Network Working Group P. Chown
Request for Comments: 3268 Skygate Technology
Category: Standards Track June 2002
Advanced Encryption Standard (AES) Ciphersuites for Transport Layer
Security (TLS)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document proposes several new ciphersuites. At present, the
symmetric ciphers supported by Transport Layer Security (TLS) are
RC2, RC4, International Data Encryption Algorithm (IDEA), Data
Encryption Standard (DES), and triple DES. The protocol would be
enhanced by the addition of Advanced Encryption Standard (AES)
ciphersuites.
Overview
At present, the symmetric ciphers supported by TLS are RC2, RC4,
IDEA, DES, and triple DES. The protocol would be enhanced by the
addition of AES [AES] ciphersuites, for the following reasons:
1. RC2, RC4, and IDEA are all subject to intellectual property
claims. RSA Security Inc. has trademark rights in the names RC2
and RC4, and claims that the RC4 algorithm itself is a trade
secret. Ascom Systec Ltd. owns a patent on the IDEA algorithm.
2. Triple DES is much less efficient than more modern ciphers.
3. Now that the AES process is completed there will be commercial
pressure to use the selected cipher. The AES is efficient and has
withstood extensive cryptanalytic efforts. The AES is therefore a
desirable choice.
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4. Currently the DHE ciphersuites only allow triple DES (along with
some "export" variants which do not use a satisfactory key
length). At the same time the DHE ciphersuites are the only ones
to offer forward secrecy.
This document proposes several new ciphersuites, with the aim of
overcoming these problems.
Cipher Usage
The new ciphersuites proposed here are very similar to the following,
defined in [TLS]:
TLS_RSA_WITH_3DES_EDE_CBC_SHA
TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA
TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA
TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA
TLS_DH_anon_WITH_3DES_EDE_CBC_SHA
All the ciphersuites described here use the AES in cipher block
chaining (CBC) mode. Furthermore, they use SHA-1 [SHA-1] in an HMAC
construction as described in section 5 of [TLS]. (Although the TLS
ciphersuite names include the text "SHA", this actually refers to the
modified SHA-1 version of the algorithm.)
The ciphersuites differ in the type of certificate and key exchange
method. The ciphersuites defined here use the following options for
this part of the protocol:
CipherSuite Certificate type (if applicable)
and key exchange algorithm
TLS_RSA_WITH_AES_128_CBC_SHA RSA
TLS_DH_DSS_WITH_AES_128_CBC_SHA DH_DSS
TLS_DH_RSA_WITH_AES_128_CBC_SHA DH_RSA
TLS_DHE_DSS_WITH_AES_128_CBC_SHA DHE_DSS
TLS_DHE_RSA_WITH_AES_128_CBC_SHA DHE_RSA
TLS_DH_anon_WITH_AES_128_CBC_SHA DH_anon
TLS_RSA_WITH_AES_256_CBC_SHA RSA
TLS_DH_DSS_WITH_AES_256_CBC_SHA DH_DSS
TLS_DH_RSA_WITH_AES_256_CBC_SHA DH_RSA
TLS_DHE_DSS_WITH_AES_256_CBC_SHA DHE_DSS
TLS_DHE_RSA_WITH_AES_256_CBC_SHA DHE_RSA
TLS_DH_anon_WITH_AES_256_CBC_SHA DH_anon
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For the meanings of the terms RSA, DH_DSS, DH_RSA, DHE_DSS, DHE_RSA
and DH_anon, please refer to sections 7.4.2 and 7.4.3 of [TLS].
The AES supports key lengths of 128, 192 and 256 bits. However, this
document only defines ciphersuites for 128- and 256-bit keys. This
is to avoid unnecessary proliferation of ciphersuites. Rijndael
actually allows for 192- and 256-bit block sizes as well as the 128-
bit blocks mandated by the AES process. The ciphersuites defined
here all use 128-bit blocks.
The new ciphersuites will have the following definitions:
CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x2F };
CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x30 };
CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x31 };
CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x32 };
CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x33 };
CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA = { 0x00, 0x34 };
CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x35 };
CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x36 };
CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x37 };
CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x38 };
CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x39 };
CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA = { 0x00, 0x3A };
Security Considerations
It is not believed that the new ciphersuites are ever less secure
than the corresponding older ones. The AES is believed to be secure,
and it has withstood extensive cryptanalytic attack.
The ephemeral Diffie-Hellman ciphersuites provide forward secrecy
without any known reduction in security in other areas. To obtain
the maximum benefit from these ciphersuites:
1. The ephemeral keys should only be used once. With the TLS
protocol as currently defined there is no significant efficiency
gain from reusing ephemeral keys.
2. Ephemeral keys should be destroyed securely when they are no
longer required.
3. The random number generator used to create ephemeral keys must not
reveal past output even when its internal state is compromised.
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[TLS] describes the anonymous Diffie-Hellman (ADH) ciphersuites as
deprecated. The ADH ciphersuites defined here are not deprecated.
However, when they are used, particular care must be taken:
1. ADH provides confidentiality but not authentication. This means
that (if authentication is required) the communicating parties
must authenticate to each other by some means other than TLS.
2. ADH is vulnerable to man-in-the-middle attacks, as a consequence
of the lack of authentication. The parties must have a way of
determining whether they are participating in the same TLS
connection. If they are not, they can deduce that they are under
attack, and presumably abort the connection.
For example, if the parties share a secret, it is possible to
compute a MAC of the TLS Finished message. An attacker would have
to negotiate two different TLS connections; one with each
communicating party. The Finished messages would be different in
each case, because they depend on the parties' public keys (among
other things). For this reason, the MACs computed by each party
would be different.
It is important to note that authentication techniques which do
not use the Finished message do not usually provide protection
from this attack. For example, the client could authenticate to
the server with a password, but it would still be vulnerable to
man-in-the-middle attacks.
Recent research has identified a chosen plaintext attack which
applies to all ciphersuites defined in [TLS] which use CBC mode.
This weakness does not affect the common use of TLS on the World
Wide Web, but may affect the use of TLS in other applications.
When TLS is used in an application where this attack is possible,
attackers can determine the truth or otherwise of a hypothesis
that particular plaintext data was sent earlier in the session.
No key material is compromised.
It is likely that the CBC construction will be changed in a future
revision of the TLS protocol.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use other technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
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IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
During the development of the AES, NIST published the following
statement on intellectual property:
SPECIAL NOTE - Intellectual Property
NIST reminds all interested parties that the adoption of AES is
being conducted as an open standards-setting activity.
Specifically, NIST has requested that all interested parties
identify to NIST any patents or inventions that may be required
for the use of AES. NIST hereby gives public notice that it may
seek redress under the antitrust laws of the United States against
any party in the future who might seek to exercise patent rights
against any user of AES that have not been disclosed to NIST in
response to this request for information.
Acknowledgements
I would like to thank the ietf-tls mailing list contributors who have
made helpful suggestions for this document.
References
[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[AES] National Institute of Standards and Technology,
"Specification for the Advanced Encryption Standard (AES)"
FIPS 197. November 26, 2001.
[SHA-1] FIPS PUB 180-1, "Secure Hash Standard," National Institute
of Standards and Technology, U.S. Department of Commerce,
April 17, 1995.
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Author's Address
Pete Chown
Skygate Technology Ltd
8 Lombard Road
London
SW19 3TZ
United Kingdom
Phone: +44 20 8542 7856
EMail: pc@skygate.co.uk
Chown Standards Track [Page 6]
RFC 3268 AES Ciphersuites for TLS June 2002
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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