Prohibiting RC4 Cipher Suites
TLS clients and servers must never negotiate the RC4 stream cipher, in any TLS version.
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
AI ELI5
On this page
§1Why RC4 Had to Go
RC4 is a stream cipher. The RFC notes that it is widely supported and often preferred by TLS servers, yet it has long been known to have a variety of cryptographic weaknesses.
Recent cryptanalysis exploits biases in the RC4 keystream to recover plaintexts that are encrypted repeatedly. The RFC says these results are on the verge of becoming practically exploitable: they currently require 2^26 sessions or 13x2^30 encryptions. Its conclusion is that RC4 can no longer be seen as providing a sufficient level of security for TLS sessions.
Imagine a lock that lets a patient burglar learn a bit more each time you use it. After enough tries the burglar can read your secrets, so you should stop using that lock.
§2The Three Rules
The RFC uses RFC 2119 requirement words and makes three changes to TLS, which apply to all TLS versions it covers (RFC 5246, RFC 4346 and RFC 2246).
- TLS clients MUST NOT include RC4 cipher suites in the
ClientHellomessage. - TLS servers MUST NOT select an RC4 cipher suite when a client sends such a suite in the
ClientHello. - If a client offers only RC4 cipher suites, the server MUST terminate the handshake. It MAY send the
insufficient_securityfatal alert.
Clients must not ask for the weak lock, servers must not pick it even if asked, and if the only thing offered is the weak lock, the server ends the conversation.
§3How This Changes the Handshake
In TLS, the client lists the cipher suites it supports in its ClientHello and the server picks one. This RFC removes RC4 from both sides of that choice: the client must not list RC4 suites, and a server that sees one anyway must not choose it.
If that leaves nothing acceptable, the server does not fall back to RC4. It ends the handshake, optionally with an insufficient_security alert, so the connection fails instead of running with a weak cipher.
It is like ordering from a menu. The weak dish is taken off the menu, and if it is the only thing you ask for, the waiter says sorry and nobody eats.
§4The Prohibited Cipher Suites
Appendix A lists the 18 RC4 cipher suites defined for TLS. They cover several key exchange families, including RSA, anonymous Diffie-Hellman, Kerberos, pre-shared key, and elliptic-curve variants (ECDH, ECDHE).
Examples are TLS_RSA_WITH_RC4_128_SHA, TLS_RSA_WITH_RC4_128_MD5 and TLS_ECDHE_RSA_WITH_RC4_128_SHA. Suites with a 40-bit key, such as TLS_RSA_EXPORT_WITH_RC4_40_MD5, are on the list too.
The RFC includes a full list of every menu item that uses the weak lock, so nobody has to guess which ones are banned.
§5Security Considerations
The security section is a single statement: the document helps maintain the security guarantees of TLS by prohibiting RC4-based cipher suites, which do not provide a sufficiently high level of security.
Taking away the weak lock keeps the whole door as safe as people expect it to be.