Use of RSA Keys with SHA-2 256 and 512 in Secure Shell (SSH)

Document Type Active Internet-Draft (curdle WG)
Last updated 2017-03-27 (latest revision 2017-02-27)
Replaces draft-rsa-dsa-sha2-256
Stream IETF
Intended RFC status (None)
Formats plain text pdf html bibtex
Stream WG state In WG Last Call
Document shepherd No shepherd assigned
IESG IESG state I-D Exists
Consensus Boilerplate Unknown
Telechat date
Responsible AD (None)
Send notices to (None)
Internet-Draft                                                  D. Bider
Updates: 4252, 4253 (if approved)                        Bitvise Limited
Intended status: Standards Track                       February 27, 2017
Expires: August 27, 2017

      Use of RSA Keys with SHA-2 256 and 512 in Secure Shell (SSH)


  This memo defines an algorithm name, public key format, and signature
  format for use of RSA keys with SHA-2 512 for server and client
  authentication in SSH connections.


  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), its areas, and its working groups.  Note that other
  groups may also distribute working documents as Internet-Drafts.

  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."

  The list of current Internet-Drafts can be accessed at
  The list of Internet-Draft Shadow Directories can be accessed at


  Copyright (c) 2017 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
  ( 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.

Bider                                                           [Page 1]
Internet-Draft         RSA Keys with SHA-2 in SSH          February 2017

1.  Overview and Rationale

  Secure Shell (SSH) is a common protocol for secure communication on
  the Internet. In [RFC4253], SSH originally defined the signature
  methods "ssh-rsa" for server and client authentication using RSA with
  SHA-1, and "ssh-dss" using 1024-bit DSA and SHA-1.
  A decade later, these signature methods are considered deficient.
  For US government use, NIST has disallowed 1024-bit RSA and DSA, and
  use of SHA-1 for signing [800-131A].
  This memo defines a new algorithm name allowing for interoperable use
  of RSA keys with SHA-2 256 and SHA-2 512, and a mechanism for servers
  to inform SSH clients of signature algorithms they support and accept.

1.1.  Requirements Terminology

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  document are to be interpreted as described in [RFC2119].

2.  Public Key Algorithms

  This memo adopts the style and conventions of [RFC4253] in specifying
  how use of a signature algorithm is indicated in SSH.

  The following new signature algorithms are defined:
    rsa-sha2-256    RECOMMENDED    sign    Raw RSA key
    rsa-sha2-512    OPTIONAL       sign    Raw RSA key

  These signature algorithms are suitable for use both in the SSH transport
  layer [RFC4253] for server authentication, and in the authentication
  layer [RFC4252] for client authentication.

  Since RSA keys are not dependent on the choice of hash function, both
  new algorithms reuse the public key format of the existing "ssh-rsa"
  algorithm as defined in [RFC4253]:

    string    "ssh-rsa"
    mpint     e
    mpint     n
  All aspects of the "ssh-rsa" format are kept, including the encoded
  string "ssh-rsa", in order to allow users' existing RSA keys to be
  used with the new signature formats, without requiring re-encoding,
  or affecting already trusted key fingerprints.
  Signing and verifying using these algorithms is performed according to
  the RSASSA-PKCS1-v1_5 scheme in [RFC3447] using SHA-2 [FIPS-180-4] as
  hash; MGF1 as mask function; and salt length equal to hash size.

Bider                                                           [Page 2]
Internet-Draft         RSA Keys with SHA-2 in SSH          February 2017
  For the algorithm "rsa-sha2-256", the hash used is SHA-2 256.
  For the algorithm "rsa-sha2-512", the hash used is SHA-2 512.
Show full document text