Prohibiting RC4 Cipher Suites
Prohibit TLS clients and servers from negotiating RC4 cipher suites.
Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, DOI 10.17487/RFC7465, February 2015, <https://www.rfc-editor.org/info/rfc7465>.
@misc{rfc7465,
series = {Request for Comments},
number = 7465,
howpublished = {RFC 7465},
publisher = {RFC Editor},
doi = {10.17487/RFC7465},
url = {https://www.rfc-editor.org/info/rfc7465},
author = {A. Popov},
title = {{Prohibiting RC4 Cipher Suites}},
pagetotal = 6,
year = 2015,
month = feb,
} RFC text
Contents
Internet Engineering Task Force (IETF) A. Popov
Request for Comments: 7465 Microsoft Corp.
Updates: 5246, 4346, 2246 February 2015
Category: Standards Track
ISSN: 2070-1721
Prohibiting RC4 Cipher Suites
Abstract
This document requires that Transport Layer Security (TLS) clients
and servers never negotiate the use of RC4 cipher suites when they
establish connections. This applies to all TLS versions. This
document updates RFCs 5246, 4346, and 2246.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7465.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Changes to TLS . . . . . . . . . . . . . . . . . . . . . . . 2
3. Security Considerations . . . . . . . . . . . . . . . . . . . 3
4. References . . . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Normative References . . . . . . . . . . . . . . . . . . 3
4.2. Informative References . . . . . . . . . . . . . . . . . 3
Appendix A. RC4 Cipher Suites . . . . . . . . . . . . . . . . . 5
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
RC4 is a stream cipher that is described in [SCH]; it is widely
supported, and often preferred by TLS servers. However, RC4 has long
been known to have a variety of cryptographic weaknesses, e.g., see
[PAU], [MAN], and [FLU]. Recent cryptanalysis results [ALF] exploit
biases in the RC4 keystream to recover repeatedly encrypted
plaintexts.
These recent results are on the verge of becoming practically
exploitable; currently, they require 2^26 sessions or 13x2^30
encryptions. As a result, RC4 can no longer be seen as providing a
sufficient level of security for TLS sessions.
This document requires that TLS ([RFC5246] [RFC4346] [RFC2246])
clients and servers never negotiate the use of RC4 cipher suites.
1.1. Requirements Language
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. Changes to TLS
Because of the RC4 deficiencies noted in Section 1, the following
apply:
o TLS clients MUST NOT include RC4 cipher suites in the ClientHello
message.
o TLS servers MUST NOT select an RC4 cipher suite when a TLS client
sends such a cipher suite in the ClientHello message.
o If the TLS client only offers RC4 cipher suites, the TLS server
MUST terminate the handshake. The TLS server MAY send the
insufficient_security fatal alert in this case.
Appendix A lists the RC4 cipher suites defined for TLS.
3. Security Considerations
This document helps maintain the security guarantees of the TLS
protocol by prohibiting the use of the RC4-based cipher suites
(listed in Appendix A), which do not provide a sufficiently high
level of security.
4. References
4.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999,
<http://www.rfc-editor.org/info/rfc2246>.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006,
<http://www.rfc-editor.org/info/rfc4346>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
4.2. Informative References
[ALF] AlFardan, N., Bernstein, D., Paterson, K., Poettering, B.,
and J. Schuldt, "On the Security of RC4 in TLS and WPA",
USENIX Security Symposium, July 2013,
<https://www.usenix.org/conference/usenixsecurity13/
security-rc4-tls>.
[FLU] Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the
Key Scheduling Algorithm of RC4", Selected Areas of
Cryptography: SAC 2001, Lecture Notes in Computer Science
Vol. 2259, pp 1-24, 2001.
[MAN] Mantin, I. and A. Shamir, "A Practical Attack on Broadcast
RC4", Fast Software Encryption: FSE 2001, Lecture Notes in
Computer Science Vol. 2355, pp 152-164, 2002.
[PAU] Paul, G. and S. Maitra, "Permutation after RC4 Key
Scheduling Reveals the Secret Key", Selected Areas of
Cryptography: SAC 2007, Lecture Notes on Computer Science,
Vol. 4876, pp 360-337, 2007.
