Key-Exposure Protection in Public Auditing with User Revocation in Cloud Storage

With the development of cloud data storage, more and more data owners are choosing to store their data in the Cloud and share them as a group. To protect integrity of sharing data, data are signed before they are stored on the cloud. When a user is revoked from the group, the revoked user's signature can be converted to the existing group member's signature by the cloud to preserve the revocation's efficiency. Accordingly, the public auditing should be done by the third party auditor using the existing group member's public key. As a basic secure requirement, the cloud sever should not know the existing group member's private key even if he obtains the revoked user's private key. In this paper, we propose a new public auditing protocol in which a public verifier is always able to audit the integrity of shared data even if some part of shared data has been re-signed by the cloud. By integrating the proxy re-signature with random masking technique, the new public auditing protocol satisfies the basic secure requirement. In addition, we prove the security of the new protocol, and finally compare it with other existing public auditing protocols and show that the new mechanism provides a good key-exposure protection for the existed public auditing protocol for shared data without losing the communication and computation efficiency.