[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-baushke-ssh-dh-group-sha2) 00 01 02 03 04 05 06 07 08 09

Internet Engineering Task Force                               M. Baushke
Internet-Draft                                    Juniper Networks, Inc.
Updates: 4250 (if approved)                                July 30, 2017
Intended status: Standards Track
Expires: January 31, 2018


 Key Exchange (KEX) Method Updates and Recommendations for Secure Shell
                                 (SSH)
                   draft-ietf-curdle-ssh-kex-sha2-09

Abstract

   This document is intended to update the recommended set of key
   exchange methods for use in the Secure Shell (SSH) protocol to meet
   evolving needs for stronger security.  This document updates RFC
   4250.

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 http://datatracker.ietf.org/drafts/current/.

   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 January 31, 2018.

Copyright Notice

   Copyright (c) 2017 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   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




Baushke                 Expires January 31, 2018                [Page 1]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Overview and Rationale  . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Key Exchange Methods  . . . . . . . . . . . . . . . . . . . .   3
     3.1.  curve25519-sha256 . . . . . . . . . . . . . . . . . . . .   4
     3.2.  curve448-sha512 . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  diffie-hellman-group-exchange-sha1  . . . . . . . . . . .   4
     3.4.  diffie-hellman-group-exchange-sha256  . . . . . . . . . .   4
     3.5.  diffie-hellman-group1-sha1  . . . . . . . . . . . . . . .   4
     3.6.  diffie-hellman-group14-sha1 . . . . . . . . . . . . . . .   4
     3.7.  diffie-hellman-group14-sha256 . . . . . . . . . . . . . .   5
     3.8.  diffie-hellman-group15-sha512 . . . . . . . . . . . . . .   5
     3.9.  diffie-hellman-group16-sha512 . . . . . . . . . . . . . .   5
     3.10. diffie-hellman-group17-sha512 . . . . . . . . . . . . . .   5
     3.11. diffie-hellman-group18-sha512 . . . . . . . . . . . . . .   5
     3.12. ecdh-sha2-nistp256  . . . . . . . . . . . . . . . . . . .   5
     3.13. ecdh-sha2-nistp384  . . . . . . . . . . . . . . . . . . .   6
     3.14. ecdh-sha2-nistp521  . . . . . . . . . . . . . . . . . . .   6
     3.15. gss-gex-sha1-*  . . . . . . . . . . . . . . . . . . . . .   6
     3.16. gss-group1-sha1-* . . . . . . . . . . . . . . . . . . . .   6
     3.17. gss-group14-sha1-*  . . . . . . . . . . . . . . . . . . .   6
     3.18. gss-group14-sha256-*  . . . . . . . . . . . . . . . . . .   7
     3.19. gss-group15-sha512-*  . . . . . . . . . . . . . . . . . .   7
     3.20. gss-group16-sha512-*  . . . . . . . . . . . . . . . . . .   7
     3.21. gss-group17-sha512-*  . . . . . . . . . . . . . . . . . .   7
     3.22. gss-group18-sha512-*  . . . . . . . . . . . . . . . . . .   7
     3.23. gss-nistp256-sha256-* . . . . . . . . . . . . . . . . . .   8
     3.24. gss-nistp384-sha384-* . . . . . . . . . . . . . . . . . .   8
     3.25. gss-nistp521-sha512-* . . . . . . . . . . . . . . . . . .   8
     3.26. gss-curve25519-sha256-* . . . . . . . . . . . . . . . . .   8
     3.27. gss-curve448-sha512-* . . . . . . . . . . . . . . . . . .   8
     3.28. rsa1024-sha1  . . . . . . . . . . . . . . . . . . . . . .   8
     3.29. rsa2048-sha256  . . . . . . . . . . . . . . . . . . . . .   8
   4.  Selecting an appropriate hashing algorithm  . . . . . . . . .   8
   5.  Summary Guidance for Key Exchange Method Names  . . . . . . .   9
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14





Baushke                 Expires January 31, 2018                [Page 2]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


1.  Overview and Rationale

   Secure Shell (SSH) is a common protocol for secure communication on
   the Internet.  In [RFC4253], SSH originally defined two Key Exchange
   Method Names that MUST be implemented.  Over time, what was once
   considered secure, is no longer considered secure.  The purpose of
   this RFC is to recommend that some published key exchanges be
   deprecated as well as recommending some that SHOULD and one that MUST
   be adopted.  This document updates [RFC4250].

