Internet Engineering Task Force                               M. Baushke
Internet-Draft                                    Juniper Networks, Inc.
Updates: 4253, 4432 (if approved)                          March 6, 2017
Intended status: Standards Track
Expires: September 7, 2017


 More Modular Exponential (MODP) Diffie-Hellman (DH) Key Exchange (KEX)
                     Groups for Secure Shell (SSH)
                 draft-ietf-curdle-ssh-modp-dh-sha2-02

Abstract

   This document defines added Modular Exponential (MODP) Groups for the
   Secure Shell (SSH) protocol using SHA-2 hashes.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 7, 2017.

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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.

   Please send comments on this draft to ietf-ssh@NetBSD.org and ietf-
   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 [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.

   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



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   The SHA-2 family of secure hash algorithms are defined in
   [FIPS-180-4].

   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.

   The method of key exchange used for the name "gss-group14-sha256-*"
   is the same as that for "gss-group14-sha1-*" except that the SHA2-256
   hash algorithm is used.

   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].

   IANA is requested to add to the Key Exchange Method Names algorithm
   registry 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-
   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.






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   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

   Many users seem to be interested in the perceived safety of using
   larger MODP groups and hashing with SHA2-based algorithms.

6.  References

6.1.  Normative References

   [FIPS-180-4]
              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4, August 2015,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

   [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>.

   [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>.

   [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>.








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6.2.  Informative References

   [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>.

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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