Peer-to-peer (P2P) networks are vulnerable to peers who cheat, propagate malicious code, leech on the network, or simply
do not cooperate. The traditional security techniques developed for the centralized distributed systems like client-server networks are
insufficient for P2P networks by the virtue of their centralized nature. The absence of a central authority in a P2P network poses unique
challenges for reputation management in the network. These challenges include identity management of the peers, secure reputation data management, Sybil attacks, and above all, availability of reputation data. In this paper, we present a cryptographic protocol for ensuring secure and timely availability of the reputation data of a peer to other peers at extremely low costs. The past behavior of the
peer is encapsulated in its digital reputation, and is subsequently used to predict its future actions. As a result, a peer’s reputation
motivates it to cooperate and desist from malicious activities. The cryptographic protocol is coupled with self-certification and
cryptographic mechanisms for identity management and countering Sybil attack. We illustrate the security and the efficiency of the
system analytically and by means of simulations in a completely decentralized Gnutella-like P2P network.