Provable Security - With the rapid development of cloud storage technology, effectively verifying the cloud data’s integrity becomes a major issue in cloud storage. Recently, Nayak and Tripathy proposed their cloud auditing protocol. Although the protocol is efficient, the protocol is not secure as Yu et al’s attack shows. Even if the server does not store the users’ data, it is possible to forge the provable data possession proof and pass the integrity audit. We point out the issue in their article and describe in detail the process by which the cloud server forging the proof. We finally give an improved and more secure scheme and analysis its security also.