Cloud Computing is a promising service platform to access computing resources on demand over the Internet. It moves the data and applications to the large data centres where data and services can be managed in a much better manner than locally. However, its use puts forth a lot of challenging issues related to security, privacy and reliability of the overall system. In this paper, we focus on the problem of data-integrity verification(by a third party auditor) for the client's data residing on a cloud storage server(CSS). Here, we optimize an existing third party auditing protocol and make it resistant to replace, replay and forge attacks launched by malicious insiders at cloud storage server. We also propose a protocol to perform efficient block-level and fine-grained dynamic-data update operations on data stored on cloud using a modified Chameleon Authentication Tree. Extensive security and performance analysis shows that our protocols are efficient and can resist aforementioned attacks.
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