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.

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
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   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.

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.

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>.

   [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: logan@hackers.mu


   Mark D. Baushke
   Juniper Networks, Inc.

   Email: mdb@juniper.net