On the Security of Schnorr's Pseudo Random Generator

At Eurocrypt 88 Schnorr [8] proposed a pseudo random generator for which he claimed that it could not be distinguished from a truly random source with less than 2°(n) output bits, even when unlimited computing power was available. We show that this generator can, in fact, be distinguished with only 4n bits of output. Moreover, we present an efficient (linear-time) algorithm which recovers the key from a substring only slightly larger than the generator’s keysize. Consequently, the generator is insecure.