Providing security to the stored data on the cloud is one of the important challenges in cloud computing. Encrypted data which is stored on the cloud may be viewed or modified by the cloud service provider. To overcome this problem many techniques have been developed but, those cannot guarantee accurately about the security of the stored data. These modifications of the data by the service provider or by others should also be known to the data owner. For such purpose, data tagging technique can be used to audit the data.Traditionally, owners can check the data integrity based on two-party storage auditing protocols. In cloud storage system, however, it is inappropriate to let either side of cloud service providers or owners conduct such auditing, because none of them could be guaranteed to provide unbiased auditing result. In this situation, trusted auditor is a natural choice for the storage auditing in cloud computing. A trusted auditor that has expertise and capabilities can do a more efficient work and convince both cloud service providers and owners.