SHA-2 Data Integrity Verification for the Secure Shell (SSH) Transport Layer Protocol
RFC 6668

Document Type RFC - Proposed Standard (July 2012; No errata)
Updates RFC 4253
Was draft-dbider-sha2-mac-for-ssh (individual in sec area)
Last updated 2015-10-14
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IESG IESG state RFC 6668 (Proposed Standard)
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Responsible AD Sean Turner
IESG note Jeffrey Hutzelman ( is the document shepherd.
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Internet Engineering Task Force (IETF)                          D. Bider
Request for Comments: 6668                               Bitvise Limited
Updates: 4253                                                 M. Baushke
Category: Standards Track                         Juniper Networks, Inc.
ISSN: 2070-1721                                                July 2012

                 SHA-2 Data Integrity Verification for
            the Secure Shell (SSH) Transport Layer Protocol


   This memo defines algorithm names and parameters for use in some of
   the SHA-2 family of secure hash algorithms for data integrity
   verification in the Secure Shell (SSH) protocol.  It also updates RFC
   4253 by specifying a new RECOMMENDED data integrity algorithm.

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

Copyright Notice

   Copyright (c) 2012 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
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   ( in effect on the date of
   publication of this document.  Please review these documents
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   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.

Bider & Baushke              Standards Track                    [Page 1]
RFC 6668              Sha2-Transport Layer Protocol            July 2012

1.  Overview and Rationale

   The Secure Shell (SSH) [RFC4251] is a very common protocol for secure
   remote login on the Internet.  Currently, SSH defines data integrity
   verification using SHA-1 and MD5 algorithms [RFC4253].  Due to recent
   security concerns with these two algorithms ([RFC6194] and [RFC6151],
   respectively), implementors and users request support for data
   integrity verification using some of the SHA-2 family of secure hash

1.1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Data Integrity Algorithms

   This memo adopts the style and conventions of [RFC4253] in specifying
   how the use of new data integrity algorithms are indicated in SSH.

   The following new data integrity algorithms are defined:

   hmac-sha2-256     RECOMMENDED   HMAC-SHA2-256
                                   (digest length = 32 bytes,
                                    key length    = 32 bytes)

   hmac-sha2-512     OPTIONAL      HMAC-SHA2-512
                                   (digest length = 64 bytes,
                                    key length    = 64 bytes)

                          Figure 1

   The Hashed Message Authentication Code (HMAC) mechanism was
   originally defined in [RFC2104] and has been updated in [RFC6151].

   The SHA-2 family of secure hash algorithms is defined in

   Sample code for the SHA-based HMAC algorithms are available in
   [RFC6234].  The variants, HMAC-SHA2-224 and HMAC-SHA2-384 algorithms,
   were considered but not added to this list as they have the same
   computational requirements of HMAC-SHA2-256 and HMAC-SHA2-512,
   respectively, and do not seem to be much used in practice.

Bider & Baushke              Standards Track                    [Page 2]
RFC 6668              Sha2-Transport Layer Protocol            July 2012

   Test vectors for use of HMAC with SHA-2 are provided in [RFC4231].
   Users, implementors, and administrators may choose to put these new
   MACs into the proposal ahead of the REQUIRED hmac-sha1 algorithm
   defined in [RFC4253] so that they are negotiated first.

3.  IANA Considerations

   This document augments the MAC Algorithm Names in [RFC4253] and

   IANA has updated the "Secure Shell (SSH) Protocol Parameters"
   registry with the following entries:

   MAC Algorithm Name      Reference       Note
   hmac-sha2-256           RFC 6668        Section 2
   hmac-sha2-512           RFC 6668        Section 2

                        Figure 2

4.  Security Considerations

   The security considerations of RFC 4253 [RFC4253] apply to this
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