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Internet Engineering Task Force                            L. Velvindron
Internet-Draft                                                Hackers.mu
Updates: 4419 (if approved)                                   M. Baushke
Intended status: Standards Track                  Juniper Networks, Inc.
Expires: December 23, 2017                                 June 21, 2017


     Increase SSH minimum recommended DH modulus size to 2048 bits
               draft-ietf-curdle-ssh-dh-group-exchange-04

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 modulus length of k bits, where the recommended
   minumum value is 1024 bits.  Recent security research has shown that
   a minimum value of 1024 bits is insufficient against state-sponsored
   actors.  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 which
   allowed for DH moduli less than 2048 bits.

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
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   This Internet-Draft will expire on December 23, 2017.

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



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   carefully, as they describe your rights and restrictions with respect
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   This document may contain material from IETF Documents or IETF
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   than English.

1.  Introduction

   [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] so that the minimum recommended size
   be 2048 bits.  This recommendation is based on recent research
   [LOGJAM] on DH Group weaknesses.

1.1.  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].

2.  2048 bits DH Group

   Recent research [LOGJAM] strongly suggests that DH groups that are
   1024 bits can be broken by state actors, and possibly an organization
   with enough computing resources.  The authors show how they are able
   to break 768 bits DH group and extrapolate the attack to 1024 bits DH
   groups.  In their analysis, they show that breaking 1024 bits can be
   done with enough computing resources.  This document provides the
   following recommendation: SSH Servers and SSH clients SHOULD support
   groups with a modulus length of k bits where 2048 <= k <= 8192.

   [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: section 3 Paragraph 9:



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   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.  This document also updates
   [RFC4419] Section 3 Paragraph 11 as follows: In all cases, the size
   of the group SHOULD be at least 2048 bits.

3.  Interoperability

   This document keeps the [RFC4419] requirement "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 updates the sentence that follows to read: "In all cases,
   the size of the returned group SHOULD be at least 2048 bits."

4.  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 server
   implementation could potentially use Backdoored Diffie-Hellman primes
   using the methods described in [Backdoor-DH] to provide the g,p
   values to be used.  Or, they could just send the calculated secret
   through a covert channel of some sort to a passive listener.

5.  IANA Considerations

   This document contains no considerations for IANA.

6.  References

6.1.  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>.

6.2.  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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   [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>.

   [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>.

Authors' Addresses

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

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


   Mark D. Baushke
   Juniper Networks, Inc.

   Email: mdb@juniper.net





















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