One of the basic primitives in cryptography and other areas of computer science is a pseudo-random generator. The usefulness of a pseudo-random generator is demonstrated by the fact that it can be used to construct a private key cryptosystem that is secure even against chosen plaintext attack. A pseudo-random generator can also be used to conserve random bits and allows reproducibility of results in Monte Carlo simulation experiments. Intuitively, a pseudo-random generator is a polynomial time computable function g that stretches a short random string x into a much longer string g(x) that ''looks'' just like a random string to any polynomial time adversary that is allowed to examine g(x).1 Thus, a pseudo-random number generator can be used to efficiently convert a small amount of true randomness into a much longer string that is indistinguishable from a truly random string of the same length to any polynomial time adversary. On the other hand, there seem to be a variety of natural examples of another basic primitive; the one-way function. Intuitively, a function f is one-way if: (1) given any x, f(x) can be computed in polynomial time; (2) given f(x) for a randomly chosen x, it is not possible on average to find an inverse x' such that f(x') = f(x) in polynomial time. It has not been Proven that there are any one-way functions, but there am a number of problems from number theory, coding theory, graph theory and combinatorial theory that are candidates for problems that might eventually be proven to be one-way functions. We show how to construct a pseudo-random generator from any one-way function. The journal version of this work (in preparation) is the combination of the results announced in the conference papers [ILL, Impagliazzo Levin Luby] and [H, Hastad].
