There are lots of entities in the ubiquitous computing environment. For the traditional public key Infrastructure (PKI), every entity should be signed a valid certificate by the certificate authentication center. However, it's hard to construct a centralized trust management framework and assign a valid certificate for every entity in the ubiquitous computing environment because of large numbers of dynamic entities. Trust negotiation (TN) is an important means to establish trust between strangers in ubiquitous computing systems through the exchange of digital credentials and mobile access control policies specifying what combinations of credentials a stranger must submit. Current existing TN technologies, such as TrustBuilder and KeyNote, focused on how to solve a certain problem by using some special techniques. In this paper, we present a formal framework for expressing trust negotiation. The framework specifies the basic concepts, elements and the semantics of TN. By analyzing TN, we point out how to build a TN system in practice.
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