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Deprecating RC4 in Secure Shell (SSH)
RFC 8758 also known as BCP 227

Document Type RFC - Best Current Practice (April 2020)
Updates RFC 4253
Author Loganaden Velvindron
Last updated 2020-04-28
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Benjamin Kaduk
Send notices to (None)
RFC 8758

Internet Engineering Task Force (IETF)                     L. Velvindron
Request for Comments: 8758                       
BCP: 227                                                      April 2020
Updates: 4253                                                           
Category: Best Current Practice                                         
ISSN: 2070-1721

                 Deprecating RC4 in Secure Shell (SSH)


   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

Copyright Notice

   Copyright (c) 2020 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
   ( 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
     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
   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",
   "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

   | 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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

5.2.  Informative References

   [IANA]     "Secure Shell (SSH) Protocol Parameters",

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <>.

   [RFC4345]  Harris, B., "Improved Arcfour Modes for the Secure Shell
              (SSH) Transport Layer Protocol", RFC 4345,
              DOI 10.17487/RFC4345, January 2006,

   [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
              DOI 10.17487/RFC7465, February 2015,

   [RFC8429]  Kaduk, B. and M. Short, "Deprecate Triple-DES (3DES) and
              RC4 in Kerberos", BCP 218, RFC 8429, DOI 10.17487/RFC8429,
              October 2018, <>.

   [SCHNEIER] Schneier, B., "Applied Cryptography Second Edition:
              Protocols, Algorithms, and Source in Code in C", John
              Wiley and Sons New York, NY, 1996.


   The author would like to thank Eric Rescorla, Daniel Migault, and
   Rich Salz.

Author's Address

   Loganaden Velvindron