TLS Working Group P. Eronen
Internet-Draft Nokia
Expires: February 16, 2005 H. Tschofenig
Siemens
August 18, 2004
Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)
draft-ietf-tls-psk-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies three sets of new ciphersuites for the
Transport Layer Security (TLS) protocol to support authentication
based on pre-shared keys. These pre-shared keys are symmetric keys,
shared in advance among the communicating parties. The first set of
ciphersuites uses only symmetric key operations for authentication.
The second set uses a Diffie-Hellman exchange authenticated with a
pre-shared key; and the third set combines public key authentication
of the server with pre-shared key authentication of the client.
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1. Introduction
Usually TLS uses public key certificates [3] or Kerberos [11] for
authentication. This document describes how to use symmetric keys
(later called pre-shared keys or PSKs), shared in advance among the
communicating parties, to establish a TLS connection.
There are basically two reasons why one might want to do this:
o First, TLS may be used in performance-constrained environments
where the CPU power needed for public key operations is not
available.
o Second, pre-shared keys may be more convenient from a key
management point of view. For instance, in closed environments
where the connections are mostly configured manually in advance,
it may be easier to configure a PSK than to use certificates.
Another case is when the parties already have a mechanism for
setting up a shared secret key, and that mechanism could be used
to "bootstrap" a key for authenticating a TLS connection.
This document specifies three sets of new ciphersuites for TLS.
These ciphersuites use new key exchange algorithms, and re-use
existing cipher and MAC algorithms from [3] and [2]. A summary of
these ciphersuites is shown below.
CipherSuite Key Exchange Cipher Hash
TLS_PSK_WITH_RC4_128_SHA PSK RC4_128 SHA
TLS_PSK_WITH_3DES_EDE_CBC_SHA PSK 3DES_EDE_CBC SHA
TLS_PSK_WITH_AES_128_CBC_SHA PSK AES_128_CBC SHA
TLS_PSK_WITH_AES_256_CBC_SHA PSK AES_256_CBC SHA
TLS_DHE_PSK_WITH_RC4_128_SHA DHE_PSK RC4_128 SHA
TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA DHE_PSK 3DES_EDE_CBC SHA
TLS_DHE_PSK_WITH_AES_128_CBC_SHA DHE_PSK AES_128_CBC SHA
TLS_DHE_PSK_WITH_AES_256_CBC_SHA DHE_PSK AES_256_CBC SHA
TLS_RSA_PSK_WITH_RC4_128_SHA RSA_PSK RC4_128 SHA
TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA RSA_PSK 3DES_EDE_CBC SHA
TLS_RSA_PSK_WITH_AES_128_CBC_SHA RSA_PSK AES_128_CBC SHA
TLS_RSA_PSK_WITH_AES_256_CBC_SHA RSA_PSK AES_256_CBC SHA
The first set of ciphersuites (with PSK key exchange algorithm),
defined in Section 2 use only symmetric key algorithms, and are thus
especially suitable for performance-constrained environments.
The ciphersuites in Section 3 (with DHE_PSK key exchange algorithm)
use a PSK to authenticate a Diffie-Hellman exchange. These
ciphersuites give some additional protection against dictionary
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attacks, and also provide Perfect Forward Secrecy (PFS).
The third set of ciphersuites (with RSA_PSK key exchange algorithm),
defined in Section 4, combine public key based authentication of the
server (using RSA and certificates) with mutual authentication using
a PSK.
1.1 Applicability statement
The ciphersuites defined in this document are intended for a rather
limited set of applications, usually involving only a very small
number of clients and servers. Even in such environments, other
alternatives may be more appropriate.
If the main goal is to avoid PKIs, another possibility worth
considering is to use self-signed certificates with public key
fingerprints. Instead of manually configuring a shared secret in,
for instance, some configuration file, a fingerprint (hash) of the
other party's public key (or certificate) could be placed there
instead.
It is also possible to use the SRP (Secure Remote Password)
ciphersuites for shared secret authentication [13]. SRP was designed
to be used with passwords, and incorporates protection against
dictionary attacks. However, it is computationally more expensive
than the PSK ciphersuites in Section 2.
