[openssl-users] RFC 7919 DH parameters and OpenSSL DH_check()

[Kurt Roeckx]

On Thu, Jan 03, 2019 at 12:18:05PM -0800, Andy Schmidt wrote:
> I am adding the RFC 7919 Diffie-Hellman parameters to our TLS servers, and
> I've found that these parameters won't pass OpenSSL's Diffie Hellman
> parameter check function DH_check(). The return code is
> DH_NOT_SUITABLE_GENERATOR. Looking at the source code, it appears to fail
> because the remainder of the prime divided by 24 is not 11. That its, p mod
> 24 != 11. I have a couple of questions:
> 
> What relationship between the prime p and the generator g is this checking
> for? I thought that since p was a safe prime, as long as the generator g
> wasn't 1 the only choice is between the full group and the subgroup of the
> squares?
> 
> I would like to use DH_check() to attempt to ensure that Diffie Hellman
> parameters haven't been tampered on operating systems that don't have
> digital signatures for executable binaries.

See:
https://crypto.stackexchange.com/questions/12961/diffie-hellman-parameter-check-when-g-2-must-p-mod-24-11


Kurt