Recovering Private Keys Generated With Weak PRNGs

Abstract. Suppose that the private key of discrete logarithm-based or factoring-based public-key primitive is obtained by concatenating the outputs of a linear congruential generator. How seriously is the scheme weakened as a result? While linear congruential generators are cryptographically very weak “pseudorandom ” number generators, the answer to that question is not immediately obvious, since an adversary in such a setting does not get to examine the outputs of the congruential generator directly, but can only obtain an implicit hint about them—namely the public key. In this paper, we take a closer look at that problem, and show that, in most cases, an attack does exist to retrieve the key much faster than with a naive exhaustive search on the seed of the generator. The problem is similar to the one considered by Bellare, Goldwasser and Micciancio regarding DSA and “pseudorandomness”, and this line of work arguably has renewed relevance in view of the sensitive role that random number generation has been found to play in a number of recent noted papers, such as the one by Lenstra et al. at CRYPTO 2012