In peer-to-peer (P2P) reputation systems,each peer's trustworthiness is evaluated based on its pseudonym's rating values given by other peers. Since it is assumed that each peer has a long lived pseudonym,all the transactions conducted by the same peer may be linked by its pseudonym. Therefore,one of the fundamental challenges in P2P reputation systems is to protect peers' identity privacy. In this paper,we present two independent anonymity protocols to achieve all the peers' anonymity by changing pseudonym with the help of a trusted third party (TTP) server. Compared with RuP (Reputation using Pseudonym),an existing representative scheme,our protocols reduce the server's cost in two different ways. First,we propose a protocol using blind signature scheme as in RuP. The protocol improves the blind signature scheme and assessment of macro-node values,and reduces the server's cost by half in terms of encryption and decryption operations and message overhead. Second,we propose another protocol,group-confusion protocol,to further reduce the server's cost.
