A New Relation Attack on the Optimistic Mix-Net

Mix-net is a cryptographic technique that is used to hide the origin of messages in computer network communications. The optimistic mix-net proposed by Golle et al. is a very practical and efficient construction which has a very fast mixing process when all players behave honestly. A new type of relation attack on Golle´s optimistic mix-net and its modified editions is presented in this paper. In particular a malicious mix server can break any given user´s anonymity in previous mix-sessions by using current session as a decryption oracle at the cost of being accused of cheating. The attack suggests that the optimist mix-net must use different keys in each mix-session and this demerit would deliver a significant blow to its practical value.