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 [1].
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2. PSK key exchange algorithm
This section defines the PSK key exchange algorithm and associated
ciphersuites. These ciphersuites use only symmetric key algorithms.
It is assumed that the reader is familiar with ordinary TLS
handshake, shown below. The elements in parenthesis are not included
when PSK key exchange algorithm is used.
Client Server
------ ------
ClientHello -------->
ServerHello
(Certificate)
ServerKeyExchange
(CertificateRequest)
<-------- ServerHelloDone
(Certificate)
ClientKeyExchange
(CertificateVerify)
ChangeCipherSpec
Finished -------->
ChangeCipherSpec
<-------- Finished
Application Data <-------> Application Data
The client indicates its willingness to use pre-shared key
authentication by including one or more PSK ciphersuites in the
ClientHello message. If the TLS server also wants to use pre-shared
keys, it selects one of the PSK ciphersuites, places the selected
ciphersuite in the ServerHello message, and includes an appropriate
ServerKeyExchange message (see below). The Certificate and
CertificateRequest payloads are omitted from the response.
Both clients and servers may have pre-shared keys with several
different parties. The client indicates which key to use by
including a "PSK identity" in the ClientKeyExchange message (note
that unlike in [6], the session_id field in ClientHello message keeps
its usual meaning). To help the client in selecting which identity
to use, the server can provide a "PSK identity hint" in the
ServerKeyExchange message (note that if no hint is provided, a
ServerKeyExchange message is still sent).
This document does not specify the format of the PSK identity or PSK
identity hint; neither is specified how exactly the client uses the
hint (if it uses it at all). The parties have to agree on the
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identities when the shared secret is configured (however, see Section
6 for related security considerations). In situations where the
identity is entered by a person, it is RECOMMENDED that the input is
processed using an appropriate stringprep [9] profile and encoded in
octets using UTF-8 encoding [14]. One possible stringprep profile is
described in [8].
The format of the ServerKeyExchange and ClientKeyExchange messages is
shown below.
struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case psk: /* NEW */
opaque psk_identity_hint<0..2^16-1>;
};
} ServerKeyExchange;
struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case psk: /* NEW */
opaque psk_identity<0..2^16-1>;
} exchange_keys;
} ClientKeyExchange;
The premaster secret is formed as follows: If the PSK is N octets
long, concatenate N zero octets and the PSK.
Note: This effectively means that only the HMAC-SHA1 part of the
TLS PRF is used, and the HMAC-MD5 part is not used. See [7] for a
more detailed rationale.
The TLS handshake is authenticated using the Finished messages as
usual.
If the server does not recognize the PSK identity, it MAY respond
with an "unknown_psk_identity" alert message. Alternatively, if the
server wishes to hide the fact that the PSK identity was not known,
it MAY continue the protocol as if the PSK identity existed but the
key was incorrect: that is, respond with a "decrypt_error" alert.
3. DHE_PSK key exchange algorithm
This section defines additional ciphersuites that use a PSK to
authenticate a Diffie-Hellman exchange. These ciphersuites give some
additional protection against dictionary attacks, and also provide
Perfect Forward Secrecy (PFS). See Section 6 for discussion of
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related security considerations.
When these ciphersuites are used, the ServerKeyExchange and
ClientKeyExchange also include the Diffie-Hellman parameters. The
PSK identity and identity hint fields have the same meaning as in the
previous section.
The format of the ServerKeyExchange and ClientKeyExchange messages is
shown below.
struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case diffie_hellman_psk: /* NEW */
opaque psk_identity_hint<0..2^16-1>;
ServerDHParams params;
};
} ServerKeyExchange;
struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case diffie_hellman_psk: /* NEW */
opaque psk_identity<0..2^16-1>;
ClientDiffieHellmanPublic public;
} exchange_keys;
} ClientKeyExchange;
The premaster secret is formed as follows: concatenate the value
produced by the Diffie-Hellman exchange (with leading zero bytes
stripped as in other Diffie-Hellman based ciphersuites) and the PSK,
in this order.
