Increase the Secure Shell Minimum Recommended Diffie-Hellman Modulus Size to 2048 Bits
RFC 8270

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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.

Velvindron & Baushke         Standards Track                    [Page 1]
RFC 8270            Recommended Minimum Modulus Size       December 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
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   the Trust Legal Provisions and are provided without warranty as
   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.

Velvindron & Baushke         Standards Track                    [Page 2]
RFC 8270            Recommended Minimum Modulus Size       December 2017

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.