Internet Engineering Task Force                               M. Baushke
Internet-Draft                                    Juniper Networks, Inc.
Updates: 4250, 4253 (if approved)                         April 14, 2017
Intended status: Standards Track
Expires: October 16, 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.

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   This document may contain material from IETF Documents or IETF
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   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.

   The United States Information Assurance Directorate at the National
   Security Agency has published a FAQ [MFQ-U-OO-815099-15] suggesting
   both: a) DH groups using less than 3072-bits, and b) the use of SHA-2
   based hashes less than SHA2-384, are no longer sufficient for
   transport of Top Secret information.  For this reason, the new MODP
   groups are being introduced starting with the MODP 3072-bit group 15
   are all using SHA2-512 as the hash algorithm.

   The DH 2048-bit MODP group 14 is already present in most SSH
   implementations and most implementations already have a SHA2-256
   implementation, so diffie-hellman-group14-sha256 is provided as an
   easy to implement and faster to use key exchange for small embedded

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

   [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].

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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.

   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 SHA2-256 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] 3071-bit MODP Group 15, 4096-bit MODP Group 16, 6144-bit
   MODP Group 17, and 8192-bit MODP Group 18.

   The SHA2-512 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

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   [TO BE REMOVED: This registration should take place at the following
   location: <

5.  Security Considerations

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

   The security considerations of [RFC3526] suggest that these MODP
   groups have security strengths given in this table.  They are based
   on [RFC3766] Determining Strengths For Public Keys Used For
   Exchanging Symmetric Keys.

   Group modulus security strength estimates (RFC3526)

   | Group  | Modulus  | Strength Estimate 1 | Strength Estimate 2 |
   |        |          +----------+----------+----------+----------+
   |        |          |          | exponent |          | exponent |
   |        |          | in bits  | size     | in bits  | size     |
   |  14    | 2048-bit |      110 |     220- |      160 |     320- |
   |  15    | 3072-bit |      130 |     260- |      210 |     420- |
   |  16    | 4096-bit |      150 |     300- |      240 |     480- |
   |  17    | 6144-bit |      170 |     340- |      270 |     540- |
   |  18    | 8192-bit |      190 |     380- |      310 |     620- |

                                 Figure 2

   Using a fixed set of Diffie-Hellman parameters makes them a high
   value target for precomputation.  Generating additional sets of
   primes to be used, or moving to larger values is a mitigation against
   this issue.  Care should be taken to avoid backdoored primes ([SNFS])
   by using "nothing up my sleve" parameters.

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,

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   [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, <>.

6.2.  Informative References

              Internet Assigned Numbers Authority (IANA), "Secure Shell
              (SSH) Protocol Parameters: Key Exchange Method Names",
              March 2017, <

              "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,

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   [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,

   [SNFS]     Fried, , Gaudry, , Heninger, , and Thome, "A kilobit
              hidden SNFS discrete logarithm computation", 2016,

Author's Address

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

   Phone: +1 408 745 2952

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