   This document adds recommendations for adoption of Key Exchange
   Methods which MUST, SHOULD, MAY, SHOULD NOT, and MUST NOT be
   implemented.  New key exchange methods will use the SHA-2 family of
   hashes and are drawn from these ssh-curves from
   [I-D.ietf-curdle-ssh-curves] and new-modp from the
   [I-D.ietf-curdle-ssh-modp-dh-sha2] and gss-keyex
   [I-D.ietf-curdle-gss-keyex-sha2].

   [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].

3.  Key Exchange Methods

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

   This RFC also collects Key Exchange Method Names in various existing
   RFCs [RFC4253], [RFC4419], [RFC4432], [RFC4462], [RFC5656],
   [I-D.ietf-curdle-ssh-modp-dh-sha2], [I-D.ietf-curdle-gss-keyex-sha2],
   and [I-D.ietf-curdle-ssh-curves] and provides a suggested suitability
   for implementation of MUST, SHOULD, SHOULD NOT, and MUST NOT.  Any
   method not explicitly listed, MAY be implemented.

   This document is intended to provide guidance as to what Key Exchange
   Algorithms are to be considered for new or updated SSH
   implementations.  This document will be superseded when one or more
   of the listed algorithms are considered too weak to continue to use
   securely, in which case they will likely be downgraded to SHOULD NOT
   or MUST NOT.  Or, when newer methods have been analyzed and found to
   be secure with wide enough adoption to upgrade their recommendation
   from MAY to SHOULD or MUST.




Baushke                 Expires January 31, 2018                [Page 3]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


3.1.  curve25519-sha256

   The Curve25519 provides strong security and is efficient on a wide
   range of architectures with properties that allow better
   implementation properties compared to traditional elliptic curves.
   The use of SHA2-256 for integrity is a reasonable one for this
   method.  This Key Exchange Method has multiple implementations and
   SHOULD be implemented in any SSH interested in using elliptic curve
   based key exchanges.

3.2.  curve448-sha512

   The Curve448 provides very strong security.  It is probably stronger
   and more work than is currently needed.  This method MAY be
   implemented.

3.3.  diffie-hellman-group-exchange-sha1

   This set of ephemerally generated key exchange groups uses SHA-1 as
   defined in [RFC4419].  However, SHA-1 has security concerns provided
   in [RFC6194].  It is recommended that these key exchange groups NOT
   be used.  This key exchange SHOULD NOT be used.

3.4.  diffie-hellman-group-exchange-sha256

   This set of ephemerally generated key exchange groups uses SHA2-256
   as defined in [RFC4419].  [I-D.ietf-curdle-ssh-dh-group-exchange]
   mandates implementations avoid any MODP group with less than 2048
   bits.  This key exchange MAY be used.

3.5.  diffie-hellman-group1-sha1

   This method uses [RFC7296] Oakley Group 2 (a 1024-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 (see [LOGJAM]
   and [NIST-SP-800-131Ar1]), this method is considered insecure.  This
   method is being moved from MUST to SHOULD NOT instead of MUST NOT
   only to allow a transition time to get off of it.  There are many old
   implementations out there that may still need to use this key
   exchange, it should be removed from server implementations as quickly
   as possible.

3.6.  diffie-hellman-group14-sha1

   This method uses [RFC3526] group14 (a 2048-bit MODP group) which is
   still a reasonable size.  This key exchange group uses SHA-1 which
   has security concerns [RFC6194].  However, this group is still strong
   enough and is widely deployed.  This method is being moved from MUST



Baushke                 Expires January 31, 2018                [Page 4]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


   to SHOULD to aid in transition to stronger SHA-2 based hashes.  This
   method will transition to SHOULD NOT when SHA-2 alternatives are more
   generally available.

