TL;DR The LogJam downgrade attack does not apply to MODP groups in the IKE protocol, only to TLS, so IKE or IPsec is not impacted.

If you are using libreswan you are not vulnerable to weak MODP groups and using MODP2048 per default unless specifically configured for a lower MODP group.
If you are using openswan with IKEv2 you are using MODP2048, but if you are using IKEv1 you are using MODP1536 which is still much stronger than MODP768 or MODP1024.

Libreswan as a client to a weak server will allow MODP1024 in IKEv1 as the least secure option, and MODP1536 in IKEv2 as the least secure option.
Openswan does not properly implement INVALID_KE, so it cannot connect to another DH group than the one it started out as, so it runs the risk of getting locked out if the server side bumps their minimum MODP group to 2048. openswan defaults to MODP1536 in IKEv1 and MODP2048 in IKEv2

(minor update: Hugh Redelmeier corrected me that freeswan never supported modp768 – the smallest modp was 1024)

A bunch of smart crypto research released a paper describing the LogJam attack. It is a cryptographic attack on weak DiffieHellman groups. It talks about DH in various protocols. It is an interesting read. When you break the DH layer, you obtain the secret session keys needed to decrypt the various protocol (TLS, IPsec, SSH, etc) sessions. The conclusion from the paper is that any DH group smaller than 1024 should not be used as even non-nationstate attackers can obtain the resources needed to break these. They further conclude that they believe it very likely that the standarized 1024 bit DH MODP group has been broken as well by the NSA. It involves large scale computing clusters and sieving and doing a lot of heating up the planet. They can do this because in crypto standards, everyone uses the same MODP prime values for the DH exchange.

First, a small clarification. When the paper talks about IPsec, they mostly mean the IKE keying protocol, not the ESP packet encryption protocol. IKE is responsible for the secure keying operation that generates symmetric session keys used for the actual packet encryption in the ESP protocol. IKE is the userland daemon, ESP is the kernel level encryption/decryption.

The DH values for IKE are listed in various RFC’s, and can be found in the IANA registry for the older (but more used) IKEv1 GROUP values and for the newer but less used IKEv2 GROUP values. For the published attack, the only type of groups relevant are the MODP groups. The ECP groups do not use exponentiation and thus are not vulnerable to factoring primes.

IKEv1 MODP groups

In IKEv1, the most common modp groups are modp1024 (DH2) and modp1536 (DH5). Some people still want to use modp768 (DH1) but it has strongly decreased over time. Most IKEv1 stacks also support modp2048 (DH14) up to modp8192 (DH18) although notably Cisco administrators in general seem to have stuck with modp1024 and modp1536, just as they have stuck with 3DES and MD5/SHA1. This is mostly because the technical knowledge to configure Cisco’s for VPN is so difficult, that once a Standard Operating Procedure(SOP) is in place, no one wants to, or is allowed to, modify the configuration used. (It’s also the tail end of Cisco asking for extra money to support AES many years ago)

FreeS/WAN never supported modp768 – John Gilmore had good ideas about scaling and NSA computing powers, he was one of the people behind Deep Crack, the EFF 1DES cracker. Openswan which was forked from FreeS/WAN in 2003 had support for modp768 (and 1DES) but did not compile it in per default and neither did any of the Linux distributions that shipped it. But people needed it frequently enough to interoperate that the code was added, but disabled per default. Notably, embedded people using the *swan’s would enable it due to customer demand. It annoyed us so much that sometime around 2005 (I can’t find it in the changelog) we just ripped it out. Of course, it did not prevent people from patching it back in, which some did. Most likely, many unattended embedded devices out there are still doing modp768 bassed VPNs, and as LogJammer shows, that’s within reach of academia and banana republics – so count on these being decrypted routinely by the Five Eyes, Russia, China, and probably most other Western TLA’s.

The default IKEv1 modp group for openswan (and libreswan) was 1536. In July 2014, libreswan changed this to MODP2048. I believe openswan is still using modp1536 (and if you are still using openswan, I recommend that you switch ASAP, see this git commit activity comparison

IKEv2 MODP groups

IKEv2 has seen a very very slow start. I’ve only seen it really being deployed in the last two to three years. Most of these deployments use at least MODP2048, do not support MODP768, but are usually willing to do MODP1536 or even MODP1024. The latter is mostly done for fear of compatibility issues. But also to support an upgrade path for all devices. If your VPN connection supports “either IKEv1 or IKEv2” and your clients are still connecting with IKEv1, then most surely you will need to support MODP1536 and even MODP1024, or you will be losing your older VPN clients.

In April of 2008, as the then release manager of Openswan, we released version 2.6.12 which changed the IKEv2 default to MODP2048. This was a controversial decision – one developer quit the openswan project over this change because he deemed it would cause too many interoperability failures because many IKE implementations did not properly implement INVALID_KE renegotiation or simply did not support MODP2048. No problems emerged after the change, but most likely because simply no one but the certification checkbox people were even testing or running IKEv2 at the time. Since then, libreswan (but not openswan) has implemented proper INVALID_KE renegotiation.

RFC 5114 MODP groups

There is a semi-mysterious RFC 5114 – Additional Diffie-Hellman Groups document. It introduces new MODP groups not with higher sizes, but just with different primes. The justification in the RFC states

The purpose of this document is to provide the parameters and test
data for eight additional groups, in a format consistent with existing RFCs.

It lists one new 1024 bit MODP group and two 2048 bit MODP groups, when we already had RFC 3526, which has MODP groups up to 8192 bit. The odd thing is that when I talked to people in the IPsec community, no one really knew why this document was started. Nothing triggered this document, no one really wanted these, but no one really objected to it either, so the document (originating from Defense contractor BBN) made it to RFC status. Due to this strange history, the RFC 5114 MODP groups are not included in any default proposal set and MUST be manually configured before libreswan will use these groups. We had to support these groups due to demands for RFC compliance and certifications.

IKE and IPsec are not that bad you know

Not that I want to go into a nahnahnahnahnahnah mode, but to those people that keep dissing IKE and IPsec, note that we were again not vulnerable unlike TLS. And when it comes down to cipher mode attacks, IKE has proven to be far more secure than TLS has been in the last three years. So please stop beating up on IKE/IPsec citing some old papers from the late 90’s. Kthanks