Increase SSH minimum recommended DH modulus size to 2048 bits
draft-ietf-curdle-ssh-dh-group-exchange-03
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (curdle WG) | |
|---|---|---|---|
| Authors | Loganaden Velvindron , Mark D. Baushke | ||
| Last updated | 2017-06-21 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
| Reviews |
OPSDIR Last Call review
(of
-05)
Has Nits
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||
| Stream | WG state | In WG Last Call | |
| Document shepherd | Daniel Migault | ||
| Shepherd write-up | Show Last changed 2017-06-12 | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | Daniel Migault <daniel.migault@ericsson.com> |
draft-ietf-curdle-ssh-dh-group-exchange-03
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 22, 2017 June 20, 2017
Increase SSH minimum recommended DH modulus size to 2048 bits
draft-ietf-curdle-ssh-dh-group-exchange-03
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.
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This Internet-Draft will expire on December 22, 2017.
Copyright Notice
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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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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 [RFC 4419] Section 3 Paragraph 11 as follows: In all cases,
the size of the group SHOULD be at least 2048 bits.
3. Interoperability
As state in [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.
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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