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

Internet Engineering Task Force (IETF)                          D. Bider
Request for Comments: 8332                               Bitvise Limited
Updates: 4252, 4253                                           March 2018
Category: Standards Track
ISSN: 2070-1721


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

Abstract

   This memo updates RFCs 4252 and 4253 to define new public key
   algorithms for use of RSA keys with SHA-256 and SHA-512 for server
   and client authentication in SSH connections.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8332.

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Table of Contents

   1.  Overview and Rationale  . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
     1.2.  Wire Encoding Terminology . . . . . . . . . . . . . . . .   3
   2.  Public Key Format vs. Public Key Algorithm  . . . . . . . . .   3
   3.  New RSA Public Key Algorithms . . . . . . . . . . . . . . . .   4
     3.1.  Use for Server Authentication . . . . . . . . . . . . . .   5
     3.2.  Use for Client Authentication . . . . . . . . . . . . . .   5
     3.3.  Discovery of Public Key Algorithms Supported by Servers .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     5.1.  Key Size and Signature Hash . . . . . . . . . . . . . . .   7
     5.2.  Transition  . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  PKCS #1 v1.5 Padding and Signature Verification . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Overview and Rationale

   Secure Shell (SSH) is a common protocol for secure communication on
   the Internet.  In [RFC4253], SSH originally defined the public key
   algorithms "ssh-rsa" for server and client authentication using RSA
   with SHA-1, and "ssh-dss" using 1024-bit DSA and SHA-1.  These
   algorithms are now considered deficient.  For US government use, NIST
   has disallowed 1024-bit RSA and DSA, and use of SHA-1 for signing
   [NIST.800-131A].

   This memo updates RFCs 4252 and 4253 to define new public key
   algorithms allowing for interoperable use of existing and new RSA
   keys with SHA-256 and SHA-512.

1.1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Wire Encoding Terminology

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

2.  Public Key Format vs. 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; and
   C.  procedures used to calculate, encode, and verify a signature.

   This document uses the term "public key format" to identify only A
   and B in isolation.  The term "public key algorithm" continues to
   identify all three aspects -- A, B, and C.

3.  New RSA Public Key Algorithms

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

   The following new public key 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 public key 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 public key algorithms, 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 [RFC8017] using SHA-2 [SHS] as
   hash.

   For the algorithm "rsa-sha2-256", the hash used is SHA-256.
   For the algorithm "rsa-sha2-512", the hash used is SHA-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 that
   contains an octet string S (which is the output of RSASSA-PKCS1-v1_5)
   and that has the same length (in octets) as the RSA modulus.  When S
   contains leading zeros, there exist signers that will send a shorter
   encoding of S that omits them.  A verifier MAY accept shorter
   encodings of S with one or more leading zeros omitted.

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" public key format.

   For example, as defined in [RFC4252] and [RFC4253], an SSH
   "publickey" authentication request using an "rsa-sha2-512" signature
   would be properly encoded as follows:

     byte      SSH_MSG_USERAUTH_REQUEST
     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

   If the client includes the signature field, the client MUST encode
   the same algorithm name in the signature as in
   SSH_MSG_USERAUTH_REQUEST -- either "rsa-sha2-256" or "rsa-sha2-512".
   If a server receives a mismatching request, it MAY apply arbitrary
   authentication penalties, including but not limited to authentication
   failure or disconnect.

   OpenSSH 7.2 (but not 7.2p2) incorrectly encodes the algorithm in the
   signature as "ssh-rsa" when the algorithm in SSH_MSG_USERAUTH_REQUEST
   is "rsa-sha2-256" or "rsa-sha2-512".  In this case, the signature
   does actually use either SHA-256 or SHA-512.  A server MAY, but is
   not required to, accept this variant or another variant that
   corresponds to a good-faith implementation and is considered safe to
   accept.

3.3.  Discovery of Public Key Algorithms Supported by Servers

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

   Servers that accept rsa-sha2-* signatures for client authentication
   SHOULD implement the extension negotiation mechanism defined in
   [RFC8308], 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.  When the new
   rsa-sha2-* algorithms have been sufficiently widely adopted to
   warrant disabling "ssh-rsa", clients MAY default to one of the new
   algorithms.

