Re: Logjam attack / Imperfect Forward Secrecy: How Diffie-Hellman Fails in Practice

[Paul Wouters <pwouters () redhat com>]

On 05/20/2015 07:57 AM, Yves-Alexis Perez wrote:
> I guess most people will already have seen that, but just in case,
> because it might interest readers (even though it's not specifically
> about open source stuff).
>
> https://weakdh.org/
> https://weakdh.org/imperfect-forward-secrecy.pdf

Note that it really points to TLS. While they mention other protocols, IKE / IPsec are not really (as) vulnerable. The 
original IKE design from November 1998
has a minimum MODP group of 768 bits. Why is TLS till allowing 512 bits SEVENTEEN years later?

I did a write up on MODP and IKE/IPsec from the freeswan/openswan/libreswan point of view:


https://nohats.ca/wordpress/blog/2015/05/20/weakdh-and-ike-ipsec/

    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

[...]

Paul