In this paper, we propose and evaluate a distributed protocol to manage trust diffusion in ad hoc networks. In this protocol, each node i maintains a \trust value" about an other node j which is computed both as a result of the exchanges with node j itself and as a function of the opinion that other nodes have about j. These two aspects are respectively weighted by a trust index that measures the trust quality the node has in its own experiences and by a trust index representing the trust the node has in the opinions of the other nodes. Simulations have been realized to validate the robustness of this protocol against three kinds of attacks: simple coalitions, Trojan attacks and detonator attacks.