Cryptanalysis of Mutual Anonymity Protocols for Hybrid Peer to Peer Systems

Xiao et al. proposed several approaches to achieve both initiator and responder mutual anonymity in hybrid P2P systems for file sharing. It uses trusted index servers to prepare a cover path, and both initiator and responder cannot identify each other. The system with trusted index servers is necessary for transferring file and has better efficiency and reliability. Xiao et al. proposed a center-directing protocol that reduces traffic resource with encrypt and decrypt operation. However, this protocol still has the same security problem with Mix-based protocol for transferring files. However, in this paper, we shall point out that Xiao et al.’s protocol is still vulnerable to the man-in-the-middle attack.