4. RSA_PSK key exchange algorithm
The ciphersuites in this section use RSA and certificates to
authenticate the server, in addition to using a PSK.
As in normal RSA ciphersuites, the server must send a Certificate
message. The format of the ServerKeyExchange and ClientKeyExchange
messages is shown below.
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struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case rsa_psk: /* NEW */
opaque psk_identity_hint<0..2^16-1>;
};
} ServerKeyExchange;
struct {
select (KeyExchangeAlgorithm) {
/* other cases for rsa, diffie_hellman, etc. */
case rsa_psk: /* NEW */
opaque psk_identity<0..2^16-1>;
EncryptedPreMasterSecret;
} exchange_keys;
} ClientKeyExchange;
The premaster secret is formed as follows: concatenate the 48-byte
value generated by the client (and sent to the server in
ClientKeyExchange message) and the PSK, in this order.
5. IANA considerations
(This depends on whether this document is published before or after
TLS 1.1.)
(If after 1.1) This document does not create any new namespaces to be
maintained by IANA, but it requires new values in the ciphersuite
namespace defined in TLS 1.1 specification.
(If before 1.1) There are no IANA actions associated with this
document. For easier reference in the future, the ciphersuite
numbers defined in this document are summarized below.
CipherSuite TLS_PSK_WITH_RC4_128_SHA = { 0x00, 0xTBD };
CipherSuite TLS_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0xTBD };
CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0xTBD };
CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0xTBD };
CipherSuite TLS_DHE_PSK_WITH_RC4_128_SHA = { 0x00, 0xTBD };
CipherSuite TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0xTBD };
CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0xTBD };
CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0xTBD };
CipherSuite TLS_RSA_PSK_WITH_RC4_128_SHA = { 0x00, 0xTBD };
CipherSuite TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0xTBD };
CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0xTBD };
CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0xTBD };
This document also defines a new TLS alert message,
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unknown_psk_identity(TBD). Since IANA does not maintain a registry
of TLS alert messages, no IANA action is needed for this.
6. Security Considerations
As with all schemes involving shared keys, special care should be
taken to protect the shared values and to limit their exposure over
time.
6.1 Perfect forward secrecy (PFS)
The PSK and RSA_PSK ciphersuites defined in this document do not
provide Perfect Forward Secrecy (PFS). That is, if the shared secret
key (in PSK ciphersuites), or both the shared secret key and the RSA
private key (in RSA_PSK ciphersuites), is somehow compromised, an
attacker can decrypt old conversations.
The DHE_PSK ciphersuites provide Perfect Forward Secrecy if a fresh
DH private key is generated for each handshake.
6.2 Brute-force and dictionary attacks
Use of a fixed shared secret of limited entropy (such as a password)
may allow an attacker to perform a brute-force or dictionary attack
to recover the secret. This may be either an off-line attack
(against a captured TLS handshake messages), or an on-line attack
where the attacker attempts to connect to the server and tries
different keys.
For the PSK ciphersuites, an attacker can get the information
required for an off-line attack by eavesdropping a TLS handshake, or
by getting a valid client to attempt connection with the attacker (by
tricking the client to connect to wrong address, or intercepting a
connection attempt to the correct address, for instance).
For the DHE_PSK ciphersuites, an attacker can obtain the information
by getting a valid client to attempt connection with the attacker.
Passive eavesdropping alone is not sufficient.
For the RSA_PSK ciphersuites, only the server (authenticated using
RSA and certificates) can obtain sufficient information for an
off-line attack.
It is RECOMMENDED that implementations that allow the administrator
to manually configure the PSK also provide a functionality for
generating a new random PSK, taking [4] into account.
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6.3 Identity privacy
The PSK identity is sent in cleartext. While using a user name or
other similar string as the PSK identity is the most straightforward
option, it may lead to problems in some environments since an
eavesdropper is able to identify the communicating parties. Even
when the identity does not reveal any information itself, reusing the
same identity over time may eventually allow an attacker to perform
traffic analysis to identify the parties. It should be noted that
this is no worse than client certificates, since they are also sent
in cleartext.