[SCH] Schneier, B., "Applied Cryptography: Protocols,
Algorithms, and Source Code in C", 2nd Edition, 1996.
Appendix A. RC4 Cipher Suites
The following cipher suites defined for TLS use RC4:
o TLS_RSA_EXPORT_WITH_RC4_40_MD5
o TLS_RSA_WITH_RC4_128_MD5
o TLS_RSA_WITH_RC4_128_SHA
o TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
o TLS_DH_anon_WITH_RC4_128_MD5
o TLS_KRB5_WITH_RC4_128_SHA
o TLS_KRB5_WITH_RC4_128_MD5
o TLS_KRB5_EXPORT_WITH_RC4_40_SHA
o TLS_KRB5_EXPORT_WITH_RC4_40_MD5
o TLS_PSK_WITH_RC4_128_SHA
o TLS_DHE_PSK_WITH_RC4_128_SHA
o TLS_RSA_PSK_WITH_RC4_128_SHA
o TLS_ECDH_ECDSA_WITH_RC4_128_SHA
o TLS_ECDHE_ECDSA_WITH_RC4_128_SHA
o TLS_ECDH_RSA_WITH_RC4_128_SHA
o TLS_ECDHE_RSA_WITH_RC4_128_SHA
o TLS_ECDH_anon_WITH_RC4_128_SHA
o TLS_ECDHE_PSK_WITH_RC4_128_SHA
Acknowledgements
This document was inspired by discussions with Magnus Nystrom, Eric
Rescorla, Joseph Salowey, Yaron Sheffer, Nagendra Modadugu, and
others on the TLS mailing list.
Author's Address
Andrei Popov
Microsoft Corp.
One Microsoft Way
Redmond, WA 98052
USA
EMail: andreipo@microsoft.com
ISO Plain Language
On this page
§1Why RC4 Is Prohibited
RC4 is a stream cipher that was widely supported by TLS servers. The RFC notes that cryptographic weaknesses had been known for some time.
More recent analysis exploited biases in the RC4 keystream to recover plaintexts encrypted repeatedly. The RFC says the attacks then required 2^26 sessions or 13x2^30 encryptions, and concludes RC4 no longer provides sufficient security for TLS.
Research found weaknesses in RC4 that could expose text encrypted with it many times. The RFC therefore prohibits using RC4 to protect TLS sessions.
§2The Three TLS Requirements
The rules apply to all TLS versions named in the RFC. A TLS client MUST NOT include RC4 cipher suites in its ClientHello.
A TLS server MUST NOT select an RC4 suite, even if the client includes one. If the client offers only RC4 suites, the server MUST terminate the handshake and MAY send the fatal insufficient_security alert.
A client cannot request RC4, and a server cannot choose it. If RC4 is the client's only offer, the server ends the connection setup.
§2What Happens During the Handshake
During a TLS handshake, the client lists supported cipher suites in ClientHello, and the server selects one of them. This RFC removes RC4 suites from the acceptable choices.
If the server cannot select an allowed suite, it does not fall back to RC4. It terminates the handshake, so the connection is not established with that cipher.
The server chooses from the suites offered by the client, but it must ignore any RC4 suite. A handshake ends if there is no other acceptable choice.
§4.1The RC4 Suites Covered
Appendix A lists the TLS cipher suites that use RC4. The list includes RSA, anonymous Diffie-Hellman, Kerberos, pre-shared key, and elliptic-curve suite families.
Examples include TLS_RSA_WITH_RC4_128_SHA, TLS_RSA_WITH_RC4_128_MD5, and TLS_ECDHE_RSA_WITH_RC4_128_SHA. Export suites with a 40-bit key are included as well.
The appendix names the RC4 suites explicitly, including suites from several key-exchange families. A TLS implementation can use this list to identify the prohibited choices.
§3Maintaining TLS Security
The security consideration is direct: prohibiting RC4-based cipher suites helps maintain the security guarantees of TLS. The RFC states that those suites do not provide a sufficiently high level of security.
The prohibition is achieved through the client and server requirements in Section 2, including terminating the handshake when the client offers only RC4 suites.
The prohibition keeps TLS from using cipher suites the RFC considers too weak. A connection must fail rather than negotiate RC4 as a fallback.