6668
PROPOSED STANDARD
SHA-2 Data Integrity Verification for the Secure Shell (SSH) Transport Layer Protocol
Authors: D. Bider, M. Baushke
Date: July 2012
Working Group: NON WORKING GROUP
Stream: IETF
Updates:
RFC 4253
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. [STANDARDS-TRACK]
RFC 6668
PROPOSED STANDARD
Internet Engineering Task Force (IETF) D. Bider
Request for Comments: 6668 Bitvise Limited
Updates: <a href="./rfc4253">4253</a> M. Baushke
Category: Standards Track Juniper Networks, Inc.
ISSN: 2070-1721 July 2012
<span class="h1">SHA-2 Data Integrity Verification for</span>
<span class="h1">the Secure Shell (SSH) Transport Layer Protocol</span>
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 <a href="./rfc4253">RFC</a>
<a href="./rfc4253">4253</a> 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 <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="https://www.rfc-editor.org/info/rfc6668">http://www.rfc-editor.org/info/rfc6668</a>.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
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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.
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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Overview and Rationale</span>
The Secure Shell (SSH) [<a href="./rfc4251" title=""The Secure Shell (SSH) Protocol Architecture"">RFC4251</a>] is a very common protocol for secure
remote login on the Internet. Currently, SSH defines data integrity
verification using SHA-1 and MD5 algorithms [<a href="./rfc4253" title=""The Secure Shell (SSH) Transport Layer Protocol"">RFC4253</a>]. Due to recent
security concerns with these two algorithms ([<a href="./rfc6194" title=""Security Considerations for the SHA-0 and SHA-1 Message-Digest Algorithms"">RFC6194</a>] and [<a href="./rfc6151" title=""Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms"">RFC6151</a>],
respectively), implementors and users request support for data
integrity verification using some of the SHA-2 family of secure hash
algorithms.
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Requirements Terminology</span>
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 [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Data Integrity Algorithms</span>
This memo adopts the style and conventions of [<a href="./rfc4253" title=""The Secure Shell (SSH) Transport Layer Protocol"">RFC4253</a>] 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 [<a href="./rfc2104" title=""HMAC: Keyed- Hashing for Message Authentication"">RFC2104</a>] and has been updated in [<a href="./rfc6151" title=""Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms"">RFC6151</a>].
The SHA-2 family of secure hash algorithms is defined in
[<a href="#ref-FIPS-180-3">FIPS-180-3</a>].
Sample code for the SHA-based HMAC algorithms are available in
[<a href="./rfc6234" title=""US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)"">RFC6234</a>]. 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.
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Test vectors for use of HMAC with SHA-2 are provided in [<a href="./rfc4231" title=""Identifiers and Test Vectors for HMAC- SHA-224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512"">RFC4231</a>].
Users, implementors, and administrators may choose to put these new
MACs into the proposal ahead of the REQUIRED hmac-sha1 algorithm
defined in [<a href="./rfc4253" title=""The Secure Shell (SSH) Transport Layer Protocol"">RFC4253</a>] so that they are negotiated first.
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. IANA Considerations</span>
This document augments the MAC Algorithm Names in [<a href="./rfc4253" title=""The Secure Shell (SSH) Transport Layer Protocol"">RFC4253</a>] and
[<a href="./rfc4250" title=""The Secure Shell (SSH) Protocol Assigned Numbers"">RFC4250</a>].
IANA has updated the "Secure Shell (SSH) Protocol Parameters"
registry with the following entries:
MAC Algorithm Name Reference Note
hmac-sha2-256 <a href="./rfc6668#section-2">RFC 6668 Section 2</a>
hmac-sha2-512 <a href="./rfc6668#section-2">RFC 6668 Section 2</a>
Figure 2
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Security Considerations</span>
The security considerations of <a href="./rfc4253">RFC 4253</a> [<a href="./rfc4253" title=""The Secure Shell (SSH) Transport Layer Protocol"">RFC4253</a>] apply to this
document.
