DH Group Exchange Fallback

Joseph S Testa II jtesta at positronsecurity.com
Sun Sep 24 03:32:11 AEST 2017

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 

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, 

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