Active Attacks on Reputable Mix Networks

A mix network is a cryptographic construction for anonymous communications. In addition to anonymity, a reputable mix network first defined by Golle offers a reputation property: the mix-net can prove that every message it outputs corresponds to an input submitted by a user without revealing which input. This property can shield the mix-net from liability in the event that an output message is objectionable or illegal. In this work we analyze two reputable ElGamal based mix-net schemes proposed by Golle and present two active attacks for them. Our attacks rely on the homomorphism properties of RSA signature and ElGamal cryptosystem and can break the reputation properties of those schemes. We also show how to counter our attacks by using secure hash functions.