Transport Layer Security Working D. Taylor
Group Forge Research Pty Ltd
Internet-Draft November 29, 2002
Expires: May 30, 2003
Using SRP for TLS Authentication
draft-ietf-tls-srp-04
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This memo presents a technique for using the SRP [2] (Secure Remote
Password) protocol as an authentication method for the TLS
[1](Transport Layer Security) protocol.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. SRP Authentication in TLS . . . . . . . . . . . . . . . . . 4
2.1 Modifications to the TLS Handshake Sequence . . . . . . . . 4
2.1.1 Message Sequence . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 Session Re-use . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 SRP Verifier Message Digest Selection . . . . . . . . . . . 5
2.3 Changes to the Handshake Message Contents . . . . . . . . . 5
2.3.1 Client hello . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3.2 Server certificate . . . . . . . . . . . . . . . . . . . . . 5
2.3.3 Server key exchange . . . . . . . . . . . . . . . . . . . . 5
2.3.4 Client key exchange . . . . . . . . . . . . . . . . . . . . 6
2.4 Calculating the Pre-master Secret . . . . . . . . . . . . . 6
2.5 Cipher Suite Definitions . . . . . . . . . . . . . . . . . . 6
2.6 New Message Structures . . . . . . . . . . . . . . . . . . . 7
2.6.1 ExtensionType . . . . . . . . . . . . . . . . . . . . . . . 7
2.6.2 Client Hello . . . . . . . . . . . . . . . . . . . . . . . . 7
2.6.3 Server Key Exchange . . . . . . . . . . . . . . . . . . . . 7
2.6.4 Client Key Exchange . . . . . . . . . . . . . . . . . . . . 9
3. Security Considerations . . . . . . . . . . . . . . . . . . 10
References . . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . 11
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
Full Copyright Statement . . . . . . . . . . . . . . . . . . 13
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1. Introduction
At the time of writing, TLS uses public key certificiates with RSA/
DSA digital signatures, or Kerberos, for authentication.
These authentication methods do not seem well suited to the
applications now being adapted to use TLS (IMAP [4], FTP [6], or
TELNET [7], for example). Given these protocols (and others like
them) are designed to use the user name and password method of
authentication, being able to safely use user names and passwords to
authenticate the TLS connection provides a much easier route to
additional security than implementing a public key infrastructure in
certain situations.
SRP is an authentication method that allows the use of user names and
passwords over unencrypted channels without revealing the password to
an eavesdropper. SRP also supplies a shared secret at the end of the
authetication sequence that can be used to generate encryption keys.
This document describes the use of the SRP authentication method for
TLS.
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 RFC 2119.
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2. SRP Authentication in TLS
2.1 Modifications to the TLS Handshake Sequence
The advent of SRP-6 [3] allows the SRP protocol to be implemented
using the standard sequence of handshake messages defined in [1].
The parameters to various messages are given in the following
diagram.
2.1.1 Message Sequence
Handshake Message Flow for SRP Authentication
Client Server
| |
Client Hello (I) ------------------------> |
| <---------------------------- Server Hello
| <---------------------------- Certificate*
| <---------------------------- Server Key Exchange (N, g, s, B)
| <---------------------------- Server Hello Done
Client Key Exchange (A) -----------------> |
[Change cipher spec] |
Finished --------------------------------> |
| [Change cipher spec]
| <---------------------------- Finished
| |
Application Data <--------------> Application Data
* Indicates optional or situation-dependent messages that are not
always sent.
The identifiers given after each message name refer to the SRP
variables included in that message. The variables I, N, g, s, A, and
B are defined in [3].
An extended client hello message, as defined in [8], is used to send
the client identifier (the user name).
Servers MAY add an SRP extension to the server hello message. For
the cipher suites defined in this document no information is carried
in the SRP extension in the server hello message. The option to add
an SRP extension to the server hello message is given in case it is
required in future.
2.1.2 Session Re-use
The short handshake mechanism for re-using sessions for new
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connections, and renegotiating keys for existing connections will
still work with the SRP authentication mechanism and handshake.
When a client attemps to re-use a session that uses SRP
authentication, it MUST include the SRP extension carrying the user
name (I) in the client hello message, in case the server cannot or
will not allow re-use of the session, meaning a full handshake
sequence is required.
If the server does agree to re-use an existing session the server
MUST ignore the information in the SRP extension of the client hello
message, except for its inclusion in the finished message hashes.
This is to ensure attackers cannot replace the authenticated identity
without supplying the proper authentication information.
2.2 SRP Verifier Message Digest Selection
Implementations conforming to this document MUST use the SHA-1
message digest with the SRP algorithm.
2.3 Changes to the Handshake Message Contents
This section describes the changes to the TLS handshake message
contents when SRP is being used for authentication. The definitions
of the new message contents and the on-the-wire changes are given in
Section 2.6.
2.3.1 Client hello
The user name is appended to the standard client hello message using
the hello message extension mechanism defined in [8].
2.3.2 Server certificate
The server MUST send a certificate if it agrees to an SRP cipher
suite that requires the server to provide additional authentication
in the form of a digital signature. See Section 2.5 for details of
which ciphersuites defined in this document require a server
certificate to be sent.
Because the server's certificate is only used for generating a
digital signature in SRP cipher suites, the certificate sent MUST
contain a public key that can be used for generating digital
signatures.
2.3.3 Server key exchange
The server key exchange message contains the prime (N), the generator
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(g), and the salt value (s) read from the SRP password file based on
the value of (I) received in the client hello extension. The server
key exchange message also contains the server's public key (B).
If the server has sent a certificate message, the server key exchange
message MUST be signed.
