Two Stage Public Auditing for Shared Data with Efficient User Revocation in the Cloud

With cloud computing, users can remotely store their data into the cloud and use on-demand high-quality applications by using a shared pool of configurable computing resources by using data outsourcing users are relieved from the burden of data storage and maintenance. When users put their data (of large size) on the cloud, the data integrity protection is challenging. To ensure shared data integrity can be verified publicly, users in the group need to compute signatures on all the blocks in shared data. Different blocks in shared data are generally signed by different users due to data modifications performed by different users. The straightforward method, which allows an existing user to download the corresponding part of shared data and re-sign it during user revocation, is inefficient due to the large size of shared data in the cloud. In this paper, we propose a novel public auditing mechanism for the integrity of shared data with efficient user revocation in mind. By utilizing the idea of collusion resistant multi- proxy re-signatures, we allow the cloud to resign blocks on behalf of existing users during user revocation. Collusion-resistant proxy re-signature schemes generally have two levels of signatures. In addition, a public verifier is always able to audit the integrity of shared data without retrieving the entire data from the cloud. Moreover, our mechanism is able to support batch auditing by verifying multiple auditing tasks simultaneously.