An important concept in network security is trust, interpreted as a relation among entities that participate in various protocols. Trust relations are based on evidence related to the previous interactions of entities within a protocol. In this work, we are focusing on the evaluation process of trust evidence in Ad Hoc Networks. Because of the dynamic nature of Ad Hoc Networks, trust evidence may be uncertain and incomplete. Also, no pre-established infrastructure can be assumed. The process is formulated as a path problem on a directed graph, where nodes represent entities, and edges represent trust relations. Using the theory of semirings, we show how two nodes can establish an indirect trust relation without previous direct interaction. The results are robust in the presence of attackers. We give intuitive requirements for any trust evaluation algorithm. The performance of the scheme is evaluated on three topologies.