Weak DH primes and openssh

David McBride dwm37 at cam.ac.uk
Sat May 23 00:30:43 AEST 2015

On Fri, May 22, 2015 at 12:27:01, Darren Tucker <dtucker at zip.com.au> wrote:

> Note that PuTTY does do Diffie-Hellman Group Exchange, but until very
> recently (ie after their 0.64 release) they didn't do the one that was
> actually standardized in RFC4419.  OpenSSH recently removed support for
> that non-standard one and as a result we don't offer DHGEX to PuTTY
> versions <= 0.64 so they'll fall back to group14 (2k bit fix group).

I think this is wrong.

This commit [0] from 2005 appears to show the addition of
diffie-hellman-group-exchange-sha256 support to PuTTY.

I've also just successfully connected to a local test OpenSSH server
(v6.7p1, as packaged by Debian) with only
diffie-hellman-group-exchange-sha256 enabled with an older release of
PuTTY (0.63) without any issue.

Indeed, PuTTY explicitly reported in its event log that it performed
key-exchange using Diffie-Hellman group exchange and SHA-256, so I'm
quite sure this is working!

Unless there's more than one key-exchange mechanism going by the name

Kind regards,


David McBride <dwm37 at cam.ac.uk>
Unix Specialist, University Information Services

More information about the openssh-unix-dev mailing list