Abstract: Reputation protocol is used to compute feedback by summing up the individual reputations of other system users．The feedback is then used to make a decision．It is always the case that the entities in a reputation system will provide dishonest feedback due to fear of exposing their privacy which would lead to retaliatory acts from opponents．In this paper,a privacy preserving reputation protocol enable an entity to acquire fair reputation from other entities．In this protocol Shamir Secret Sharing and Verifiable Secret Sharing with homomorphic property are employed to enable the dealer and secret holders to verify the correctness of the shared secret．Moreover a random selection of the participating secret holders is achieved．And using the shared secret key,the secret reputation feedback is attained by decryption．Further more,the nice privacy preserving properties of the semantically secure ElGamal and Cramer Shoup Cryptosystem are used to enable the secret holders to submit their secret reputation feedback in a privacy preserving way and securely access a database of current reputation feedback maintained by a reputation engine.

Key words: ElGamal cryptosystem,Cramer-Shoup cryptosystem,Shamir threhold secret sharing,Homomorphic encryption,Reputation system