Cloud computing has been envisioned as the next-generation architecture of IT enterprise. It moves the databases and application software to the centralized large data centers, where the management of the services and data may not be fully trustworthy. These unique paradigms bring about many new security challenges, which have not been fine understood. This paper study the problem of ensuring the integrity of data storage in cloud computing. In particular, the authors believe the task of allowing a Third Party Auditor (TPA), on behalf of the cloud client, to prove the integrity of the dynamic data stored in the cloud.