Introduction
A cyber-physical social system (CPSS) integrates the social world with the cyber world and physical world in order to promote the resources in the network from the single space to tri-space [1]. There are some typical paradigms of CPSS, including mobile social networks [2], vehicular networks [3], and so on. Users equipped with various sensors can sense, cache, compute, and share the information with each other. However, since information forwarding among users is a transaction between strangers, the security of transactions has to be guaranteed.