Internet-Draft                                                  D. Bider
Expires: May 12, 2016                                    Bitvise Limited
                                                       November 12, 2015

      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.


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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 the 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.

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  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.

  The resulting signature is encoded as follows:

    string    "rsa-sha2-256" / "rsa-sha2-512"
    string    rsa_signature_blob

  The value for 'rsa_signature_blob' is encoded as a string containing
  S - an octet string which is the output of RSASSA-PKCS1-v1_5, of
  length equal to the length in octets of the RSA modulus.

2.1.  Use for server authentication

  To express support and preference for one or both of these algorithms
  for server authentication, the SSH client or server includes one or
  both algorithm names, "rsa-sha2-256" and/or "rsa-sha2-512", in the
  name-list field "server_host_key_algorithms" in the SSH_MSG_KEXINIT
  packet [RFC4253]. If one of the two host key algorithms is negotiated,
  the server sends an "ssh-rsa" public key as part of the negotiated key
  exchange method (e.g. in SSH_MSG_KEXDH_REPLY), and encodes a signature
  with the appropriate signature algorithm name - either "rsa-sha2-256",
  or "rsa-sha2-512".

2.2.  Use for client authentication

  To use this algorithm for client authentication, the SSH client sends
  an SSH_MSG_USERAUTH_REQUEST message [RFC4252] encoding the "publickey"
  method, and encoding the string field "public key algorithm name" with
  the value "rsa-sha2-256" or "rsa-sha2-512". The "public key blob"
  field encodes the RSA public key using the "ssh-rsa" algorithm name.
  The signature field, if present, encodes a signature using an
  algorithm name that matches the SSH authentication request - either
  "rsa-sha2-256", or "rsa-sha2-512".

3.  Discovery of signature algorithms supported by servers

  When a public key format can use multiple signature algorithms, it can
  be useful for a mechanism to exist which a client can use to discover
  signature algorithms accepted by a server for user authentication
  without resorting to trial and error in authentication requests.

  Such a mechanism is defined in [SSH-EXT-INFO], which describes general
  purpose extension negotiation for SSH, and specifies discovery of
  signature algorithms as a usage case.

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4.  IANA Considerations

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

  IANA is requested to update the "Secure Shell (SSH) Protocol
  Parameters" registry with the following entry:

    Public Key Algorithm Name      Reference          Note
    rsa-sha2-256                   [this document]    Section 2
    rsa-sha2-512                   [this document]    Section 2

5.  Security Considerations

  The security considerations of [RFC4253] apply to this document.

  The National Institute of Standards and Technology (NIST) Special
  Publication 800-131A [800-131A] disallows the use of RSA and DSA keys
  shorter than 2048 bits for US government use after 2013. Keys of 2048
  bits or larger are considered acceptable.

  The same document disallows the SHA-1 hash function, as used in the
  "ssh-rsa" and "ssh-dss" algorithms, for digital signature generation
  after 2013. The SHA-2 family of hash functions is seen as acceptable.

6.  Why no DSA?

  A draft version of this memo also defined an algorithm name for use of
  2048-bit and 3072-bit DSA keys with a 256-bit subgroup and SHA-2 256
  hashing. It is possible to implement DSA securely by generating "k"
  deterministically as per [RFC6979]. However, a plurality of reviewers
  were concerned that implementers would not pay heed, and would use
  cryptographic libraries that continue to generate "k" randomly. This
  is vulnerable to biased "k" generation, and extremely vulnerable to
  "k" reuse. The relative speed advantage of DSA signing compared to RSA
  signing was not perceived to outweigh this shortcoming, especially
  since algorithms based on elliptic curves are faster yet.

  Due to these disrecommendations, this document abstains from defining
  an algorithm name for large DSA keys, and recommends RSA instead.

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7.  References

7.1.  Normative References

              National Institute of Standards and Technology (NIST),
              United States of America, "Secure Hash Standard (SHS)",
              FIPS Publication 180-4, August 2015,

  [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

  [RFC3447]   Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

  [RFC4252]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, January 2006.

  [RFC4253]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, January 2006.

6.2.  Informative References

  [800-131A]  National Institute of Standards and Technology (NIST),
              "Transitions: Recommendation for Transitioning the Use of
              Cryptographic Algorithms and Key Lengths", 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.

  [RFC6979]   Pornin, T., "Deterministic Usage of the Digital
              Signature Algorithm (DSA) and Elliptic Curve Digital
              Signature Algorithm (ECDSA)", RFC 6979, August 2013.

              Bider, D., "Extension Negotiation in Secure Shell (SSH)",
              draft-ssh-ext-info-00, November 2015,

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Author's Address

  Denis Bider
  Bitvise Limited
  Suites 41/42, Victoria House
  26 Main Street

  Phone: +506 8315 6519


  Thanks to Jon Bright, Niels Moeller, Stephen Farrell, Mark D. Baushke,
  Jeffrey Hutzelman, Hanno Boeck, Peter Gutmann, and Damien Miller for
  comments and suggestions.

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