SSH 2048-bit DH Minimum: Increase the Secure Shell Minimum Recommended Diffie-Hellman Modulus Size to 2048 Bits
SSH servers and clients should now use Diffie-Hellman groups of 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
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 . . . . . . . . . . . . . . . . . . . . . . . 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
AI ELI5
On this page
§1Why 1024 Bits Is Not Enough
RFC 4419 specifies a recommended minimum Diffie-Hellman (DH) modulus group size of 1024 bits for SSH. Recent research (Logjam) strongly suggests that 1024-bit DH groups can be broken by state-sponsored actors and any organization with enough computing resources. The authors showed how they break 768-bit groups and extrapolated the attack to 1024 bits.
This document therefore raises the minimum recommended size to 2048 bits, and notes that the minimum may need to increase to 3072 for forward-looking users.
A small padlock was fine once, but thieves now have better tools, so we need a bigger one.
§2The New Size Rules
SSH servers and clients SHOULD support a minimum acceptable group size of 2048 bits for the min value of the SSH_MSG_KEY_DH_GEX_REQUEST message. Clients SHOULD be able to send 3072 bits as the preferred size n.
The RFC replaces two passages of RFC 4419. Servers and clients SHOULD support moduli of k bits where 2048 <= k <= 8192, with recommended minimum values for min and max of 2048 and 8192. Setting k to 3072 SHOULD be possible. In all cases, the group size SHOULD be at least 2048 bits.
Both sides should accept only big locks, at least 2048 sizes, and be ready to use an even bigger 3072 one.
§3Interoperability
RFC 4419 says the server should return the smallest group it knows that is larger than the size the client requested, and if it knows none, SHOULD return the largest group it knows. That stays. The following sentence is updated: in all cases the size of the returned group SHOULD be at least 2048 bits, and 3072 SHOULD be possible.
The server still hands over the smallest lock that is big enough, but never one smaller than 2048.
§4Security Considerations
A hostile server could use backdoored DH primes to supply the g and p values, or send the calculated secret through a covert channel to a passive listener. A malicious client could cause a denial of service by making multiple connections with groups smaller than 2048 bits.
For that reason, operating systems SHOULD NOT log DH groups smaller than 2048 bits, since doing so would create an additional attack surface. No IANA actions are required.
A bad server might hand out a rigged lock, and a bad client might cause trouble by asking over and over, so do not write every small request in the logbook.