Internet-Draft                                                  D. Bider
Updates: 4252, 4253 (if approved)                        Bitvise Limited
Intended status: Standards Track                          March 29, 2017
Expires: September 29, 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 hashing 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 introduces a distinction between public key and signature
  algorithms in SSH, and defines new signature algorithm names allowing
  for interoperable use of existing and new RSA keys with SHA-2 hashing.

1.1.  Requirements Terminology

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

1.2.  Wire Encoding Terminology

  The wire encoding types in this document - "boolean", "byte",
  "string", "mpint" - have meanings as described in [RFC4251].

2.  Signature Algorithm as Distinct Aspect of Public Key Algorithm

  In [RFC4252], the concept "public key algorithm" is used to establish
  a relationship between one algorithm name, and:

  A. Procedures used to generate and validate a private/public keypair.
  B. A format used to encode a public key.
  C. Procedures used to calculate, encode, and verify a signature.

  This document narrows the term "public key algorithm" to mean A and B,
  though it can still potentially imply C when a public key algorithm is
  associated with only one signature algorithm. A new term, "signature
  algorithm", is introduced to refer specifically to C.

  This affects the meaning of the field "server_host_key_algorithms" in
  the message SSH_MSG_KEXINIT ([RFC4253]). With this document, this
  field now refers specifically to signature, not public key algorithms.

  This also affects the message SSH_MSG_USERAUTH_REQUEST when used with
  the "publickey" authentication method as defined in [RFC4252]. With
  this document, the definition of this message is updated as follows:

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      string    user name in ISO-10646 UTF-8 encoding [RFC3629]
      string    service name in US-ASCII
      string    "publickey"
      boolean   FALSE
      string    signature algorithm name
      string    public key blob

  The format of the message remains unchanged. The change is in the line
  which now reads "signature algorithm name". This used to read "public
  key algorithm name".

  These changes do not affect key types other than RSA. Other public key
  algorithms continue to use one signature algorithm of the same name.

  There is no impact on existing implementations that support RSA keys
  only as "ssh-rsa". Such implementations continue to use the public key
  algorithm "ssh-rsa", and the signature algorithm of the same name.

3.  New RSA Signature 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 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, the
  new signature algorithms are defined as aspects of the existing
  "ssh-rsa" public key algorithm. This means the new algorithms reuse
  the "ssh-rsa" public key format 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". This allows 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.

3.1.  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".

3.2.  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 MUST match the SSH authentication request - either
  "rsa-sha2-256", or "rsa-sha2-512".

  For example, an SSH "publickey" authentication request using an
  "rsa-sha2-512" signature would be properly encoded as follows:

    string    user name
    string    service name
    string    "publickey"
    boolean   TRUE
    string    "rsa-sha2-512"
    string    public key blob:
        string    "ssh-rsa"
        mpint     e
        mpint     n
    string    signature:
        string    "rsa-sha2-512"
        string    rsa_signature_blob

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3.3.  Discovery of signature algorithms supported by servers

  Implementation experience has shown that there are servers which apply
  authentication penalties to clients attempting signature algorithms
  which the SSH server does not support.

  Servers that accept rsa-sha2-* signatures for client authentication
  SHOULD implement the extension negotiation mechanism defined in
  [SSH-EXT-INFO], including especially the "server-sig-algs" extension.

  When authenticating with an RSA key against a server that does not
  implement the "server-sig-algs" extension, clients MAY default to an
  ssh-rsa signature to avoid authentication penalties.

4.  IANA Considerations

  IANA is requested to update the "Secure Shell (SSH) Protocol
  Parameters" registry, to extend the table Public Key Algorithm Names:

  - To the immediate right of the column Public Key Algorithm Name,
    a new column is to be added, titled Signature Algorithm Name. For
    existing entries, the column Signature Algorithm Name should be
    assigned the same value found under Public Key Algorithm Name.

  - Immediately following the existing entry for "ssh-rsa", two sibling
    entries are to be added:

    P. K. Alg. Name    Sig. Alg. Name    Reference          Note
    ssh-rsa            rsa-sha2-256      [this document]    Section 3
    ssh-rsa            rsa-sha2-512      [this document]    Section 3

5.  Security Considerations

  The security considerations of [RFC4251] 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. 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.

  This document is based on the premise that RSA is used in environments
  where a gradual, compatible transition to improved algorithms will be
  better received than one that is abrupt and incompatible. It advises
  that SSH implementations add support for new RSA signature algorithms
  along with SSH_MSG_EXT_INFO and the "server-sig-algs" extension to
  allow coexistence of new deployments with older versions that support
  only "ssh-rsa". Nevertheless, implementations SHOULD start to disable
  "ssh-rsa" in their default configurations as soon as they have reason
  to believe that new RSA signature algorithms have been widely adopted.

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 continue to use libraries that

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  generate "k" randomly. This is vulnerable to biased "k" generation,
  and extremely vulnerable to "k" reuse. This document therefore
  disrecommends DSA, in favor of RSA and elliptic curve cryptography.

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.

  [RFC4251]   Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

  [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.

7.2.  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-ietf-curdle-ssh-ext-info-03.txt, March 2017,

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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, Damien Miller, Mat
  Berchtold, and Roumen Petrov for reviews, comments, and suggestions.

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