Cloud computing can reduce the high cost of setting up computing infrastructure. It provides an IT architecture through which the lightweight portable devices can access data on the internet flexibly. In cloud scenario, the data can be accessed through virtual machines and at the same time the capacity of the software is increased. Since the data may reside in any machine wherever it is near or far from the users, a large number of security and privacy challenges emerge unavoidably. Also, when the breakdown of a server happens, the data storing on it is lost certainly.