DH Group Exchange Fallback
Tim.Broberg at servicenow.com
Mon Sep 25 03:40:12 AEST 2017
Yes, using explicitly disabled algorithms is a very surprising behavior.
On 9/23/17, 10:32 AM, "Joseph S Testa II" <jtesta at positronsecurity.com> wrote:
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. 
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
There's currently no way to ensure 100% that 2048-bit DH is disabled.
 See NIST Special Publication 800-57, Part 1, Revision 4, p. 53,
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