Local View Attack on Anonymous Communication
We consider anonymous communication protocols based on onions: each message is sent in an encrypted form through a path chosen at random by its sender, and the message is re-coded by each server on the path. Recently, it has been shown that if the anonymous paths are long enough, then the protocols provide provable security for some adversary models. However, it was assumed that all users choose intermediate servers uniformly at random from the same set of servers.
We show that if a single user chooses only from a constrained subset of possible intermediate servers, anonymity level may dramatically decrease. A thumb rule is that if Alice is aware of much less than 50% of possible intermediate servers, then the anonymity set for her message becomes surprisingly small with high probability. Moreover, for each location in the anonymity set an adversary may compute probability that it gets a message of Alice. Since there are big differences in these probabilities, in most cases the true destination of the message from Alice is in a small group of locations with the highest probabilities.
Our results contradict some beliefs that the protocols mentioned guarantee anonymity provided that the set of possible intermediate servers for each user is large.
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