3.7.  diffie-hellman-group14-sha256

   This key exchange uses the group14 (a 2048-bit MODP group) along with
   a SHA-2 (SHA2-256) hash.  This represents the smallest Finite Field
   Cryptography (FFC) Diffie-Hellman (DH) key exchange method considered
   to be secure.  It is a reasonably simple transition to move from
   SHA-1 to SHA-2.  This method MUST be implemented.

3.8.  diffie-hellman-group15-sha512

   Note: The use of this 3072-bit MODP group would be equally justified
   to use SHA2-384 as the hash rather than SHA2-512.  However, some
   small implementations would rather only worry about two rather than
   three new hashing functions.  This group does not really provide much
   additional head room over the 2048-bit group14 FFC DH and the
   predominate open source implementations are not adopting it.  This
   method MAY be implemented.

3.9.  diffie-hellman-group16-sha512

   The use of FFC DH is well understood and trusted.  Adding larger
   modulus sizes and protecting with SHA2-512 should give enough head
   room to be ready for the next scare that someone has pre-computed it.
   This modulus (4096-bit) is larger than that required by [CNSA-SUITE]
   and should be sufficient to inter-operate with more paranoid nation-
   states.  This method SHOULD be implemented.

3.10.  diffie-hellman-group17-sha512

   The use of this 6144-bit MODP group is going to be slower than what
   may be desirable.  It is provided to help those who wish to avoid
   using ECC algorithms.  This method MAY be implemented.

3.11.  diffie-hellman-group18-sha512

   The use of this 8192-bit MODP group is going to be slower than what
   may be desirable.  It is provided to help those who wish to avoid
   using ECC algorithms.  This method MAY be implemented.

3.12.  ecdh-sha2-nistp256

   Elliptic Curve Diffie-Hellman (ECDH) are often implemented because
   they are smaller and faster than using large FFC primes with
   traditional Diffie-Hellman (DH).  However, given [CNSA-SUITE] and



Baushke                 Expires January 31, 2018                [Page 5]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


   [safe-curves], this curve may not be as useful and strong as desired
   for handling TOP SECRET information for some applications.  The SSH
   development community is divided on this and many implementations do
   exist.  If traditional ECDH key exchange methods are implemented,
   then this method SHOULD be implemented.  It is advisable to match the
   ECDSA and ECDH algorithms to use the same family of curves.

3.13.  ecdh-sha2-nistp384

   This ECDH method should be implemented because it is smaller and
   faster than using large FFC primes with traditional Diffie-Hellman
   (DH).  Given [CNSA-SUITE], it is considered good enough for TOP
   SECRET.  If traditional ECDH key exchange methods are implemented,
   then this method SHOULD be implemented.

3.14.  ecdh-sha2-nistp521

   This ECDH method may be implemented because it is smaller and faster
   than using large FFC primes with traditional Diffie-Hellman (DH).  It
   is not listed in [CNSA-SUITE], so it is not currently appropriate for
   TOP SECRET.  This method MAY be implemented.

3.15.  gss-gex-sha1-*

   This set of ephemerally generated key exchange groups uses SHA-1
   which has security concerns [RFC6194].  It is recommended that these
   key exchange groups NOT be used.  This key exchange SHOULD NOT be
   used.  It is intended that it move to MUST NOT as soon as the
   majority of server implementations no longer offer it.  It should be
   removed from server implementations as quickly as possible.

3.16.  gss-group1-sha1-*

   This method suffers from the same problems of diffie-hellman-
   group1-sha1.  It uses [RFC7296] Oakley Group 2 (a 1024-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 (see
   [LOGJAM] and [NIST-SP-800-131Ar1]), this method is considered
   insecure.  This method SHOULD NOT be implemented.  It is intended
   that it move to MUST NOT as soon as the majority of server
   implementations no longer offer it.  It should be removed from server
   implementations as quickly as possible.

3.17.  gss-group14-sha1-*

   This generated key exchange groups uses SHA-1 which has security
   concerns [RFC6194].  If GSS-API key exchange methods are being used,
   then this one SHOULD be implemented until such time as SHA-2 variants



Baushke                 Expires January 31, 2018                [Page 6]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


   may be implemented and deployed.  This method will transition to
   SHOULD NOT when SHA-2 alternatives are more generally available.  No
   other standard indicated that this method was anything other than
   optional even though it was implemented in all GSS-API systems.  This
   method MAY be implemented.

