Internet Engineering Task Force (IETF)                     L. Velvindron
Internet-Draft
Request for Comments: 8270                                    Hackers.mu
Updates: 4419 (if approved)                                                 M. Baushke
Intended status:
Category: Standards Track                         Juniper Networks, Inc.
Expires: March 25, 2018                               September 21,
ISSN: 2070-1721                                            November 2017

             Increase SSH minimum recommended DH modulus size the Secure Shell Minimum Recommended
                Diffie-Hellman Modulus Size to 2048 bits
               draft-ietf-curdle-ssh-dh-group-exchange-06 Bits

Abstract

   The Diffie-Hellman (DH) Group Exchange for the Secure Shell (SSH)
   Transport layer Protocol
   transport-layer protocol specifies that servers and clients should
   support groups with a minimum modulus length group size of k bits, where the recommended
   minimum value is 1024 bits.
   Recent security research has shown that
   a the minimum value of 1024
   bits is insufficient to protect against state-sponsored
   actors, actors and possibly
   any organization with enough computing resources.  As such, this document formally updates the specification
   such that the minimum recommended value for k is 2048 bits and the
   group size is 2048 bits at minimum.  This RFC updates RFC4419
   RFC 4419, which allowed for DH moduli less than 2048 bits. bits; now, 2048
   bits is the minimum acceptable group size.

Status of This Memo

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   This Internet-Draft will expire on March 25, 2018.
   https://www.rfc-editor.org/info/rfc8270.

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   document authors.  All rights reserved.

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1.  Introduction

   [RFC4419] specifies a recommended minimum DH modulus group size of
   1024 bits for k,
   which is the modulus length of the DH Group. 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
   be is 2048 bits.  This recommendation
   is based on recent research [LOGJAM] on DH Group group weaknesses.

1.1.  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 RFC 2119 [RFC2119].

2.  2048 bits
   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 actors, state-sponsored actors and possibly any
   organization with enough computing resources.  The authors show how
   they are able to break 768 bits 768-bit DH group groups and extrapolate the attack
   to 1024 bits 1024-bit DH groups.  In their analysis, they show that breaking
   1024 bits can be done with enough sufficient computing resources.  This
   document provides the following recommendation: SSH Servers servers and SSH
   clients SHOULD support groups with a modulus length minimum aceptable group size of k bits where
   2048 <= k
   <= 8192, where it is possible to set k 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 should bits for the need arise preferred acceptable group
   size "n" in the coming years. 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. group.  It also suggests that 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  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 SHOULD be able to be set to
      3072 by an
      implementation should SHOULD be possible, as the need may arise in the coming
      years.

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

3.

4.  Interoperability

   This document keeps the [RFC4419] following requirement "The 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." and knows.

   Also, it updates the subsequent sentence that follows to read: "In as follows:

      In all cases, the size of the returned group SHOULD be at least
      2048 bits."

4. 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" Secure Shell SSH server implementation could
   potentially use Backdoored Diffie-Hellman backdoored DH primes using the methods described in
   [Backdoor-DH] to provide the g,p g and p values to be used.  Or, they 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 which that are less than 2048 bits in size on purpose. size.
   Therefore,
   Operating Systems operating systems SHOULD NOT log DH groups that are less
   than 2048 bits in size, as it would create an additional attack
   surface.

5.

6.  IANA Considerations

   This document contains no considerations for IANA.

6. does not require any IANA actions.

7.  References

6.1.

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>.

6.2.

   [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
   MU
   Mauritius

   Phone: +230 59762817
   Email: logan@hackers.mu

   Mark D. Baushke
   Juniper Networks, Inc.

   Email: mdb@juniper.net