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Key Exchange (KEX) Method Updates and Recommendations for Secure Shell (SSH)

The information below is for an old version of the document.
Document Type This is an older version of an Internet-Draft that was ultimately published as an RFC.
Author Mark D. Baushke
Last updated 2016-09-07
Replaces draft-baushke-ssh-dh-group-sha2
Stream Internet Engineering Task Force (IETF)
OPSDIR Last Call Review Incomplete, due 2021-02-24
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Internet Engineering Task Force                               M. Baushke
Internet-Draft                                    Juniper Networks, Inc.
Updates: 4253, 4419, 4432, 4462, 5656                  September 7, 2016
         (if approved)
Intended status: Standards Track
Expires: March 11, 2017

 Key Exchange (KEX) Method Updates and Recommendations for Secure Shell


   This document adds recommendations for adoption of ssh-curves from
   the [I-D.ietf-curdle-ssh-curves], adds some new Modular Exponential
   (MODP) Groups, and deprecates some previously specified Key Exchange
   Method algorithm names for the Secure Shell (SSH) protocol.  It also
   updates [RFC4253], [RFC4419], [RFC4462], and [RFC5656] by specifying
   the set key exchange algorithms that currently exist and which ones
   MUST, SHOULD, MAY, and SHOULD NOT be implemented.  New key exchange
   methods use the SHA-2 family of hashes.

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

   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 11, 2017.

Copyright Notice

   Copyright (c) 2016 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
   ( in effect on the date of

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   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
   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.  In [RFC4253], SSH originally defined the Key Exchange
   Method Name diffie-hellman-group1-sha1 which used [RFC2409] Oakley
   Group 1 (a 768-bit MODP group) and SHA-1 [RFC3174].  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 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 (IAD) at the
   National Security Agency (NSA) has published a FAQ
   [MFQ-U-OO-815099-15] suggesting that the use of Elliptic Curve
   Diffie-Hellman (ECDH) using the nistp256 curve and SHA-2 based hashes
   less than SHA2-384 are no longer sufficient for transport of Top
   Secret information.  It is for this reason that this draft moves
   ecdh-sha2-nistp256 from a REQUIRED to OPTIONAL as a key exchange
   method.  This is the same reason that the stronger MODP groups being
   introduced are using SHA2-512 as the hash algorithm.  Group14 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.  Small embedded applications may find this KEX
   desirable to use.

   The NSA Information Assurance Directorate (IAD) has also published
   the Commercial National Security Algorithm Suite (CNSA Suite)
   [CNSA-SUITE] in which the 3072-bit MODP Group 15 in RFC 3526 is
   explicitly mentioned as the minimum modulus to protect Top Secret

   It has been observed in [safe-curves] that the NIST recommended
   Elliptic Curve Prime Curves (P-256, P-384, and P-521) are perhaps not
   the best available for Elliptic Curve Cryptography (ECC) Security.
   For this reason, none of the [RFC5656] curves are marked as a MUST
   implement.  However, the requirement that "every compliant SSH ECC
   implementation MUST implement ECDH key exchange" is now taken to mean
   that if ecdsa-sha2-[identifier] is implemented, then ecdh-
   sha2-[identifier] MUST be implemented.

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   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 [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.

   A new set of Elliptic Curve Diffie-Hellman ssh-curves exist.  The
   curve25519-sha256 MUST be adopted where possible.

   As a hedge against uncertainty raised by the NSA IAD FAQ publication,
   five new MODP Diffie-Hellman based key exchanges are proposed for
   inclusion in the set of key exchange method names as well as the
   curve448-sha512 curve.

   The following new key exchange algorithms are defined:

   Key Exchange Method Name          Note
   diffie-hellman-group14-sha256     SHOULD/RECOMMENDED
   diffie-hellman-group15-sha512     MAY/OPTIONAL
   diffie-hellman-group16-sha512     SHOULD/RECOMMENDED
   diffie-hellman-group17-sha512     MAY/OPTIONAL
   diffie-hellman-group18-sha512     MAY/OPTIONAL

                                 Figure 1

   The SHA-2 family of secure hash algorithms are defined in

   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.  This new method is
   desirable for interoperability with resource-constrained devices.

   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.  All of these groups are
   within the guidelines for CNSA Suite for Top Secret.

   The SHA2-512 algorithm is to be used when "sha512" is specified as a
   part of the key exchange method name.

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4.  IANA Considerations

   This document augments the Key Exchange Method Names in [RFC4253].
   It downgrades the use of SHA-1 hashing for key exchange methods in
   [RFC4419], [RFC4432], and [RFC4462].  It also moves from MUST to MAY
   the ecdh-sha2-nistp256 given in [RFC5656].

