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Showing posts from January, 2016

OpenSSL Key Recovery Attack on DH small subgroups (CVE-2016-0701)

Usual Mandatory Disclaimer: IANAC (I am not a cryptographer) so I might likely end up writing a bunch of mistakes in this blog post...

tl;dr The OpenSSL 1.0.2 releases suffer from a Key Recovery Attack on DH small subgroups. This issue got assigned CVE-2016-0701 with a severity of High and OpenSSL 1.0.2 users should upgrade to 1.0.2f. If an application is using DH configured with parameters based on primes that are not "safe" or not Lim-Lee (as the one in RFC 5114) and either Static DH ciphersuites are used or DHE ciphersuites with the default OpenSSL configuration (in particular SSL_OP_SINGLE_DH_USE is not set) then is vulnerable to this attack.  It is believed that many popular applications (e.g. Apache mod_ssl) do set the  SSL_OP_SINGLE_DH_USE option and would therefore not be at risk (for DHE ciphersuites), they still might be for Static DH ciphersuites.
Introduction So if you are still here it means you wanna know more. And here is the thing. In my last blog post I was …

What the heck is RFC 5114?

Mandatory Disclaimer: IANAC (I am not a cryptographer) so I might likely end up writing a bunch of mistakes in this blog post...

I already talked about Diffie–Hellman (DH from now on) in TLS in my previous post: Small subgroup attack in Mozilla NSS.
As mentioned FWIW I strongly agree with Google Chrome decision to deprecate DHE .
The reason is mainly due to the Weak Diffie-Hellman attack and related paper . If you are interested in this topic there is a really nice presentation about it at 32C3 .
This shows a really nice potential attack that anyone with enough computational power (let's say NSA) can perform against DHE 1024 bits (details in the paper).
Said that for some reason I have been looking at DHE for a while now and one day I hit RFC 5114.

Now what the heck is this specification about :S ?

I found only few references about it. One funny one from here says (emphasis mine):



There is a semi-mysterious RFC 5114 – Additional Diffie-Hellman Groups document. It introduces new M…