AES for TLS: Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS)
Stronger encryption options for secure internet connections
Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS)", RFC 3268, DOI 10.17487/RFC3268, July 2002, <https://www.rfc-editor.org/info/rfc3268>.
@misc{rfc3268,
series = {Request for Comments},
number = 3268,
howpublished = {RFC 3268},
publisher = {RFC Editor},
doi = {10.17487/RFC3268},
url = {https://www.rfc-editor.org/info/rfc3268},
author = {P. Chown},
title = {{Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS)}},
pagetotal = 7,
year = 2002,
month = jul,
} RFC text
Contents
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.
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
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.
[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
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.
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
Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1The Need for Better Encryption
Transport Layer Security (TLS) is the protocol that secures internet traffic, from web browsers to email. At the time of RFC 3268, TLS supported several symmetric ciphers for encrypting data: RC2, RC4, IDEA, DES and triple DES. However, each had drawbacks. RC2, RC4 and IDEA were subject to patents or trademark claims, making them legally complicated to use. Triple DES, while reasonably secure, was computationally expensive and slow.
The RFC proposes adding AES (Advanced Encryption Standard) ciphersuites to TLS. AES was the outcome of an open standards process run by the US National Institute of Standards and Technology (NIST) and was published as FIPS 197 in 2001. It is patent-free, efficient and has withstood extensive cryptanalytic attack.
TLS kept messages secret by scrambling them, but the old scrambling methods had problems: some were patented, and one was very slow. AES is a new scrambling method that is faster, stronger and free to use.
§2The AES Ciphersuites
The RFC defines 12 new ciphersuites, all using AES in CBC (cipher block chaining) mode combined with SHA-1 for message authentication. The ciphersuites come in two groups based on key length: 6 ciphersuites for 128-bit AES and 6 for 256-bit AES. The RFC mentions that AES supports 192-bit keys as well, but the RFC only standardises 128- and 256-bit variants to avoid unnecessary proliferation.
Each of the 12 ciphersuites specifies not only the encryption algorithm but also the key exchange method. Options include RSA (using the server's RSA key for key exchange), various Diffie-Hellman modes (DH_DSS, DH_RSA, DHE_DSS, DHE_RSA) and anonymous Diffie-Hellman (DH_anon). The DHE (ephemeral Diffie-Hellman) variants are especially valuable because they provide forward secrecy: even if the server's long-term key is compromised later, past session keys remain secure.
The RFC adds 12 new ways for computers to agree on secret codes when they connect. Each way combines AES encryption with a different method of deciding on the secret. Some methods use special keys that only work once, so old conversations stay secret even if the main key is later stolen.
§3Technical Details
Each ciphersuite is identified by a two-byte code. For example, TLS_RSA_WITH_AES_128_CBC_SHA is 0x00, 0x2F and TLS_RSA_WITH_AES_256_CBC_SHA is 0x00, 0x35. All ciphersuites use 128-bit blocks (the amount of data encrypted at once), even though Rijndael, the underlying algorithm, technically supports other block sizes.
The message authentication is performed using SHA-1 in an HMAC construction (described in section 5 of RFC 2246, the TLS 1.0 specification). This means the receiving end can verify that a message was not modified in transit.
Each ciphersuite has a short code so computers can quickly say which method they want to use. All of them use 128-bit chunks of scrambling, and all check for tampering using a special fingerprint method called HMAC.
§4Security and Risks
The RFC believes the new AES ciphersuites are at least as secure as the older ones they replace. However, particular care is needed with the anonymous Diffie-Hellman variants (DH_anon), which provide confidentiality but no authentication. If authentication is required, the parties must authenticate by other means; otherwise, an attacker can perform a man-in-the-middle attack by impersonating both parties. The RFC recommends that if DH_anon is used, the parties should compute a message authentication code (MAC) over the TLS Finished message and compare them to detect such attacks.
The RFC also notes that research identified a chosen plaintext attack against all CBC-mode ciphersuites in TLS 1.0. This weakness does not affect typical web browsing but may affect other applications. The attack does not compromise key material. The RFC predicts the CBC construction will change in a future TLS revision (which it did in TLS 1.2 and later versions).
AES is strong, but the anonymous method (where computers do not check each other's identity) can be tricked by someone pretending to be both sides. There is also a subtle attack on scrambling methods like this one, but it does not steal the secret codes.
§5Status and Obsolescence
RFC 3268 defined important ciphersuites that many systems adopted in the early 2000s. However, it was obsoleted by RFC 5246 (TLS 1.2), which supersedes TLS 1.0 and introduces new ciphersuite definitions. Modern TLS implementations support AEAD ciphers (authenticated encryption) such as AES-GCM and ChaCha20-Poly1305, which are more secure and efficient than the CBC constructions defined here.
This RFC helped make TLS safer for years, but it was replaced by a better version (RFC 5246) that uses even stronger methods.