The National Institute of Standards and Technology (NIST)
publications: NIST Special Publication (SP) 800-107 [<a href="#ref-800-107" title=""Recommendation for Applications Using Approved Hash Algorithms"">800-107</a>] and
NIST SP 800-131A [<a href="#ref-800-131A" title=""Transitions: Recommendation for the Transitioning of the Use of Cryptographic Algorithms and Key Lengths"">800-131A</a>] 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.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. References</span>
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. Normative References</span>
[<a id="ref-FIPS-180-3">FIPS-180-3</a>]
National Institute of Standards and Technology (NIST),
United States of America, "Secure Hash Standard (SHS)",
FIPS PUB 180-3, October 2008, <<a href="http://csrc.nist.gov/publications/fips/fips180-3/fips180-3_final.pdf">http://csrc.nist.gov/</a>
<a href="http://csrc.nist.gov/publications/fips/fips180-3/fips180-3_final.pdf">publications/fips/fips180-3/fips180-3_final.pdf</a>>.
[<a id="ref-RFC2104">RFC2104</a>] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", <a href="./rfc2104">RFC 2104</a>, February
1997.
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<span class="grey"><a href="./rfc6668">RFC 6668</a> Sha2-Transport Layer Protocol July 2012</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC4231">RFC4231</a>] Nystrom, M., "Identifiers and Test Vectors for HMAC-
SHA-224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
<a href="./rfc4231">RFC 4231</a>, December 2005.
[<a id="ref-RFC4253">RFC4253</a>] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", <a href="./rfc4253">RFC 4253</a>, January 2006.
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. Informative References</span>
[<a id="ref-800-107">800-107</a>] National Institute of Standards and Technology (NIST),
"Recommendation for Applications Using Approved Hash
Algorithms", NIST Special Publication 800-107, February
2009, <<a href="http://csrc.nist.gov/publications/nistpubs/800-107/NIST-SP-800-107.pdf">http://csrc.nist.gov/publications/</a>
<a href="http://csrc.nist.gov/publications/nistpubs/800-107/NIST-SP-800-107.pdf">nistpubs/800-107/NIST-SP-800-107.pdf</a>>.
[<a id="ref-800-131A">800-131A</a>] 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,
<<a href="http://csrc.nist.gov/publications/nistpubs/800-131A/sp800-131A.pdf">http://csrc.nist.gov/publications/nistpubs/800-131A/</a>
<a href="http://csrc.nist.gov/publications/nistpubs/800-131A/sp800-131A.pdf">sp800-131A.pdf</a>>.
[<a id="ref-RFC4250">RFC4250</a>] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Assigned Numbers", <a href="./rfc4250">RFC 4250</a>, January 2006.
[<a id="ref-RFC4251">RFC4251</a>] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", <a href="./rfc4251">RFC 4251</a>, January 2006.
[<a id="ref-RFC6151">RFC6151</a>] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
<a href="./rfc6151">RFC 6151</a>, March 2011.
[<a id="ref-RFC6194">RFC6194</a>] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", <a href="./rfc6194">RFC 6194</a>, March 2011.
[<a id="ref-RFC6234">RFC6234</a>] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", <a href="./rfc6234">RFC 6234</a>, May 2011.
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Authors' Addresses
Denis Bider
Bitvise Limited
Suites 41/42, Victoria House
26 Main Street
GI
Phone: +1 869 762 1410
EMail: [email protected]
URI: <a href="http://www.bitvise.com/">http://www.bitvise.com/</a>
Mark D. Baushke
Juniper Networks, Inc.
1194 N Mathilda Av
Sunnyvale, CA 94089-1206
US
Phone: +1 408 745 2952
EMail: [email protected]
URI: <a href="http://www.juniper.net/">http://www.juniper.net/</a>
Bider & Baushke Standards Track [Page 5]
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