   It is desirable to also include the ssh-curves from the
   [I-D.ietf-curdle-ssh-curves] in this list.  The "curve25519-sha256"
   is currently available in some Secure Shell implementations under the
   name "" and is the best candidate for a
   fast, safe, and secure key exchange method.

   IANA is requested to update the SSH algorithm registry with the
   following entries:

   Key Exchange Method Name              Reference     Note
   diffie-hellman-group-exchange-sha1    RFC4419       SHOULD NOT
   diffie-hellman-group-exchange-sha256  RFC4419       MAY
   diffie-hellman-group1-sha1            RFC4253       SHOULD NOT
   diffie-hellman-group14-sha1           RFC4253       SHOULD
   ecdh-sha2-nistp256                    RFC5656       MAY
   ecdh-sha2-nistp384                    RFC5656       SHOULD
   ecdh-sha2-nistp521                    RFC5656       SHOULD
   ecdh-sha2-*                           RFC5656       MAY
   ecmqv-sha2                            RFC5656       MAY
   gss-gex-sha1-*                        RFC4462       SHOULD NOT
   gss-group1-sha1-*                     RFC4462       SHOULD NOT
   gss-group14-sha1-*                    RFC4462       MAY
   gss-*                                 RFC4462       MAY
   rsa1024-sha1                          RFC4432       SHOULD NOT
   rsa2048-sha256                        RFC4432       MAY
   diffie-hellman-group14-sha256         This Draft    SHOULD
   diffie-hellman-group15-sha512         This Draft    MAY
   diffie-hellman-group16-sha512         This Draft    SHOULD
   diffie-hellman-group17-sha512         This Draft    MAY
   diffie-hellman-group18-sha512         This Draft    MAY
   curve25519-sha256                     ssh-curves    MUST
   curve448-sha512                       ssh-curves    MAY

                                 Figure 2

   The Note column in the above table is an implementation suggestion/
   recommendation for the listed key exchange method.  It is up to the
   end-user as to what algorithms they choose to be able to negotiate.

   The guidance of his document is that the SHA-1 algorithm hashing
   SHOULD NOT be used.  If it is used, it should only be provided for

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   backwards compatibility, should not be used in new designs, and
   should be phased out of existing key exchanges as quickly as possible
   because of its known weaknesses.  Any key exchange using SHA-1 SHOULD
   NOT be in a default key exchange list if at all possible.  If they
   are needed for backward compatibility, they SHOULD be listed after
   all of the SHA-2 based key exchanges.

   The RFC4253 REQUIRED diffie-hellman-group14-sha1 method SHOULD be
   retained for compatibility with older Secure Shell implementations.
   It is intended that this key exchange be phased out as soon as

5.  Acknowledgements

   Thanks to the following people for review and comments: Denis Bider,
   Peter Gutmann, Damien Miller, Niels Moeller, Matt Johnston, Iwamoto
   Kouichi, Simon Josefsson, Dave Dugal, Daniel Migault.

   Thanks to the following people for code to implement interoperable
   exchanges using some of these groups as found in an this draft:
   Darren Tucker for OpenSSH and Matt Johnston for Dropbear.  And thanks
   to Iwamoto Kouichi for information about RLogin, Tera Term (ttssh)
   and Poderosa implementations also adopting new Diffie-Hellman groups
   based on this draft.

6.  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 3

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

7.  References

7.1.  Normative References

              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4, August 2015,

   [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,

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <>.

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7.2.  Informative References

              "Information Assurance by the National Security Agency",
              "Commercial National Security Algorithm Suite", September
              2016, <

              Adamantiadis, A. and S. Josefsson, "Secure Shell (SSH) Key
              Exchange Method using Curve25519 and Curve448", draft-
              ietf-curdle-ssh-curves-00 (work in progress), March 2016.

              "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,

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,

   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,

   [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,

   [RFC4419]  Friedl, M., Provos, N., and W. Simpson, "Diffie-Hellman
              Group Exchange for the Secure Shell (SSH) Transport Layer
              Protocol", RFC 4419, DOI 10.17487/RFC4419, March 2006,

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   [RFC4432]  Harris, B., "RSA Key Exchange for the Secure Shell (SSH)
              Transport Layer Protocol", RFC 4432, DOI 10.17487/RFC4432,
              March 2006, <>.

   [RFC4462]  Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
              "Generic Security Service Application Program Interface
              (GSS-API) Authentication and Key Exchange for the Secure
              Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May
              2006, <>.

   [RFC5656]  Stebila, D. and J. Green, "Elliptic Curve Algorithm
              Integration in the Secure Shell Transport Layer",
              RFC 5656, DOI 10.17487/RFC5656, December 2009,

   [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,

              Bernstein, and Lange, "SafeCurves: choosing safe curves
              for elliptic-curve cryptography.", February 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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