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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 8268

Internet Engineering Task Force                               M. Baushke
Internet-Draft                                    Juniper Networks, Inc.
Updates: 4250, 4253 (if approved)                          June 20, 2017
Intended status: Standards Track
Expires: December 22, 2017

 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.

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
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   This Internet-Draft will expire on December 22, 2017.

Copyright Notice

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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

1.  Overview and Rationale

   Secure Shell (SSH) is a common protocol for secure communication on
   the Internet.  Due to recent 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.

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

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   The following new key exchange algorithms are defined:

   Key Exchange Method Name

                                 Figure 1

   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.  IANA Considerations

   This document augments the Key Exchange Method Names in [RFC4253] and

   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-

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5.  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.

6.  References

6.1.  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, <http://www.rfc-editor.org/info/rfc4253>.

6.2.  Informative References

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

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              "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,

   [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,

   [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,

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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