6.4 Implementation notes
The implementation notes in [10] about correct implementation and use
of RSA (including Section 7.4.7.1) and Diffie-Hellman (including
Appendix F.1.1.3) apply to the DHE_PSK and RSA_PSK ciphersuites as
well.
7. Acknowledgments
The protocol defined in this document is heavily based on work by Tim
Dierks and Peter Gutmann, and borrows some text from [6] and [2].
Valuable feedback was also provided by Philip Ginzboorg, Peter
Gutmann, David Jablon, Nikos Mavroyanopoulos, Bodo Moeller, and Mika
Tervonen.
When the first version of this draft was almost ready, the authors
learned that something similar had been proposed already in 1996
[12]. However, this draft is not intended for web password
authentication, but rather for other uses of TLS.
The DHE_PSK and RSA_PSK ciphersuites borrow heavily from [5].
8. Open issues
o Identity privacy could be provided (in DHE_PSK/RSA_PSK versions)
by encrypting the psk_identity payload with keys derived from the
DH value/RSA-encrypted random (but not PSK). But perhaps this
would be an unnecessary complication.
o The way the PSK is combined with DH value (and is then used to
calculate the Finished message) is not exactly the traditional
way. It should be OK with TLS-PRF, though.
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9. References
9.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
Transport Layer Security (TLS)", RFC 3268, June 2002.
[3] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[4] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
9.2 Informative References
[5] Badra, M., Cherkaoui, O., Hajjeh, I. and A. Serhrouchni,
"Pre-Shared-Key key Exchange methods for TLS",
draft-badra-tls-key-exchange-00 (work in progress), August
2004.
[6] Gutmann, P., "Use of Shared Keys in the TLS Protocol",
draft-ietf-tls-sharedkeys-02 (expired), October 2003.
[7] Krawczyk, H., "Re: TLS shared keys PRF", message on
ietf-tls@lists.certicom.com mailing list 2004-01-13,
http://www.imc.org/ietf-tls/mail-archive/msg04098.html.
[8] Zeilenga, K., "SASLprep: Stringprep profile for user names and
passwords", draft-ietf-sasl-saslprep-10 (work in progress),
July 2004.
[9] Hoffman, P. and M. Blanchet, "Preparation of Internationalized
Strings ("stringprep")", RFC 3454, December 2002.
[10] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.1",
draft-ietf-tls-rfc2246-bis-08 (work in progress), August 2004.
[11] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher Suites
to Transport Layer Security (TLS)", RFC 2712, October 1999.
[12] Simon, D., "Addition of Shared Key Authentication to Transport
Layer Security (TLS)", draft-ietf-tls-passauth-00 (expired),
November 1996.
[13] Taylor, D., Wu, T., Mavroyanopoulos, N. and T. Perrin, "Using
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SRP for TLS Authentication", draft-ietf-tls-srp-07 (work in
progress), June 2004.
[14] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
3629, November 2003.
Authors' Addresses
Pasi Eronen
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group
Finland
EMail: pasi.eronen@nokia.com
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany
EMail: Hannes.Tschofenig@siemens.com
Appendix A. Changelog
(This section should be removed by the RFC Editor before
publication.)
Changes from draft-ietf-tls-psk-00 to -01:
o Added DHE_PSK and RSA_PSK key exchange algorithms, and updated
other text accordingly
o Removed SHA-1 hash from PSK key exchange premaster secret
construction (since premaster secret doesn't need to be 48 bytes).
o Added unknown_psk_identity alert message.
o Updated IANA considerations section.
Changes from draft-eronen-tls-psk-00 to draft-ietf-tls-psk-00:
o Updated dictionary attack considerations based on comments from
David Jablon.
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o Added a recommendation about using UTF-8 in the identity field.
o Removed Appendix A comparing this document with
draft-ietf-tls-sharedkeys-02.
o Removed IPR comment about SPR.
o Minor editorial changes.
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