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
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Bider & Baushke Standards Track [Page 1]RFC 6668 Sha2-Transport Layer Protocol July 20121. 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.
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"