SSH 2048-bit DH Minimum: Increase the Secure Shell Minimum Recommended Diffie-Hellman Modulus Size to 2048 Bits
Raises SSH Diffie-Hellman group size recommendations to at least 2048 bits.
Velvindron, L. and M. Baushke, "Increase the Secure Shell Minimum Recommended Diffie-Hellman Modulus Size to 2048 Bits", RFC 8270, DOI 10.17487/RFC8270, December 2017, <https://www.rfc-editor.org/info/rfc8270>.
@misc{rfc8270,
series = {Request for Comments},
number = 8270,
howpublished = {RFC 8270},
publisher = {RFC Editor},
doi = {10.17487/RFC8270},
url = {https://www.rfc-editor.org/info/rfc8270},
author = {L. Velvindron and M. Baushke},
title = {{Increase the Secure Shell Minimum Recommended Diffie-Hellman Modulus Size to 2048 Bits}},
pagetotal = 5,
year = 2017,
month = dec,
} RFC text
Contents
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.
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
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
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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 . . . . . . . . . . . . . . . . . . . . . . . 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.
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.
[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.
5. 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. References
7.1. 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>.
[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.net
ISO Plain Language
On this page
§1Why the Minimum Increased
RFC 4419 recommended a minimum Diffie-Hellman modulus size of 1024 bits for SSH group exchange. The research cited by RFC 8270 showed how 768-bit groups could be broken and extrapolated that attack to 1024-bit groups, given sufficient computing resources. This memo updates the recommended minimum to 2048 bits.
The earlier SSH recommendation allowed 1024-bit groups. The cited research led this memo to recommend larger groups.
§3Group Size Recommendations
SSH servers and clients SHOULD support groups with a modulus length from 2048 through 8192 bits. The recommended minimum and maximum values are 2048 and 8192, and setting the group size to 3072 SHOULD be possible. In all cases, the group SHOULD be at least 2048 bits.
Clients SHOULD be able to send 3072 as the preferred acceptable size in the group-exchange request. The RFC says a minimum of 3072 bits may be appropriate for forward-looking users.
Servers and clients should support groups from 2048 to 8192 bits. Clients should also be able to request 3072 bits as their preferred size.
§4Selecting a Group
The server should return the smallest group it knows that is larger than the size requested by the client. If it knows no group larger than the request, it SHOULD return the largest group it knows. The returned group SHOULD be at least 2048 bits, and setting the group size to 3072 SHOULD be possible.
The server chooses the smallest group it has that exceeds the requested size. The returned group should still meet the 2048-bit minimum recommendation.
§5Security Considerations
The RFC warns that a hostile SSH server could provide backdoored Diffie-Hellman primes or send a calculated secret through a covert channel. A malicious client could also cause denial of service by making repeated connections using groups smaller than 2048 bits. Operating systems SHOULD NOT log groups smaller than 2048 bits because doing so would create an additional attack surface.
The RFC describes risks from a hostile server and from repeated small-group requests. It recommends that operating systems not log groups below the stated size.