rfc4346-bis]
Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.2",
draft-ietf-tls-rfc4346-bis-10 (work in progress), March
2008.
[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.
[RFC4785] Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK)
Ciphersuites with NULL Encryption for Transport Layer
Security (TLS)", RFC 4785, January 2007.
Author's Addresses
Mohamad Badra
LIMOS Laboratory - UMR6158, CNRS
France
Email: badra@isima.fr
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