3.18.  gss-group14-sha256-*

   This key exchange uses the group14 (a 2048-bit MODP group) along with
   a SHA-2 (SHA2-256) hash.  This represents the smallest Finite Field
   Cryptography (FFC) Diffie-Hellman (DH) key exchange method considered
   to be secure.  It is a reasonably simple transition to move from
   SHA-1 to SHA-2.  If the GSS-API is to be used, then this method
   SHOULD be implemented.

3.19.  gss-group15-sha512-*

   The use of this 3072-bit MODP group does not really provide much
   additional head room over the 2048-bit group14 FFC DH.  If the GSS-
   API is to be used, then this method MAY be implemented.

3.20.  gss-group16-sha512-*

   The use of FFC DH is well understood and trusted.  Adding larger
   modulus sizes and protecting with SHA2-512 should give enough head
   room to be ready for the next scare that someone has pre-computed.
   This modulus (4096-bit) is larger than that required by [CNSA-SUITE]
   and should be sufficient to inter-operate with more paranoid nation-
   states.  If the GSS-API is to be used, then this method SHOULD be
   implemented.

3.21.  gss-group17-sha512-*

   The use of this 6144-bit MODP group is going to be slower than what
   may be desirable.  It is provided to help those who wish to avoid
   using ECC algorithms.  If the GSS-API is to be used, then this method
   MAY be implemented.

3.22.  gss-group18-sha512-*

   The use of this 8192-bit MODP group is going to be slower than what
   may be desirable.  It is provided to help those who prefer to avoid
   using ECC algorithms.  If the GSS-API is to be used, then this method
   MAY be implemented.







Baushke                 Expires January 31, 2018                [Page 7]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


3.23.  gss-nistp256-sha256-*

   If the GSS-API is to be used with ECC algorithms, then this method
   SHOULD be implemented.

3.24.  gss-nistp384-sha384-*

   If the GSS-API is to be used with ECC algorithms, then this method
   SHOULD be implemented to permit TOP SECRET information to be
   communicated.

3.25.  gss-nistp521-sha512-*

   If the GSS-API is to be used with ECC algorithms, then this method
   MAY be implemented.

3.26.  gss-curve25519-sha256-*

   If the GSS-API is to be used with ECC algorithms, then this method
   SHOULD be implemented.

3.27.  gss-curve448-sha512-*

   If the GSS-API is to be used with ECC algorithms, then this method
   MAY be implemented.

3.28.  rsa1024-sha1

   The security of RSA 1024-bit modulus keys is not good enough any
   longer.  A key size should be 2048-bits.  This generated key exchange
   groups uses SHA-1 which has security concerns [RFC6194].  This method
   MUST NOT be implemented.

3.29.  rsa2048-sha256

   An RSA 2048-bit modulus key with a SHA2-256 hash.  This method MAY be
   implemented.

4.  Selecting an appropriate hashing algorithm

   As may be seen from the above, the Key Exchange Methods area all
   using either SHA256 or SHA512 with the exception of the ecdh-
   sha2-nistp384 which uses SHA384.

   The cited CNSA Suite specifies the use of SHA384 and says that SHA256
   is no longer good enough for TOP SECRET.  Nothing is said about the
   use of SHA512.  It may be that the internal state of 1024 bits in
   both SHA384 and SHA512 makes the SHA384 more secure because it does



Baushke                 Expires January 31, 2018                [Page 8]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


   not leak an additional 128 bits of state.  Of course, use of SHA384
   also reduces the security strength to 192 bits instead of being 256
   bits or more.  This seems to contradict the desire to double the
   symmetric key strength in order to try to be safe from Post Quantum
   Computing (PQC) attacks given a session key derived from the key
   exchange will be limited to the security strength of the hash being
   used.

