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<pre>Network Working Group M. Badra
Request for Comments: 5489 CNRS/LIMOS Laboratory
Category: Informational I. Hajjeh
INEOVATION
March 2009
<span class="h1">ECDHE_PSK Cipher Suites for Transport Layer Security (TLS)</span>
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 <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>).
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 <a href="./rfc4279">RFC 4279</a>, <a href="./rfc4492">RFC 4492</a>, and <a href="./rfc4785">RFC 4785</a> 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).
<span class="grey">Badra & Hajjeh Informational [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc5489">RFC 5489</a> ECDHE_PSK Cipher Suites for TLS March 2009</span>
Table of Contents
<a href="#section-1">1</a>. Introduction ....................................................<a href="#page-2">2</a>
<a href="#section-1.1">1.1</a>. Applicability Statement ....................................<a href="#page-3">3</a>
<a href="#section-1.2">1.2</a>. Conventions Used in This Document ..........................<a href="#page-3">3</a>
<a href="#section-2">2</a>. ECDHE_PSK Key Exchange Algorithm ................................<a href="#page-3">3</a>
<a href="#section-3">3</a>. ECDHE_PSK-Based Cipher Suites ...................................<a href="#page-4">4</a>
<a href="#section-3.1">3.1</a>. ECDHE_PSK Cipher Suites Using the SHA-1 Hash ...............<a href="#page-4">4</a>
<a href="#section-3.2">3.2</a>. ECDHE_PSK Cipher Suites Using SHA-2 Hashes .................<a href="#page-4">4</a>
<a href="#section-4">4</a>. ECDHE_PSK-Based Cipher Suites with NULL Encryption ..............<a href="#page-5">5</a>
4.1. ECDHE_PSK Cipher Suite Using the SHA-1 Hash with
NULL Encryption ............................................<a href="#page-5">5</a>
4.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes with
NULL Encryption ............................................<a href="#page-5">5</a>
<a href="#section-5">5</a>. Security Considerations .........................................<a href="#page-5">5</a>
<a href="#section-6">6</a>. IANA Considerations .............................................<a href="#page-6">6</a>
<a href="#section-7">7</a>. Acknowledgments .................................................<a href="#page-6">6</a>
<a href="#section-8">8</a>. Normative References ............................................<a href="#page-6">6</a>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
<a href="./rfc4279">RFC 4279</a> 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.
<a href="./rfc4785">RFC 4785</a> 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.
<a href="./rfc4492">RFC 4492</a> 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 <a href="./rfc4279">RFC 4279</a>, <a href="./rfc4492">RFC 4492</a>,
and <a href="./rfc4785">RFC 4785</a> prior to studying this document.
<span class="grey">Badra & Hajjeh Informational [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc5489">RFC 5489</a> ECDHE_PSK Cipher Suites for TLS March 2009</span>
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Applicability Statement</span>
The cipher suites defined in this document can be negotiated,
whatever the negotiated TLS version is.
The applicability statement in [<a href="./rfc4279" title=""Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)"">RFC4279</a>] applies to this document as
well.
<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>. Conventions Used in This Document</span>
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 [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. ECDHE_PSK Key Exchange Algorithm</span>
The cipher suites described in this document make use of the elliptic
curve (EC) parameter negotiation mechanism defined in <a href="./rfc4492">RFC 4492</a>. 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 [<a href="./rfc4279" title=""Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)"">RFC4279</a>] (i.e., the ServerKeyExchange and
ClientKeyExchange messages include the EC Diffie-Hellman parameters
in the form specified in Sections <a href="#section-5.4">5.4</a> and <a href="#section-5.7">5.7</a> of [<a href="./rfc4492" title=""Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)"">RFC4492</a>]). The PSK
identity and identity hint fields have the same meaning and encoding
as specified in [<a href="./rfc4279" title=""Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)"">RFC4279</a>] (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;
<span class="grey">Badra & Hajjeh Informational [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc5489">RFC 5489</a> ECDHE_PSK Cipher Suites for TLS March 2009</span>
The premaster secret is formed as follows. First, perform the ECDH
computation as described in <a href="./rfc4492#section-5.10">Section 5.10 of [RFC4492]</a>. 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 <a href="./rfc4279#section-2">Section 2 of [RFC4279]</a>), with "other_secret"
containing Z.
