DH Group Exchange Fallback

[Joseph S Testa II]

On 09/22/2017 06:55 PM, Tim Broberg wrote:
> Do I understand correctly, that you find the security of group 14 unacceptable and yet you left it enabled?

In the end, I'm trying to ensure a minimum equivalent of 128-bits of 
security.  Group14 is 2048-bits, which roughly translates to 112-bits. [1]

To this end, I disabled the "diffie-hellman-group14-sha1" and 
"diffie-hellman-group14-sha256" kex algorithms, but the problem is that 
the group exchange "diffie-hellman-group-exchange-sha256" is not 
respecting the admin's wishes, and falls back to group14, even when 
specifically told not to (by the admin removing 2048-bit groups in 
/etc/ssh/moduli).

There's currently no way to ensure 100% that 2048-bit DH is disabled.

    - Joe


[1] See NIST Special Publication 800-57, Part 1, Revision 4, p. 53, 
<http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-57pt1r4.pdf>.