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Versions: 00 01 02 03 04 05 06 07 08 09 rfc8268          Standards Track
Internet Engineering Task Force                               M. Baushke
Internet-Draft                                    Juniper Networks, Inc.
Updates: 4250, 4253 (if approved)                     September 15, 2017
Intended status: Standards Track
Expires: March 19, 2018

 More Modular Exponential (MODP) Diffie-Hellman (DH) Key Exchange (KEX)
                     Groups for Secure Shell (SSH)


   This document defines added Modular Exponential (MODP) Groups for the
   Secure Shell (SSH) protocol using SHA-2 hashes.  This document
   updates RFC 4250.  This document updates RFC 4253 including an errata
   fix for checking the Peer's DH Public Key.

Status of This Memo

   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).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on March 19, 2018.

Copyright Notice

   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
   (https://trustee.ietf.org/license-info) 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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Overview and Rationale

   Secure Shell (SSH) is a common protocol for secure communication on
   the Internet.  Security protocols and primitives are an active area
   for research and help to suggest updates to SSH.

   Section 3 of the [RFC4253] contains a small errata for checking the
   Peer's DH Public key.  Section 4 of this document provides the

   Due to security concerns with SHA-1 [RFC6194] and with MODP groups
   with less than 2048 bits [NIST-SP-800-131Ar1] implementer and users
   request support for larger Diffie Hellman (DH) MODP group sizes with
   data integrity verification using the SHA-2 family of secure hash
   algorithms as well as MODP groups providing more security.  The use
   of larger MODP groups and the move to the SHA-2 family of hashes are
   important features to strengthen the key exchange algorithms
   available to the SSH client and server.

   DH primes being adopted by this document are all "safe primes" such
   that p = 2q + 1 where q is also a prime.  New MODP groups are being
   introduced starting with the MODP 3072-bit group 15.  All use SHA512
   as the hash algorithm.

   The DH 2048-bit MODP group 14 is already present in most SSH
   implementations and most implementations already have a SHA256
   implementation, so diffie-hellman-group14-sha256 is provided as easy
   to implement.

   It is intended that these new MODP groups with SHA-2 based hashes
   update the [RFC4253] section 6.4 and [RFC4250] section 4.10

   The United States Information Assurance Directorate (IAD) at the
   National Security Agency (NSA) has published "Commercial National
   Security Algorithm (CNSA) Suite and Quantum Computing Frequently
   Asked Questions (FAQ)" [MFQ-U-OO-815099-15] addressed to
   organizations that run classified or unclassified national security
   systems (NSS) and vendors that build products used in NSS.

   This FAQ document indicates that NSS should no longer use:

   o  ECDH and ECDSA with NIST P-256

   o  SHA-256

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   o  AES-128

   o  RSA with 2048-bit keys

   o  Diffie-Hellman with 2048-bit keys

   The FAQ also states that NSS users should select DH groups based upon
   well established and validated parameter sets that comply with the
   minimum required sizes.  Some specific examples include:

   o  Elliptic Curves are currently restricted to the NIST P-384 group
      only for both ECDH and ECDSA, in accordance with existing NIST and
      NIAP standards.

   o  RSA moduli should have a minimum size of 3072 bits (other than the
      noted PKI exception), and keys should be generated in accordance
      with all relevant NIST standards.

   o  For Diffie-Hellman use a Diffie-Hellman prime modulus of at least
      3072 bits as specified in IETF RFC 3526 [RFC3526] (Groups 15-18).

   Although SSH may not always be used to protect Top Secret
   communications, this document adopts the use of the DH groups
   provided as an example in the FAQ as well as the use of SHA512 rather
   than SHA256 for the new DH groups.

   [TO BE REMOVED: Please send comments on this draft to

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Key Exchange Algorithms

   This document adds some new Key Exchange Algorithm Method Names in
   [RFC4253] and [RFC4250].

   This document adopts the style and conventions of [RFC4253] in
   specifying how the use of new data key exchange is indicated in SSH.

   The following new key exchange method algorithms are defined:

   o  diffie-hellman-group14-sha256

   o  diffie-hellman-group15-sha512

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   o  diffie-hellman-group16-sha512

   o  diffie-hellman-group17-sha512

   o  diffie-hellman-group18-sha512

   The SHA-2 family of secure hash algorithms are defined in [RFC6234].