   The move away from SHA256 to SHA512 for the newer key exchange
   methods is more to try to slow Grover's algorithm (a PQC attack)
   slightly.  It is also the case that SHA2-512 may, in many modern
   CPUs, be implemented more efficiently using 64-bit arithmetic than
   SHA256 which is faster on 32-bit CPUs.  The selection of SHA384 vs
   SHA512 is more about reducing the number of code point alternatives
   to negotiate.  There seemed to be consensus in favor of SHA2-512 over
   SHA2-384 for key exchanges.

5.  Summary Guidance for Key Exchange Method Names

   The Implement column is the current recommendations of this RFC.  Key
   Exchange Method Names are listed alphabetically.

         Key Exchange Method Name           Reference  Implement
         ---------------------------------- ---------- ----------
         curve25519-sha256                  ssh-curves SHOULD
         diffie-hellman-group-exchange-sha1 RFC4419    SHOULD NOT
         diffie-hellman-group1-sha1         RFC4253    SHOULD NOT
         diffie-hellman-group14-sha1        RFC4253    SHOULD
         diffie-hellman-group14-sha256      new-modp   MUST
         diffie-hellman-group16-sha512      new-modp   SHOULD
         ecdh-sha2-nistp256                 RFC5656    SHOULD
         ecdh-sha2-nistp384                 RFC5656    SHOULD
         gss-gex-sha1-*                     RFC4462    SHOULD NOT
         gss-group1-sha1-*                  RFC4462    SHOULD NOT
         gss-group14-sha256-*               gss-keyex  SHOULD
         gss-group16-sha512-*               gss-keyex  SHOULD
         gss-nistp256-sha256-*              gss-keyex  SHOULD
         gss-nistp384-sha384-*              gss-keyex  SHOULD
         gss-curve25519-sha256-*            gss-keyex  SHOULD
         rsa1024-sha1                       RFC4432    MUST NOT

   The full set of official [IANA-KEX] key algorithm method names not
   otherwise mentioned in this document MAY be implemented.

   The guidance of this document is that the SHA-1 algorithm hashing
   SHOULD NOT be used.  If it is used in implementations, it should only
   be provided for backwards compatibility, should not be used in new
   designs, and should be phased out of existing key exchanges as



Baushke                 Expires January 31, 2018                [Page 9]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


   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] MUST diffie-hellman-group14-sha1 method SHOULD be
   retained for compatibility with older Secure Shell implementations.
   It is intended that this key exchange method be phased out as soon as
   possible.  It SHOULD be listed after all possible SHA-2 based key
   exchanges.

   It is believed that all current SSH implementations should be able to
   achieve an implementation of the "diffie-hellman-group14-sha256"
   method.  To that end, this is one method that MUST be implemented.

   [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>]

6.  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, Anna Johnston,
   and Tero Kivinen.

   Thanks to the following people for code to implement inter-operable
   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.

7.  Security Considerations

   This SSH protocol provides a secure encrypted channel over an
   insecure network.  It performs server host authentication, key
   exchange, encryption, and integrity protection.  It also derives a
   unique session ID that may be used by higher-level protocols.

   Full security considerations for this protocol are provided in
   [RFC4251]

   It is desirable to deprecate or remove key exchange method name that
   are considered weak.  A key exchange method may be weak because too
   few bits are used, or the hashing algorithm is considered too weak.





Baushke                 Expires January 31, 2018               [Page 10]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


   The diffie-hellman-group1-sha1 is being moved from MUST to MUST NOT.
   This method used [RFC7296] Oakley Group 2 (a 1024-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], this method is no longer considered secure.

   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.  If your systems need to be concerned with TOP
   SECRET information, then the guidance for supporting lesser security
   strength key exchanges may be omitted for your implementations.

   The MODP group14 is already required for 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 [RFC3526] is
   explicitly mentioned as the minimum modulus to protect TOP SECRET
   communications.

   It has been observed in [safe-curves] that the NIST 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 mandatory to 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.

   In a Post-Quantum Computing (PQC) world, it will be desirable to use
   larger cyclic subgroups.  To do this using Elliptic Curve
   Cryptography will require much larger prime base fields, greatly
   reducing their efficiency.  Finite Field based Cryptography already
   requires large enough base fields to accommodate larger cyclic
   subgroups.  Until such time as a PQC method of key exchange is
   developed and adopted, it may be desirable to generate new and larger
   DH groups to avoid pre-calculation attacks that are provably not
   backdoored.






