Increase SSH minimum recommended DH modulus size to 2048 bits

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 so that the minimum recommended size be 2048 bits. This recommendation is based on recent research on DH Group weaknesses.

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.

Recent research strongly suggests that DH groups that are 1024 bits can be broken by state actors, and possibly any 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, where it is possible to set k to 3072 should the need arise in the coming years. [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 as described below: 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. k SHOULD be able to be set to 3072 by an implementation should the need arise in the coming years. Section 3 Paragraph 11: In all cases, the size of the group SHOULD be at least 2048 bits, with the possibility to be set to 3072 bits should the need arise in the coming years.

This document keeps the requirement "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." and updates the sentence that follows to read: "In all cases, the size of the returned group SHOULD be at least 2048 bits."

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 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. A malicious client could cause a Denial of Service by making multiple connections which are less 2048 bits in size on purpose. Therefore, Operating Systems SHOULD NOT log DH groups less than 2048 bits in size, as it would create an additional attack surface.

This document contains no considerations for IANA.