   The method of key exchange used for the name "diffie-hellman-
   group14-sha256" is the same as that for "diffie-hellman-group14-sha1"
   except that the SHA256 hash algorithm is used.  It is recommended
   that diffie-hellman-group14-sha256 SHOULD be supported to smooth the
   transition to newer group sizes.

   The group15 through group18 names are the same as those specified in
   [RFC3526] 3072-bit MODP Group 15, 4096-bit MODP Group 16, 6144-bit
   MODP Group 17, and 8192-bit MODP Group 18.

   The SHA512 algorithm is to be used when "sha512" is specified as a
   part of the key exchange method name.

4.  Checking the Peer's DH Public Key

   Section 3 of [RFC4253] contains a small errata.  When checking e
   (client public key) and f (server public key) values, an incorrect
   range is provided.  The erroneous text is:

      Values of 'e' or 'f' that are not in the range [1, p-1] MUST NOT
      be sent or accepted by either side.  If this condition is
      violated, the key exchange fails.

   The errata is that the range should have been an open interval
   excluding the end point values. (i.e "(1, p-1)").  This document
   amends that document text as follows:

      DH Public key values MUST be checked and both conditions:

         1 < e < p-1

         1 < f < p-1

      MUST be true.  Values not within these bounds MUST NOT be sent or
      accepted by either side.  If either one of these condition is
      violated, then the key exchange fails.

   This simple check ensures:

   o  The remote peer behaves properly.

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   o  The local system is not forced into the two-element subgroup.

5.  IANA Considerations

   IANA is requested to add to the Key Exchange Method Names algorithm
   registry [IANA-KEX] with the following entries:

                 Key Exchange Method Name      Reference
                 ----------------------------- ----------
                 diffie-hellman-group14-sha256 This Draft
                 diffie-hellman-group15-sha512 This Draft
                 diffie-hellman-group16-sha512 This Draft
                 diffie-hellman-group17-sha512 This Draft
                 diffie-hellman-group18-sha512 This Draft

   [TO BE REMOVED: This registration should take place at the following
   location: <http://www.iana.org/assignments/ssh-parameters/ssh-

6.  Acknowledgements

   Thanks to the following people for review and comments: Denis Bider,
   Peter Gutmann, Damien Miller, Niels Moeller, Matt Johnston, Iwamoto
   Kouichi, Dave Dugal, Daniel Migault, Anna Johnston, Ron Frederick,
   Rich Salz, Travis Finkenauer, Eric Rescorla.

7.  Security Considerations

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

   The security considerations of [RFC3526] suggest that MODP group14
   through group18 have security strengths that range between 110 bits
   of security through 310 bits of security.  They are based on
   [RFC3766] Determining Strengths For Public Keys Used For Exchanging
   Symmetric Keys.  Care should be taken to use sufficient entropy and/
   or DRBG algorithms to maximize the true security strength of the key
   exchange and ciphers selected.

   Using a fixed set of Diffie-Hellman parameters makes them a high
   value target for pre-computation.  Generating additional sets of
   primes to be used, or moving to larger values is a mitigation against
   this issue.

8.  References

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8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3526]  Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
              Diffie-Hellman groups for Internet Key Exchange (IKE)",
              RFC 3526, DOI 10.17487/RFC3526, May 2003,

   [RFC4250]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Assigned Numbers", RFC 4250,
              DOI 10.17487/RFC4250, January 2006,

   [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>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,

8.2.  Informative References

              Internet Assigned Numbers Authority (IANA), "Secure Shell
              (SSH) Protocol Parameters: Key Exchange Method Names",
              March 2017, <http://www.iana.org/assignments/ssh-

              "National Security Agency/Central Security Service", "CNSA
              Suite and Quantum Computing FAQ", January 2016,

              Barker and Roginsky, "Transitions: Recommendation for the
              Transitioning of the Use of Cryptographic Algorithms and
              Key Lengths", NIST Special Publication 800-131A Revision
              1, November 2015,

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   [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
              Public Keys Used For Exchanging Symmetric Keys", BCP 86,
              RFC 3766, DOI 10.17487/RFC3766, April 2004,

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,

Author's Address

   Mark D. Baushke
   Juniper Networks, Inc.
   1133 Innovation Way
   Sunnyvale, CA  94089-1228

   Phone: +1 408 745 2952
   Email: mdb@juniper.net
   URI:   http://www.juniper.net/

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