Information Leakage in Mix Networks with Randomized Partial Checking

We present a previously unknown vulnerability of mix networks (mix nets) that use pseudorandom permutations and that are audited with randomized partial checking (RPC). Our method relies on two simple observations: A mix that generates pseudorandom permutations only generates a limited subset of all possible permutations; in practical cases, RPC exposes enough information to uniquely identify the mix’s permutation because of the gap between the number of permutations that the mix can generate and the total number of possible permutations. Exploiting this newly found vulnerability is difficult in practice. The only fix we see to this vulnerability is to maintain sufficient entropy used while generating the permutation. We are not aware of any applications using mix nets and RPC that can be exploited with this vulnerability.