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.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
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  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
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  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
  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: [email protected]

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