SHA-2 Data Integrity Verification for the Secure Shell (SSH) Transport Layer Protocol
draft-dbider-sha2-mac-for-ssh-05

Abstract

This memo defines algorithm names and parameters for use of some of the SHA-2 family of secure hash algorithms for data integrity verification in the Secure Shell (SSH) protocol. It also updates RFC4253 by specifying a new RECOMMENDED data integrity algorithm.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on July 02, 2012.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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1. Overview and Rationale

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 [RFC6151][RFC6194], implementors and users request support for data integrity verification using some of the SHA-2 family of of secure hash algorithms.

Please send comments on this draft to ietf-ssh@NetBSD.org.

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 defining new data integrity algorithms.

The following new data integrity algorithms are defined:

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

hmac-sha2-256-96  OPTIONAL      first 96 bits of HMAC-SHA2-256
                                (digest length = 12 bytes,
                                 key length    = 32 bytes)

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

hmac-sha2-512-96  OPTIONAL      first 96 bits of HMAC-SHA2-512
                                (digest length = 12 bytes,
                                 key length    = 64 bytes)
        

The HMAC mechanism was originally defined in [RFC2104] and has been updated in [RFC6151].

The SHA-2 family of secure hash algorithms are 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.

The truncated -96 OPTIONAL forms are present to allow applications which may be space restricted to still interoperate and make use of the new hashes.

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 would be negotiated first.

3. IANA Considerations

This document augments the MAC Algorithm Names in [RFC4253] and [RFC4250].

IANA is requested to update the SSH algorithm registry with the following entries:

MAC Algorithm Name      Reference       Note
hmac-sha2-256           This draft      Section 2
hmac-sha2-256-96        This draft      Section 2
hmac-sha2-512           This draft      Section 2
hmac-sha2-512-96        This draft      Section 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

5.2. Informative References

Authors' Addresses

Table of Contents

• 1. Overview and Rationale
• 1.1. Requirements Terminology
• 2. Data Integrity Algorithms
• 3. IANA Considerations
• 4. Security Considerations
• 5. References
• 5.1. Normative References
• 5.2. Informative References
• Authors' Addresses