struct {
opaque other_secret<0..2^16-1>;
opaque psk<0..2^16-1>;
};
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. ECDHE_PSK-Based Cipher Suites</span>
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. ECDHE_PSK Cipher Suites Using the SHA-1 Hash</span>
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
[<a href="./rfc4279" title=""Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)"">RFC4279</a>], except that they use an Elliptic Curve Diffie-Hellman
exchange [<a href="./rfc4492" title=""Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)"">RFC4492</a>] authenticated with a PSK, and:
o The Message Authentication Code (MAC) is the Hashed Message
Authentication Code (HMAC) [<a href="./rfc2104" title=""HMAC: Keyed- Hashing for Message Authentication"">RFC2104</a>] 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 [<a href="./rfc5246" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">RFC5246</a>] with SHA-256 as the hash function.
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. ECDHE_PSK Cipher Suites Using SHA-2 Hashes</span>
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 <a href="#section-3.1">Section 3.1</a>
above, except for the hash and PRF algorithms, which SHALL be as
follows:
<span class="grey">Badra & Hajjeh Informational [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc5489">RFC 5489</a> ECDHE_PSK Cipher Suites for TLS March 2009</span>
o For the cipher suite TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256:
* The MAC is HMAC [<a href="./rfc2104" title=""HMAC: Keyed- Hashing for Message Authentication"">RFC2104</a>] 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
[<a href="./rfc5246" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">RFC5246</a>] with SHA-256 as the hash function.
o For the cipher suite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384:
* The MAC is HMAC [<a href="./rfc2104" title=""HMAC: Keyed- Hashing for Message Authentication"">RFC2104</a>] 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
[<a href="./rfc5246" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">RFC5246</a>] with SHA-384 as the hash function.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. ECDHE_PSK-Based Cipher Suites with NULL Encryption</span>
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. ECDHE_PSK Cipher Suite Using the SHA-1 Hash with NULL Encryption</span>
The following cipher suite matches the cipher suites defined in
<a href="#section-3.1">Section 3.1</a>, except that we define a suite with NULL encryption.
CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA = {0xC0,0x39};
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. ECDHE_PSK Cipher Suites Using SHA-2 Hashes with NULL Encryption</span>
The following two cipher suites are the same as the corresponding
cipher suites in <a href="#section-3.2">Section 3.2</a>, 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};
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Security Considerations</span>
The security considerations described throughout [<a href="./rfc5246" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">RFC5246</a>],
[<a href="./rfc4785" title=""Pre-Shared Key (PSK) Ciphersuites with NULL Encryption for Transport Layer Security (TLS)"">RFC4785</a>], [<a href="./rfc4492" title=""Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)"">RFC4492</a>], and [<a href="./rfc4279" title=""Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)"">RFC4279</a>] 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 <a href="#section-4">Section 4</a> of this document.
<span class="grey">Badra & Hajjeh Informational [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc5489">RFC 5489</a> ECDHE_PSK Cipher Suites for TLS March 2009</span>
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.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. IANA Considerations</span>
This document defines the following new cipher suites, whose values
have been assigned from the TLS Cipher Suite registry defined in
[<a href="./rfc5246" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">RFC5246</a>].
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};
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Acknowledgments</span>
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.
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Normative References</span>
[<a id="ref-RFC2104">RFC2104</a>] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", <a href="./rfc2104">RFC 2104</a>,
February 1997.
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC4279">RFC4279</a>] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", <a href="./rfc4279">RFC 4279</a>,
December 2005.
[<a id="ref-RFC4492">RFC4492</a>] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", <a href="./rfc4492">RFC 4492</a>, May 2006.
<span class="grey">Badra & Hajjeh Informational [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc5489">RFC 5489</a> ECDHE_PSK Cipher Suites for TLS March 2009</span>
[<a id="ref-RFC4785">RFC4785</a>] Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK)
Ciphersuites with NULL Encryption for Transport Layer
Security (TLS)", <a href="./rfc4785">RFC 4785</a>, January 2007.
[<a id="ref-RFC5246">RFC5246</a>] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", <a href="./rfc5246">RFC 5246</a>, 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]
</pre>
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