[CentOS] sshd key exchange security

Fri Sep 11 15:43:49 UTC 2015
Robert Moskowitz <rgm at htt-consult.com>

On 09/11/2015 11:35 AM, Alice Wonder wrote:
> I was reading https://weakdh.org/sysadmin.html
> They also have a very interesting paper as a PDF.
> Anyway it appears that most ssh servers, when using DHE key exchange, 
> use the 1024-bit Oakley Group 2 and there is suspicion the NSA has 
> done the pre-computations needed to passively decrypt any tls 
> communication using DHE with that particular prime group.
> They recommend setting the following:
> KexAlgorithms curve25519-sha256 at libssh.org
> I don't even see that directive in my sshd config to set it, I suppose 
> it may be one that is manually added when needed but I want to verify 
> it actually means something in CentOS 7 ssh.
> Also I'm a little worried that maybe curve25519 is one of the curves 
> that Red Hat (and thus CentOS 7) doesn't support due to patent concerns.

There are no patent concerns with Dan's c25519.  But its acceptance by 
the standards communities is new.  Like really summer 2014 at the 
Toronto IETF.  Typical Dan presentation...

It HAS been around for some time and has been extensively reviewed. The 
code is really clean and easy to review and implement, even in highly 
constrained devices.

> If it is, is there a suggestion on what curve should be used instead?