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.

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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.

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 : 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

6.2. Informative References

Authors' Addresses

Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. 2048 bits DH Group
• 3. Interoperability
• 4. Security Considerations
• 5. IANA Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Authors' Addresses