# Tag Info

First you should know that Elgamal encryption and signature security is based on DDH problem (Decisional Diffie Hellman) which is tractable in some groups that CDH problem is believed to be hard (Computational Diffie Hellman). As in the case of $\mathbb{Z_q}$ in which CDH is believed to be hard but DDH is apparently tractable. Let $p = 2p_1 + 1$ where both ...