A Game-based Sybil-resistant Strategy for Reputation Systems in Self-organizing MANETs

A promising method to stimulate cooperation among the nodes of a self-organizing mobile ad hoc network is the application of reputation systems. In these systems, usually, a node uses the recommendations made by the others when evaluating the reputation of the node under consideration. This renders such systems vulnerable to the Sybil attack in which an attacker forges several identities and uses them to recommend itself as a well-behaved node. In this paper, we propose a multistage-game strategy for reputation systems that discourages Sybil attacks. The underlying notion in devising such a strategy is that a Sybil identity, to remain trustworthy, should be active and sincere in recommending the others. Thus, for an effective attack, the attacker should incur the cost of maintaining the trustworthiness of its Sybil identities. This feature can be exploited to design a reputation system in such a way that the attack becomes more costly than cooperation. It is shown that the proposed strategy makes a subgame-perfect equilibrium, which justifies its deployment in real-life networks.