Deprecating RC4 in Secure Shell (SSH)
RFC 8758

Document Type RFC - Best Current Practice (April 2020; No errata)
Updates RFC 4253
Also known as BCP 227
Last updated 2020-04-28
Replaces draft-luis140219-curdle-rc4-die-die-die
Stream IETF
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Reviews
Stream WG state Submitted to IESG for Publication
Document shepherd Daniel Migault
Shepherd write-up Show (last changed 2019-07-30)
IESG IESG state RFC 8758 (Best Current Practice)
Consensus Boilerplate Yes
Telechat date
Responsible AD Benjamin Kaduk
Send notices to Daniel Migault <daniel.migault@ericsson.com>
IANA IANA review state IANA OK - Actions Needed
IANA action state RFC-Ed-Ack


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
   (https://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
     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
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