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Keywords: pre-shared key, Diffie-Hellman, Key Exchange, Elliptic Curve Cryptography
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.
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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).
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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.
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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;
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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:
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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.
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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.
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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
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