1. Introduction
In open and uncertain environments, such as Peer-to-Peer (P2P) networks, Mobile Ad-hoc Networks (MANETs), Multi-Agent Systems (MASs) and e-commerce systems, etc., it is usual to collaborate with unfamiliar counter parties. Trust and Reputation Systems (TRSs) are effective approaches to facilitate the principal's decision-making by deducing the expectation of the counter party's behavior for specific task. The trustor usually evaluates the trustworthiness of the trustee by gathering the past direct experiences and the indirect experiences from the third parties.