The advent of cloud computing brings great changes to the IT industry. In cloud computing, users do not need to have their own expensive computing equipment nor software systems; they can use the services provided by the cloud systems in an on-demand manner for scientific computations, data storage, computer communication, and so on. As millions of users with different business background share the use of the cloud systems, security and privacy becomes a critical issue of these systems. On a different front, the challenge of security and privacy has become even more prominent following the explosive growth of online social networks in recent years. People, using online social networks, are increasingly concerned about the security and privacy of their personal data collected by the service providers.