Crowds Based on Secret-Sharing

Anonymous communication has been a long recognized problem, and various solutions of different performance have been proposed over the last decades. Manifold differently strong security notions, being specific for the sender or receiver, are found in the literature. We consider protection of both, the sender´s and receiver´s identity from each other and a coalition of intermediate relay nodes. The Crowds-system is known to provide probabilistic sender anonymity, but receiver anonymity is only given for asymptotically large networks. Assuming that the adversary notices the communication as such, we prove that the strongest form of receiver anonymity (under this assumption) is efficiently achievable for finite-size (even small) networks. Our construction is secure in the sense that a passive threshold adversary cannot disclose the receiver´s identity with a chance better than guessing this information.