Web servers that use Diffie-Hellman are configured with a precomputed set of prime numbers, called a Diffie-Hellman group. The number that is termed the "size" of the group (the modulus size, specifically) is proportional to the strength of the encryption key that will be created via the Diffie-Hellman calculations--the bigger the group size, the stronger the key. Common group sizes at the time of the attack announcement were 512 bits, 768 bits and 1024 bits.