More Modular Exponential (MODP) Diffie-Hellman (DH) Key Exchange (KEX) Groups for Secure Shell (SSH)
draft-ietf-curdle-ssh-modp-dh-sha2-04

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)
                 draft-ietf-curdle-ssh-modp-dh-sha2-04

Abstract

   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

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   This Internet-Draft will expire on October 16, 2017.

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   This document may contain material from IETF Documents or IETF
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   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.

   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
   applications.

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

   [TO BE REMOVED: Please send comments on this draft to
   curdle@ietf.org.]

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
   diffie-hellman-group14-sha256
   diffie-hellman-group15-sha512
   diffie-hellman-group16-sha512
   diffie-hellman-group17-sha512
   diffie-hellman-group18-sha512

                                 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
   [RFC4250].

   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: <http://www.iana.org/assignments/ssh-parameters/ssh-
   parameters.xhtml#ssh-parameters-16>]

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,
              <http://www.rfc-editor.org/info/rfc2119>.

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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,
              <http://www.rfc-editor.org/info/rfc3526>.

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

   [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

   [IANA-KEX]
              Internet Assigned Numbers Authority (IANA), "Secure Shell
              (SSH) Protocol Parameters: Key Exchange Method Names",
              March 2017, <http://www.iana.org/assignments/ssh-
              parameters/ssh-parameters.xhtml#ssh-parameters-16>.

   [MFQ-U-OO-815099-15]
              "National Security Agency/Central Security Service", "CNSA
              Suite and Quantum Computing FAQ", January 2016,
              <https://www.iad.gov/iad/library/ia-guidance/ia-solutions-
              for-classified/algorithm-guidance/cnsa-suite-and-quantum-
              computing-faq.cfm>.

   [NIST-SP-800-131Ar1]
              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,
              <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-131Ar1.pdf>.

   [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,
              <http://www.rfc-editor.org/info/rfc3766>.

   [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,
              <http://www.rfc-editor.org/info/rfc6194>.

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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,
              <http://www.rfc-editor.org/info/rfc6234>.

   [SNFS]     Fried, , Gaudry, , Heninger, , and Thome, "A kilobit
              hidden SNFS discrete logarithm computation", 2016,
              <http://eprint.iacr.org/2016/961.pdf>.

Author's Address

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

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

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