OpenSSL extension only supports weak (512-bit, 1024-bit) Diffie-Hellman groups
The following D-H groups are enabled per default:
These use 512-bit and 1024-bit primes respectively. These are considered weak in 2015 by all present methods of evaluating D-H group size as a security parameter:
Weak D-H groups like this were recently implicated in the Logjam attack:
512-bit D-H keys in particular can be trivially attacked by commodity hardware. I have put in a PR to the openssl gem to remove the 512-bit group:
However, the 1024-bit group is weak as well. The recommendation of the Logjam paper authors is to upgrade to a 2048-bit group at the minimum.