More efficient DDH pseudorandom generators

In this paper, we first show a DDH Lemma, which states that a multi-variable version of the decisional Diffie---Hellman problem is hard under the standard DDH assumption, where the group size is not necessarily known. Our proof, based on a self-reducibility technique, has a small reduction complexity. Using DDH Lemma, we extend the FSS pseudorandom generator of Farashahi et al. to a new one. The new generator is almost twice faster than FSS while still provably secure under the DDH assumption. Using the similar technique for the RSA modulus, we improve the Goldreich---Rosen generator. The new generator is provably secure under the factoring assumption and DDH assumption over $${\mathbb{Z}_N^*}$$ . Evidently, to achieve the same security level, different generators may have different security parameters (e.g., distinct length of modulus). We compare our generators with other generators under the same security level. For simplicity, we make comparisons without any pre-computation. As a result, our first generator is the most efficient among all generators that are provably secure under standard assumptions. It has the similar efficiency as Gennaro generator, where the latter is proven secure under a non-standard assumption. Our second generator is more efficient than Goldreich---Rosen generator.