4.  IANA Considerations

   IANA has updated the "Secure Shell (SSH) Protocol Parameters"
   registry, established with [RFC4250], to extend the table "Public Key
   Algorithm Names" [IANA-PKA] as follows.

   -  To the immediate right of the column "Public Key Algorithm Name",
      a new column has been added, titled "Public Key Format".  For
      existing entries, the column "Public Key Format" has been assigned
      the same value as under "Public Key Algorithm Name".

   -  Immediately following the existing entry for "ssh-rsa", two
      sibling entries have been added:

      P. K. Alg. Name    P. K. Format      Reference   Note
      rsa-sha2-256       ssh-rsa           RFC 8332    Section 3
      rsa-sha2-512       ssh-rsa           RFC 8332    Section 3

5.  Security Considerations

   The security considerations of [RFC4251] apply to this document.

5.1.  Key Size and Signature Hash

   The National Institute of Standards and Technology (NIST) Special
   Publication 800-131A, Revision 1 [NIST.800-131A] disallows RSA and
   DSA keys shorter than 2048 bits for US government use.  The same
   document disallows the SHA-1 hash function for digital signature
   generation, except under NIST's protocol-specific guidance.

   It is prudent to follow this advice also outside of US government
   use.

5.2.  Transition

   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 public key 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 the implementers believe that new RSA
   signature algorithms have been widely adopted.

5.3.  PKCS #1 v1.5 Padding and Signature Verification

   This document prescribes RSASSA-PKCS1-v1_5 signature padding because:

   (1)  RSASSA-PSS is not universally available to all implementations;
   (2)  PKCS #1 v1.5 is widely supported in existing SSH
        implementations;
   (3)  PKCS #1 v1.5 is not known to be insecure for use in this scheme.

   Implementers are advised that a signature with RSASSA-PKCS1-v1_5
   padding MUST NOT be verified by applying the RSA key to the
   signature, and then parsing the output to extract the hash.  This may
   give an attacker opportunities to exploit flaws in the parsing and
   vary the encoding.  Verifiers MUST instead apply RSASSA-PKCS1-v1_5
   padding to the expected hash, then compare the encoded bytes with the
   output of the RSA operation.

6.  References

6.1.  Normative References

   [SHS]      NIST, "Secure Hash Standard (SHS)", FIPS Publication
              180-4, August 2015,
              <http://dx.doi.org/10.6028/NIST.FIPS.180-4>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
              January 2006, <https://www.rfc-editor.org/info/rfc4251>.

   [RFC4252]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
              January 2006, <https://www.rfc-editor.org/info/rfc4252>.

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <https://www.rfc-editor.org/info/rfc4253>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8308]  Bider, D., "Extension Negotiation in the Secure Shell
              (SSH) Protocol", RFC 8308, DOI 10.17487/RFC8308, March
              2018, <https://www.rfc-editor.org/info/rfc8308>.

6.2.  Informative References

   [NIST.800-131A]
              NIST, "Transitions: Recommendation for Transitioning the
              Use of Cryptographic Algorithms and Key Lengths", NIST
              Special Publication 800-131A, Revision 1,
              DOI 10.6028/NIST.SP.800-131Ar1, November 2015,
              <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-131Ar1.pdf>.

   [RFC4250]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Assigned Numbers", RFC 4250,
              DOI 10.17487/RFC4250, January 2006,
              <https://www.rfc-editor.org/info/rfc4250>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [IANA-PKA]
              IANA, "Secure Shell (SSH) Protocol Parameters",
              <https://www.iana.org/assignments/ssh-parameters/>.

Acknowledgments

   Thanks to Jon Bright, Niels Moeller, Stephen Farrell, Mark D.
   Baushke, Jeffrey Hutzelman, Hanno Boeck, Peter Gutmann, Damien
   Miller, Mat Berchtold, Roumen Petrov, Daniel Migault, Eric Rescorla,
   Russ Housley, Alissa Cooper, Adam Roach, and Ben Campbell for
   reviews, comments, and suggestions.

Author's Address

   Denis Bider
   Bitvise Limited
   4105 Lombardy Court
   Colleyville, Texas  76034
   United States of America

   Email: ietf-ssh3@denisbider.com
   URI:   https://www.bitvise.com/