Section: New Results

Trust

Reputation mechanisms allow users to mutually evaluate their trust. This is achieved through the computation of a reputation score summarizing their past behaviors. Depending on these scores, users are free to accept or refuse to interact with each other. Existing solutions often rely on costly cryptographic tools that may lead to impractical solutions. We have proposed in [41] , [40] , [28] usable privacy preserving reputation mechanisms. These mechanisms are distributed and handles non-monotonic ratings. Evaluation made on our mechanism reveals it to be fully usable even with cheap on-board computers. This is a very encouraging result as it shows that privacy does not impede utility and accuracy. This has been achieved by combining distributed algorithms and cryptographic schemes. Our mechanism is independent of the reputation model, that is, our system can integrate any reputation model, preferably one using both positive and negative ratings.

In a mobile ad hoc network we have also considered the problem of designing a reputation system that allows to update and to propagate the computed reputation scores while tolerating Byzantine failures [42] . Each time a correct node uses directly a service, it can determine by itself the quality of service currently provided. This fresh and valid rating information is broadcast immediately to all its current neighbors. Then, while the mobile node moves, it can receive from other nodes other recommendations also related to the same service. Thus it updates continuously its own opinion. Meanwhile it continues to broadcast this updated information. The freshness and the validity of the received/sent information become questionable. We propose a protocol that allows a node to ignore a second hand information when this information is not fresh or not valid. In particular, fake values provided by Byzantine nodes are eliminated when they are not consistent with those gathered from correct nodes. When the quality of service stabilizes, the correct nodes are supposed to provide quite similar recommendations. In this case, we demonstrate that the proposed protocol ensures convergence to a range of possible reputation scores if a necessary condition is satisfied by the mobile nodes. Simulations are conducted in random mobility scenarios. The results show that our algorithm has a better performance than typical methods proposed in previous works.