Updates:
Internet Engineering Task Force (IETF) L. Velvindron
Request for Comments: 8758 cyberstorm.mu
BCP: 227 April 2020
Updates: 4253
Category: Best Current Practice
ISSN: 2070-1721
Deprecating RC4 in Secure Shell (SSH)
Abstract
This document deprecates RC4 in Secure Shell (SSH). Therefore, this
document formally moves RFC 4345 to Historic status.
Status of This Memo
This memo documents an Internet Best Current Practice.
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
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8758.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Requirements Language
2. Updates to RFC 4253
3. IANA Considerations
4. Security Considerations
5. References
5.1. Normative References
5.2. Informative References
Acknowledgements
Author's Address
1. Introduction
The usage of RC4 suites (also designated as "arcfour") for SSH is
specified in [RFC4253] and [RFC4345]. [RFC4253] specifies the
allocation of the "arcfour" cipher for SSH. [RFC4345] specifies and
allocates the "arcfour128" and "arcfour256" ciphers for SSH. RC4
encryption has known weaknesses [RFC7465] [RFC8429]; therefore, this
document starts the deprecation process for their use in Secure Shell
(SSH) [RFC4253]. Accordingly, [RFC4253] is updated to note the
deprecation of the RC4 ciphers, and [RFC4345] is moved to Historic
status, as all ciphers it specifies MUST NOT be used.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Updates to RFC 4253
[RFC4253] is updated to prohibit arcfour's use in SSH. [RFC4253],
Section 6.3 allocates the "arcfour" cipher by defining a list of
defined ciphers in which the "arcfour" cipher appears as optional, as
shown in Table 1.
+---------+----------+----------------------------------------------+
| arcfour | OPTIONAL | the ARCFOUR stream cipher |
| | | with a 128-bit key |
+---------+----------+----------------------------------------------+
Table 1
This document updates the status of the "arcfour" ciphers in the list
found in [RFC4253], Section 6.3 by moving it from OPTIONAL to MUST
NOT.
+---------+----------+----------------------------------------------+
| arcfour | MUST NOT | the ARCFOUR stream cipher |
| | | with a 128-bit key |
+---------+----------+----------------------------------------------+
Table 2
[RFC4253] defines the "arcfour" ciphers with the following text:
| The "arcfour" cipher is the Arcfour stream cipher with 128-bit
| keys. The Arcfour cipher is believed to be compatible with the
| RC4 cipher [SCHNEIER]. Arcfour (and RC4) has problems with weak
| keys, and should be used with caution.
This document updates [RFC4253], Section 6.3 by replacing the text
above with the following text:
| The "arcfour" cipher is the Arcfour stream cipher with 128-bit
| keys. The Arcfour cipher is compatible with the RC4 cipher
| [SCHNEIER]. Arcfour (and RC4) has known weaknesses [RFC7465]
| [RFC8429] and MUST NOT be used.
3. IANA Considerations
The IANA has updated the "Encryption Algorithm Names" subregistry in
the "Secure Shell (SSH) Protocol Parameters" registry [IANA]. The
registration procedure is IETF review, which is achieved by this
document. The registry has been updated as follows:
+---------------------------+-----------+----------+
| Encryption Algorithm Name | Reference | Note |
+===========================+===========+==========+
| arcfour | RFC 8758 | HISTORIC |
+---------------------------+-----------+----------+
| arcfour128 | RFC 8758 | HISTORIC |
+---------------------------+-----------+----------+
| arcfour256 | RFC 8758 | HISTORIC |
+---------------------------+-----------+----------+
Table 3
4. Security Considerations
This document only prohibits the use of RC4 in SSH; it introduces no
new security considerations.
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
5.2. Informative References
[IANA] "Secure Shell (SSH) Protocol Parameters",
<https://www.iana.org/assignments/ssh-parameters>.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <https://www.rfc-editor.org/info/rfc4253>.
[RFC4345] Harris, B., "Improved Arcfour Modes for the Secure Shell
(SSH) Transport Layer Protocol", RFC 4345,
DOI 10.17487/RFC4345, January 2006,
<https://www.rfc-editor.org/info/rfc4345>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC7465, February 2015,
<https://www.rfc-editor.org/info/rfc7465>.
[RFC8429] Kaduk, B. and M. Short, "Deprecate Triple-DES (3DES) and
RC4 in Kerberos", BCP 218, RFC 8429, DOI 10.17487/RFC8429,
October 2018, <https://www.rfc-editor.org/info/rfc8429>.
[SCHNEIER] Schneier, B., "Applied Cryptography Second Edition:
Protocols, Algorithms, and Source in Code in C", John
Wiley and Sons New York, NY, 1996.
Acknowledgements
The author would like to thank Eric Rescorla, Daniel Migault, and
Rich Salz.
Author's Address
Loganaden Velvindron
cyberstorm.mu
Mauritius
Email: logan@cyberstorm.mu