TLS Working Group Mohamad Badra
Internet Draft LIMOS Laboratory
Intended status: Informational September 29, 2008
Expires: March 2009
ECDHE_PSK Ciphersuites for Transport Layer Security (TLS)
draft-ietf-tls-ecdhe-psk-03.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. Applicability Statement...................................3
1.2. Conventions used in this document.........................3
2. ECDHE_PSK Key Exchange Algorithm...............................3
3. ECDHE_PSK Based Cipher Suites..................................5
3.1. ECDHE_PSK Cipher Suites Using the SHA-1 Hash..............5
3.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes................5
4. ECDHE_PSK Based Cipher Suites with NULL Encryption.............5
4.1. ECDHE_PSK Cipher Suites 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........................................6
6. IANA Considerations............................................6
7. Acknowledgments................................................6
8. References.....................................................6
8.1. Normative References......................................6
Author's Addresses................................................7
Intellectual Property Statement...................................7
Disclaimer of Validity............................................8
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1. Introduction
RFC 4279 specifies cipher suites for supporting TLS using pre-shared
symmetric keys which (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 Sections 3.1 and 4.1 can be negotiated,
whatever the negotiated TLS version is.
The ciphersuites defined in Sections 3.2 and 4.2 can be negotiated in
TLS version 1.2 or higher.
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 ciphersuites described in this document make use of the EC
parameter negotiation mechanism defined in RFC 4492. When the
ciphersuites defined in this document are used, the
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'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.
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>;
};
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3. ECDHE_PSK Based Cipher Suites
3.1. ECDHE_PSK Cipher Suites Using the SHA-1 Hash
The following cipher suites match the cipher suites defined in
[RFC4279], except that they use an Elliptic Curve Diffie-Hellman
exchange [RFC4492] authenticated with a PSK.
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};
For all four cipher suites, the PRF is the TLS PRF [RFC5246] with
SHA-1 as the hash function, and the MAC is HMAC [RFC2104] with SHA-1
as the hash function.
3.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes
CipherSuite TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384 = {0xXX,0xXX};
The above two cipher suites are the same as the corresponding AES
cipher suites in section 3.1 above, except for the hash and PRF
algorithms, which SHALL be as follows:
For TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256, the PRF is the TLS PRF
[RFC5246] with SHA-256 as the hash function, and the MAC is HMAC
[RFC2104] with SHA-256 as the hash function.
For TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384, the PRF is the TLS PRF
[RFC5246] with SHA-384 as the hash function, and the MAC is HMAC
[RFC2104] with SHA-384 as the hash function.
4. ECDHE_PSK Based Cipher Suites with NULL Encryption
4.1. ECDHE_PSK Cipher Suites 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 = {0xXX,0xXX};
4.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes with NULL Encryption
CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA256 = {0xXX,0xXX};
CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA384 = {0xXX,0xXX};
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These two cipher suites are the same as the corresponding cipher
suites in section 3.2, but with NULL encryption (instead of AES).
5. Security Considerations
The security considerations described throughout [RFC5246],
[RFC4785], [RFC4492], and [RFC4279] apply here as well. In
particular, as 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 this document.
6. IANA Considerations
This document defines the following new cipher suites, whose values
are to be assigned from the TLS Cipher Suite registry defined in
[RFC5246].
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_AES_128_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384 = {0xXX,0xXX};
CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA = {0xXX,0xXX};
CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA256 = {0xXX,0xXX};
CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA384 = {0xXX,0xXX};
7. Acknowledgments
The author appreciates Alfred Hoenes for his detailed review and
effort on issues resolving discussion. The author would like to
acknowledge 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.
8. References
8.1. Normative References
[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.
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[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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
Author's Addresses
Mohamad Badra
LIMOS Laboratory - UMR6158, CNRS
France
Email: badra@isima.fr
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