Increase SSH minimum recommended DH modulus size to 2048 bits

Document Type Active Internet-Draft (curdle WG)
Last updated 2017-07-17
Stream IETF
Intended RFC status Proposed Standard
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Internet Engineering Task Force                            L. Velvindron
Updates: 4419 (if approved)                                   M. Baushke
Intended status: Standards Track                  Juniper Networks, Inc.
Expires: January 18, 2018                                  July 17, 2017

     Increase SSH minimum recommended DH 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
   minimum value is 1024 bits.  Recent security research has shown that
   a minimum value of 1024 bits is insufficient against state-sponsored
   actors, and possibly an 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 which allowed for
   DH moduli less than 2048 bits.

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Velvindron & Baushke    Expires January 18, 2018                [Page 1]
Internet-Draft      Recommended minimum modulus size           July 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.  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",
   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.
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