Use of RSA and DSA Keys with SHA-2 256 in Secure Shell (SSH)
draft-rsa-dsa-sha2-256-00

Internet-Draft                                                  D. Bider
Expires: May 1, 2016                                     Bitvise Limited
                                                        November 1, 2015

      Use of RSA and DSA Keys with SHA-2 256 in Secure Shell (SSH)
                     draft-rsa-dsa-sha2-256-00.txt

Abstract

  This memo defines algorithm names, public key formats, and signature
  formats for use of RSA and DSA keys with SHA-2 256 for server and
  client authentication in SSH connections.

Status

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Copyright

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  document authors.  All rights reserved.

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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 DSA according to the then-available
  version of the Digital Signature Standard [FIPS-186-2]. At that time,
  DSS specified a modulus of up to 1024 bits, with a subgroup size of
  160 bits, using SHA-1 hashing.
   
  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 new algorithm names allowing for interoperable use
  of RSA and DSA keys with SHA-2 256, and use of 2048 and 3072-bit DSA.

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.  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
    dsa-sha2-256    OPTIONAL       sign    Raw 2048- or 3072-bit DSA Key

  Both signature algorithms are suitable for use both in the SSH
  transport layer [RFC4253] for server authentication, and in the SSH
  authentication layer [RFC4252] for client authentication.
   
2.1  rsa-sha2-256
   
  Since RSA keys are not dependent on the choice of hash function, the
  algorithm "rsa-sha2-256" reuses 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 format, without requiring re-encoding, or
  affecting already trusted key fingerprints.

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  Signing and verifying using this algorithm is performed according to
  the RSASSA-PSS scheme in [RFC3447] using SHA-2 256 [FIPS-180-3] as
  hash; MGF1 as mask function; and salt length equal to hash size.

  The resulting signature is encoded as follows:

    string    "rsa-sha2-256"
    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-PSS, of length
  equal to the length in octets of the RSA modulus.

2.2  dsa-sha2-256
   
  Keys used with this signature algorithm MUST use one of the following
  FIPS 186-4 options for modulus size (L) and subgroup size (N):

    L = 2048, N = 256
    L = 3072, N = 256

  At least one major platform is currently known to support large DSA
  keys only with these parameters. To help interoperability,
  applications MUST NOT use options not listed.
   
  Applications that wish to implement DSA key sizes or parameters other
  than those specified herein MUST use different algorithm names for
  such extensions. This is necessary to allow effective algorithm
  negotiation, and ensure interoperability between applications that
  may support varying sets of parameters and key sizes.
   
  This key format has the following public key encoding:

    string    "dsa-sha2-256"
    mpint     p
    mpint     q
    mpint     g
    mpint     y

  Signing and verifying using this key format is done according to the
  Digital Signature Standard [FIPS-186-4] using a 256-bit SHA-2 hash
  [FIPS-180-3]. 

  The resulting signature is encoded as follows:

    string    "dsa-sha2-256"
    string    dsa_signature_blob

  The value for 'dsa_signature_blob' is encoded as a string containing
  r, followed by s (which are 256-bit integers, without lengths or
  padding, unsigned, and in network byte order).

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3.  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 entries:

    Public Key Algorithm Name      Reference          Note
    rsa-sha2-256                   [this document]    Section 2.1
    dsa-sha2-256                   [this document]    Section 2.2

4.  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] suggests that RSA keys shorter than
  2048 bits; and DSA keys shorter than 2048 bits, and with subgroup
  sizes under 224 bits; have an encryption strength less than 112 bits.
  It disallows them for US government use after 2013. RSA key sizes of
  2048 bits or more; and DSA key sizes of 2048 bits or more, and with
  subgroup sizes of 224 bits or more; are considered acceptable.
   
  The same document disallows the SHA-1 hash function, as used in the
  "ssh-dss" algorithm, for digital signature generation after 2013. The
  SHA-2 family of hash functions, as used with the algorithm defined in
  this document, is considered acceptable.

4.1  Generation of "k" in DSA Signing

  DSA private keys are vulnerable to biases in random generation of the
  "k" parameter during signing. A small bias permits discovery of the
  private key after observing a sufficient number of signatures. Reuse
  of the same "k" for only two different messages is sufficient to
  completely compromise the key. This can be induced, for example, by
  resuming saved virtual machine state. On the contrary, a DSA private
  key is immune to these attacks if "k" is generated deterministically,
  based only on the private key and message.
   
  Applications that are able to do so SHOULD use a deterministic "k"
  as specified in [RFC6979]. Applications that cannot do this SHOULD
  feed the entropy of the message being signed into the PRNG mechanism
  used to generate "k" immediately before signing.

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5.  References

5.1.  Normative References

  [FIPS-180-3]
              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/
              publications/fips/fips180-3/fips180-3_final.pdf>.

  [FIPS-186-4]
              National Institute of Standards and Technology (NIST),
              United States of America, "Digital Signature Standard
              (DSS)", FIPS Publication 186-4, July 2013, <http://
              nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf>.

  [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.

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

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

5.2.  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, <http://csrc.nist.gov/
              publications/nistpubs/800-131A/sp800-131A.pdf>.

  [FIPS-186-2]
              National Institute of Standards and Technology (NIST),
              United States of America, "Digital Signature Standard
              (DSS)", FIPS Publication 186-2 (with Change Notice 1),
              October 2001, <http://csrc.nist.gov/publications/fips/
              archive/fips186-2/fips186-2-change1.pdf>.

  [RFC4250]   Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Assigned Numbers", RFC 4250, January 2006.

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

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Author's Address

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

  Phone: +506 8315 6519
  EMail: ietf-ssh3@denisbider.com
  URI:   https://www.bitvise.com/

Acknowledgments

  Thanks to Jeffrey Hutzelman for comments and suggestions to initial
  drafts.

  Thanks to participants on the SSH and CFRG mailing lists for 
  additional comments and suggestions.

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