2.3.4 Client key exchange
The client key exchange message carries the client's public key (A).
2.4 Calculating the Pre-master Secret
The shared secret resulting from the SRP calculations (S) (defined in
[2]) is used as the pre-master secret.
The finished messages perform the same function as the client and
server evidence messages (M1 and M2) specified in [2]. If either the
client or the server calculate an incorrect value, the finished
messages will not be understood, and the connection will be dropped
as specified in [1].
2.5 Cipher Suite Definitions
The following cipher suites are added by this draft. The usage of
AES ciphersuites is as defined in [5].
CipherSuite TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x50 };
CipherSuite TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x51 };
CipherSuite TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x52 };
CipherSuite TLS_SRP_SHA_WITH_AES_128_CBC_SHA = { 0x00,0x53 };
CipherSuite TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA = { 0x00,0x54 };
CipherSuite TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA = { 0x00,0x55 };
CipherSuite TLS_SRP_SHA_WITH_AES_256_CBC_SHA = { 0x00,0x56 };
CipherSuite TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA = { 0x00,0x57 };
CipherSuite TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA = { 0x00,0x58 };
Cipher suites that do not include a digitial signature algorithm
identifier assume the server is authenticated by its possesion of the
SRP database.
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Cipher suites that begin with TLS_SRP_SHA_RSA or TLS_SRP_SHA_DSS
require the server to send a certificate message containing a
certificate with the specified type of public key, and to sign the
server key exchange message using a matching private key.
Implementations conforming to this specification MUST implement the
TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA ciphersuite, SHOULD implement the
TLS_SRP_SHA_WITH_AES_128_CBC_SHA and TLS_SRP_SHA_WITH_AES_256_CBC_SHA
ciphersuites, and MAY implement the remaining ciphersuites.
2.6 New Message Structures
This section shows the structure of the messages passed during a
handshake that uses SRP for authentication. The representation
language used is the same as that used in [1].
2.6.1 ExtensionType
A new value, "srp(6)", has been added to the enumerated
ExtensionType, defined in [8]. This value MUST be used as the
extension number for the SRP extension.
2.6.2 Client Hello
The user name (I) is encoded in an SRPExtension structure, and sent
in an extended client hello message, using an extension of type
"srp".
enum { client, server } ClientOrServerExtension;
struct {
select(ClientOrServerExtension) {
case client:
opaque srp_I<1..2^8-1>;
case server:
/* empty struct */
}
} SRPExtension;
2.6.3 Server Key Exchange
When the value of KeyExchangeAlgorithm is set to "srp", the server's
SRP parameters are sent in the server key exchange message, encoded
in a ServerSRPParams structure.
If a certificate is sent to the client the server key exchange
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message must be signed. The following table gives the
SignatureAlgorithm value to be used for each ciphersuite.
Ciphersuite SignatureAlgorithm
TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA anonymous
TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA rsa
TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA dsa
TLS_SRP_SHA_WITH_AES_128_CBC_SHA anonymous
TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA rsa
TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA dsa
TLS_SRP_SHA_WITH_AES_256_CBC_SHA anonymous
TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA rsa
TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA dsa
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
case srp: /* new entry */
ServerSRPParams params;
Signature signed_params;
};
} ServerKeyExchange;
struct {
opaque srp_N<1..2^16-1>;
opaque srp_g<1..2^16-1>;
opaque srp_s<1..2^8-1>
opaque srp_B<1..2^16-1>;
} ServerSRPParams; /* SRP parameters */
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2.6.4 Client Key Exchange
When the value of KeyExchangeAlgorithm is set to "srp", the client's
ephemeral public key (A) is sent in the client key exchange message,
encoded in an ClientSRPPublic structure.
An extra value, srp, has been added to the enumerated
KeyExchangeAlgorithm, originally defined in TLS [1].
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
case srp: ClientSRPPublic; /* new entry */
} exchange_keys;
} ClientKeyExchange;
enum { rsa, diffie_hellman, srp } KeyExchangeAlgorithm;
struct {
opaque srp_A<1..2^16-1>;
} ClientSRPPublic;
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3. Security Considerations
If an attacker is able to steal the SRP verifier file, the attacker
can masquerade as the real host. Filesystem based X.509 certificate
installations are vulnerable to a similar attack unless the server's
certificate is issued from a PKI that maintains revocation lists, and
the client TLS code can both contact the PKI and make use of the
revocation list.
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References
[1] Dierks, T. and C. Allen, "The TLS Protocol", RFC 2246, January
1999.
[2] Wu, T., "The SRP Authentication and Key Exchange System", RFC
2945, September 2000.
[3] Wu, T., "SRP-6: Improvements and Refinements to the Secure
Remote Password Protocol", October 2002.
[4] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595, June
1999.
[5] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
Transport Layer Security (TLS)", RFC 3268, June 2002.
[6] Ford-Hutchinson, P., Carpenter, M., Hudson, T., Murray, E. and
V. Wiegand, "Securing FTP with TLS", draft-murray-auth-ftp-ssl-
09 (work in progress), April 2002.
[7] Boe, M. and J. Altman, "TLS-based Telnet Security", draft-ietf-
tn3270e-telnet-tls-06 (work in progress), April 2002.
[8] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and T.
Wright, "TLS Extensions", draft-ietf-tls-extensions-05 (work in
progress), July 2002.
Author's Address
David Taylor
Forge Research Pty Ltd
EMail: DavidTaylor@forge.com.au
URI: http://www.forge.com.au/
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Appendix A. Acknowledgements
Thanks to all on the IETF tls mailing list for ideas and analysis.
Thanks to Tom Wu for adapting the SRP protocol so it fits the
standard TLS handshake message sequence.
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