Traditionally, to handle security for stand-alone computers and small networks, user authentication and access control mechanisms would be almost enough. However, considering distributed networks such as the Internet and pervasive environments, these kinds of approaches are confronted with flexibility challenges and scalability problems. This is mainly because open environments lack a central control, and users in them are not predetermined. In such ubiquitous computing environments, issues concerning security and trust become crucial. Adding trust to the existing security infrastructures would enhance the security of these environments. Although many trust models are proposed to deal with trust issues in pervasive environments, none of them considers the semantic relations exist among pervasive elements and especially among trust categories. Employing Semantic Web concepts, we propose a computational trust model based on the ontology structure, considering the mentioned semantic relations. In this model, each entity can calculate its trust in other entities and use the calculated trust values to make decisions about granting or rejecting collaborations. Using ontology structure can make the model extendible to encompass other pervasive features such as context awareness in a simple way.
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