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 (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
Provisions Relating to IETF Documents
(http://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.
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",
"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)
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.
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 2Figure 24. Security Considerations
The security considerations of RFC 4253 [RFC4253] apply to this
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.
5.1. Normative References
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/
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
[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
[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,
[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.
Suites 41/42, Victoria House
26 Main Street
Phone: +1 869 762 1410
Mark D. Baushke
Juniper Networks, Inc.
1194 N Mathilda Av
Sunnyvale, CA 94089-1206
Phone: +1 408 745 2952