TLS Working Group Mohamad Badra
Internet Draft LIMOS Laboratory
Intended status: Informational February 1, 2008
Expires: July 2008
ECDHE_PSK Ciphersuites for Transport Layer Security (TLS)
draft-badra-ecdhe-tls-psk-03.txt
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Copyright (C) The IETF Trust (2008).
Abstract
This document extends RFC 4279 and RFC 4785 and specifies a set of
ciphersuites that use an Elliptic Curve Diffie-Hellman exchange
authenticated with a pre-shared key. These ciphersuites provide
Perfect Forward Secrecy. It also specifies one authentication-only
ciphersuites (with no encryption). This ciphersuite is useful when
authentication and integrity protection is desired, but
confidentiality is not needed or not permitted.
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The reader is expected to become familiar with RFC 4279 and RFC 4785
prior to studying this document.
1. Introduction
RFC 4279 specifies ciphersuites 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) combines public key authentication of the
server with pre-shared key authentication of the client.
RFC 4785 specifies authentication-only ciphersuites (with no
encryption).
This document specifies a set of ciphersuites that use an Elliptic
Curve Diffie-Hellman exchange authenticated with a pre-shared key.
These ciphersuites provide Perfect Forward Secrecy. This document
also specifies one authentication-only ciphersuites (with no
encryption). This ciphersuite is useful when authentication and
integrity protection is desired, but confidentiality is not needed or
not permitted.
1.1. 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 ciphersuites in this section match the ciphersuites defined in
[RFC4279], except that they use an Elliptic Curve Diffie-Hellman
exchange [RFC4492] authenticated with a pre-shared key. They are
defined as follow:
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
When the ciphersuites defined in this document are used, the
'ec_diffie_hellman_psk' case inside the ServerKeyExchange and
ClientKeyExchange structure is 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
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identity hint fields MUST have the same meaning 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 an ECDH
operation (See section 5.10 of [RFC4492]) to compute the shared
secret. Next, concatenate a uint16 containing the length of the
shared secret (in octets), the shared secret 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 the shared secret:
struct {
opaque other_secret<0..2^16-1>;
opaque psk<0..2^16-1>;
};
3. 2. ECDHE_PSK Key Exchange Algorithm with NULL Encryption
The ciphersuite in this section matches the ciphersuites defined in
[RFC4785], except that it uses an Elliptic Curve Diffie-Hellman
exchange authenticated with a pre-shared key.
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CipherSuite Key Exchange Cipher Hash
TLS_ECDHE_PSK_WITH_NULL_SHA ECDHE_PSK NULL SHA
4. Security Considerations
The security considerations described throughout [RFC4346], [RFC4785]
and [RFC4279] apply here as well.
5. IANA Considerations
This document defines the following new ciphersuites, whose values
are to be assigned from the TLS Cipher Suite registry defined in
[RFC4346].
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, 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.
[RFC4346] Dierks, T., Rescorla, E., "The TLS Protocol Version 1.1",
RFC 4346, April 200P.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279, December
2005.
[RFC4785] Blumenthal, U., Goel, P., "Pre-Shared Key (PSK)
Ciphersuites with NULL Encryption for Transport Layer
Security (TLS)", RFC 4785, January 2007.
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[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C.,
Moeller, B., "Elliptic Curve Cryptography (ECC) Cipher
Suites for Transport Layer Security (TLS)", RFC 4492, May
2006.
Author's Addresses
Mohamad Badra
LIMOS Laboratory - UMR6158, CNRS
France
Email: badra@isima.fr
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