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

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

Abstract

   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
   http://www.rfc-editor.org/info/rfc6668.

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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   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.

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
   algorithms.

1.1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
   [FIPS-180-3].

   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.

   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
   [RFC4250].

   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
   document.

   The National Institute of Standards and Technology (NIST)
   publications: NIST Special Publication (SP) 800-107 [800-107] and
   NIST SP 800-131A [800-131A] suggest that HMAC-SHA1 and HMAC-SHA2-256
   have a security strength of 128 bits and 256 bits, respectively,
   which are considered acceptable key lengths.

   Many users seem to be interested in the perceived safety of using the
   SHA2-based algorithms for hashing.

5.  References

5.1.  Normative References

   [FIPS-180-3]
              National Institute of Standards and Technology (NIST),
              United States of America, "Secure Hash Standard (SHS)",
              FIPS PUB 180-3, October 2008, <http://csrc.nist.gov/
              publications/fips/fips180-3/fips180-3_final.pdf>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4231]  Nystrom, M., "Identifiers and Test Vectors for HMAC-
              SHA-224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
              RFC 4231, December 2005.

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

5.2.  Informative References

   [800-107]  National Institute of Standards and Technology (NIST),
              "Recommendation for Applications Using Approved Hash
              Algorithms", NIST Special Publication 800-107, February
              2009, <http://csrc.nist.gov/publications/
              nistpubs/800-107/NIST-SP-800-107.pdf>.

   [800-131A] National Institute of Standards and Technology (NIST),
              "Transitions: Recommendation for the Transitioning of the
              Use of Cryptographic Algorithms and Key Lengths", DRAFT
              NIST Special Publication 800-131A, January 2011,
              <http://csrc.nist.gov/publications/nistpubs/800-131A/
              sp800-131A.pdf>.

   [RFC4250]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Assigned Numbers", RFC 4250, January 2006.

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, March 2011.

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, March 2011.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.

Authors' Addresses

   Denis Bider
   Bitvise Limited
   Suites 41/42, Victoria House
   26 Main Street
   GI

   Phone: +1 869 762 1410
   EMail: ietf-ssh2@denisbider.com
   URI:   http://www.bitvise.com/


   Mark D. Baushke
   Juniper Networks, Inc.
   1194 N Mathilda Av
   Sunnyvale, CA 94089-1206
   US

   Phone: +1 408 745 2952
   EMail: mdb@juniper.net
   URI:   http://www.juniper.net/