Internet Engineering Task Force (IETF)                     L. VelvindronRequest for Comments: 8270                                    Hackers.muUpdates: 4419                                                 M. BaushkeCategory: Standards Track                         Juniper Networks, Inc.ISSN: 2070-1721                                            December 2017             Increase the Secure Shell Minimum Recommended                Diffie-Hellman Modulus Size to 2048 BitsAbstract   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 2017Copyright 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   (https://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   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  . . . . . . . . . . . . . . . . . . . . . . .   51.  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 20173.  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.   [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:   o  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 to      3072 SHOULD be possible, as the need may arise in the coming      years.   o  Section 3, paragraph 11:      In all cases, the size of the group SHOULD be at least 2048 bits.      Setting the group size to 3072 SHOULD be possible, as the need may      arise in the coming years.4.  Interoperability   This document keeps the following requirement 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.   Also, it updates the subsequent sentence as follows:      In all cases, the size of the returned group SHOULD be at least      2048 bits.  Setting the group size to 3072 SHOULD be possible, as      the need may arise in the coming years.Velvindron & Baushke         Standards Track                    [Page 3]RFC 8270            Recommended Minimum Modulus Size       December 20175.  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" SSH server implementation could   potentially use backdoored DH primes using the methods described in   [Backdoor-DH] to provide the g and p values to be used.  Or, 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 that are less than 2048 bits in size.   Therefore, operating systems SHOULD NOT log DH groups that are less   than 2048 bits in size, as it would create an additional attack   surface.6.  IANA Considerations   This document does not require any IANA actions.7.  References7.1.  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>.   [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>.Velvindron & Baushke         Standards Track                    [Page 4]RFC 8270            Recommended Minimum Modulus Size       December 2017   [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   Mauritius   Phone: +230 59762817   Email: logan@hackers.mu   Mark D. Baushke   Juniper Networks, Inc.   Email: mdb@juniper.netVelvindron & Baushke         Standards Track                    [Page 5]