Internet Engineering Task Force                            L. Velvindron
Updates: 4419 (if approved)                                   M. Baushke
Intended status: Standards Track                  Juniper Networks, Inc.
Expires: November 17, 2017                                  May 16, 2017

         Increase minimum recommended modulus size to 2048 bits


   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

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   This Internet-Draft will expire on November 17, 2017.

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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.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 updates 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.  This
   document also updates Section 3 Paragraph 11: In all cases, ths size
   of the group SHOULD be at least 2048 bits.

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3.  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.

4.  IANA Considerations

   This document contains no considerations for IANA.

5.  References

5.1.  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,

5.2.  Informative References

   [LOGJAM]   Adrian, D., Bhargavan, K., Durumeric, Z., Gaudry, P.,
              Green, M., Halderman, J., Heninger, N., Springall, D.,
              ThomA&#131;A(C), E., Valenta, L., VanderSloot, B.,
              Wustrow, E., Zanella-BA&#131;A(C)guelin, S., and P.
              Zimmermann, "Imperfect Forward Secrecy: How Diffie-Hellman
              Fails in Practice", ACM Conference on Computer and
              Communications Security (CCS) 2015, 2015,

   [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,

Authors' Addresses

   Loganaden Velvindron
   88, Avenue De Plevitz
   Roches Brunes

   Phone: +230 59762817

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   Mark D. Baushke
   Juniper Networks, Inc.


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