Internet Engineering Task Force (IETF)                     L. Velvindron
Request for Comments: 8270                                    Hackers.mu
Updates: 4419                                                 M. Baushke
Category: Standards Track                         Juniper Networks, Inc.
ISSN: 2070-1721                                            December 2017


            Increase the Secure Shell Minimum Recommended
               Diffie-Hellman Modulus Size to 2048 Bits

Abstract

  The Diffie-Hellman (DH) Group Exchange for the Secure Shell (SSH)
  transport-layer protocol specifies that servers and clients should
  support groups with a minimum modulus group size of 1024 bits.
  Recent security research has shown that the minimum value of 1024
  bits is insufficient to protect against state-sponsored actors and
  any organization with enough computing resources.  This RFC updates
  RFC 4419, which allowed for DH moduli less than 2048 bits; now, 2048
  bits is the minimum acceptable group size.

Status of This Memo

  This is an Internet Standards Track document.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Further information on
  Internet Standards is available in Section 2 of RFC 7841.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  https://www.rfc-editor.org/info/rfc8270.

















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

Table of Contents

  1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
  2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
  3.  2048-Bit DH Group . . . . . . . . . . . . . . . . . . . . . .   3
  4.  Interoperability  . . . . . . . . . . . . . . . . . . . . . .   3
  5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
  6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
  7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
    7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
    7.2.  Informative References  . . . . . . . . . . . . . . . . .   4
  Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

  [RFC4419] specifies a recommended minimum DH modulus group size of
  1024 bits.  It also suggests that in all cases, the size of the group
  needs to be at least 1024 bits.  This document updates [RFC4419] so
  that the minimum recommended size is 2048 bits.  This recommendation
  is based on recent research [LOGJAM] on DH group weaknesses.  This
  minimum DH group size may need to be increased to 3072 for forward-
  looking users.

2.  Requirements Language

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
  "OPTIONAL" in this document are to be interpreted as described in
  BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
  capitals, as shown here.






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3.  2048-Bit DH Group

  Recent research [LOGJAM] strongly suggests that DH groups that are
  1024 bits can be broken by state-sponsored actors and any
  organization with enough computing resources.  The authors show how
  they are able to break 768-bit DH groups and extrapolate the attack
  to 1024-bit DH groups.  In their analysis, they show that breaking
  1024 bits can be done with sufficient computing resources.  This
  document provides the following recommendation: SSH servers and SSH
  clients SHOULD support groups with a minimum acceptable group size of
  2048 bits for the "min" value of the SSH_MSG_KEY_DH_GEX_REQUEST
  client message given in [RFC4419].  Further, SSH clients SHOULD be
  able to send a value of 3072 bits for the preferred acceptable group
  size "n" in the SSH_MSG_KEY_DH_GEX_REQUEST message.

  [RFC4419] specifies a recommended minimum size of 1024 bits for k,
  which is the modulus length of the DH group.  It also suggests that,
  in all cases, the size of the group needs be at least 1024 bits.
  This document updates [RFC4419] as described below:

  o  Section 3, paragraph 9:
     Servers and clients SHOULD support groups with a modulus length of
     k bits where 2048 <= k <= 8192.  The recommended minimum values
     for min and max are 2048 and 8192, respectively.  Setting k to
     3072 SHOULD be possible, as the need may arise in the coming
     years.

  o  Section 3, paragraph 11:
     In all cases, the size of the group SHOULD be at least 2048 bits.
     Setting the group size to 3072 SHOULD be possible, as the need may
     arise in the coming years.

4.  Interoperability

  This document keeps the following requirement from [RFC4419]:

     The server should return the smallest group it knows that is
     larger than the size the client requested.  If the server does not
     know a group that is larger than the client request, then it
     SHOULD return the largest group it knows.

  Also, it updates the subsequent sentence as follows:

     In all cases, the size of the returned group SHOULD be at least
     2048 bits.  Setting the group size to 3072 SHOULD be possible, as
     the need may arise in the coming years.





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RFC 8270            Recommended Minimum Modulus Size       December 2017


5.  Security Considerations

  This document discusses security issues of DH groups that are 1024
  bits in size, and formally updates the minimum size of DH groups to
  be 2048 bits.  A hostile or "owned" SSH server implementation could
  potentially use backdoored DH primes using the methods described in
  [Backdoor-DH] to provide the g and p values to be used.  Or, it could
  just send the calculated secret through a covert channel of some sort
  to a passive listener.

  A malicious client could cause a Denial of Service by intentionally
  making multiple connections that are less than 2048 bits in size.
  Therefore, operating systems SHOULD NOT log DH groups that are less
  than 2048 bits in size, as it would create an additional attack
  surface.

6.  IANA Considerations

  This document does not require any IANA actions.

7.  References

7.1.  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,
             <https://www.rfc-editor.org/info/rfc2119>.

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

  [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

  [Backdoor-DH]
             Wong, D., "How to Backdoor Diffie-Hellman", Cryptology
             ePrint Archive Report 2016/644, June 2016,
             <http://eprint.iacr.org/2016/644.pdf>.







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RFC 8270            Recommended Minimum Modulus Size       December 2017


  [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, DOI 10.1145/2810103.2813707, 2015,
             <https://weakdh.org/imperfect-forward-secrecy-ccs15.pdf>.

Authors' Addresses

  Loganaden Velvindron
  Hackers.mu
  88, Avenue De Plevitz
  Roches Brunes
  Mauritius

  Phone: +230 59762817
  Email: [email protected]


  Mark D. Baushke
  Juniper Networks, Inc.

  Email: [email protected]


























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