Baushke                 Expires January 31, 2018               [Page 11]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


8.  IANA Considerations

   IANA is requested to annotate entries in [IANA-KEX] which MUST NOT be
   implemented as being deprecated by this document.

9.  References

9.1.  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>.

   [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>.

9.2.  Informative References

   [CNSA-SUITE]
              "Information Assurance by the National Security Agency",
              "Commercial National Security Algorithm Suite", September
              2016, <https://www.iad.gov/iad/programs/iad-initiatives/
              cnsa-suite.cfm>.

   [I-D.ietf-curdle-gss-keyex-sha2]
              Sorce, S. and H. Kario, "GSS-API Key Exchange with SHA2",
              draft-ietf-curdle-gss-keyex-sha2-02 (work in progress),
              June 2017.

   [I-D.ietf-curdle-ssh-curves]
              Adamantiadis, A., Josefsson, S., and M. Baushke, "Secure
              Shell (SSH) Key Exchange Method using Curve25519 and
              Curve448", draft-ietf-curdle-ssh-curves-05 (work in
              progress), May 2017.





Baushke                 Expires January 31, 2018               [Page 12]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


   [I-D.ietf-curdle-ssh-dh-group-exchange]
              Velvindron, L. and M. Baushke, "Increase SSH minimum
              recommended DH modulus size to 2048 bits", draft-ietf-
              curdle-ssh-dh-group-exchange-05 (work in progress), July
              2017.

   [I-D.ietf-curdle-ssh-modp-dh-sha2]
              Baushke, M., "More Modular Exponential (MODP) Diffie-
              Hellman (DH) Key Exchange (KEX) Groups for Secure Shell
              (SSH)", draft-ietf-curdle-ssh-modp-dh-sha2-07 (work in
              progress), June 2017.

   [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>.

   [LOGJAM]   Adrian, D., Bhargavan, K., Durumeric, Z., Gaudry, P.,
              Green, M., Halderman, J., Heninger, N., Springall, D.,
              Thome, E., Valenta, L., VanderSloot, B., Wustrow, E.,
              Zanella-Beguelin, S., and P. Zimmermann, "Imperfect
              Forward Secrecy: How Diffie-Hellman Fails in Practice",
              ACM Conference on Computer and Communications Security
              (CCS) 2015, 2015, <https://weakdh.org/imperfect-forward-
              secrecy-ccs15.pdf>.

   [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>.

   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
              <http://www.rfc-editor.org/info/rfc3174>.






Baushke                 Expires January 31, 2018               [Page 13]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
              January 2006, <http://www.rfc-editor.org/info/rfc4251>.

   [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,
              <http://www.rfc-editor.org/info/rfc4419>.

   [RFC4432]  Harris, B., "RSA Key Exchange for the Secure Shell (SSH)
              Transport Layer Protocol", RFC 4432, DOI 10.17487/RFC4432,
              March 2006, <http://www.rfc-editor.org/info/rfc4432>.

   [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, <http://www.rfc-editor.org/info/rfc4462>.

   [RFC5656]  Stebila, D. and J. Green, "Elliptic Curve Algorithm
              Integration in the Secure Shell Transport Layer",
              RFC 5656, DOI 10.17487/RFC5656, December 2009,
              <http://www.rfc-editor.org/info/rfc5656>.

   [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>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <http://www.rfc-editor.org/info/rfc7296>.

   [safe-curves]
              Bernstein and Lange, "SafeCurves: choosing safe curves for
              elliptic-curve cryptography.", February 2016,
              <https://safecurves.cr.yp.to/>.

Author's Address











Baushke                 Expires January 31, 2018               [Page 14]


Internet-Draft KEX Method Updates/Recommendations for SSH      July 2017


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

   Email: mdb@juniper.net
   URI:   http://www.juniper.net/











































Baushke                 Expires January 31, 2018               [Page 15]


Html markup produced by rfcmarkup 1.123, available from https://tools.ietf.org/tools/rfcmarkup/