Efficient Integrity Auditing for Shared Data in the Cloud with Secure User Revocation

With the cloud storage services, users can easily form a group and share data with each other. Given the fact that the cloud is not trustable, users need to compute signatures for blocks of the shared data to allow public integrity auditing. Once a user is revoked from the group, the blocks that were previously signed by this revoked user must be re-signed by an existing user, which may result in heavy communication and computation cost for the user. Proxy re-signatures can be used here to allow the cloud to do the re-signing work on behalf of the group. However, a malicious cloud is able to use the re-signing keys to arbitrarily convert signatures from one user to another deliberately. Moreover, collusions between revoked users and a malicious cloud will disclose the secret values of the existing users. In this paper, we propose a novel public auditing scheme for the integrity of shared data with efficient and collusion-resistant user revocation utilizing the concept of Shamir secret sharing. Besides, our scheme also supports secure and efficient public auditing due to our improved polynomial-based authentication tags. The numerical analysis and experimental results demonstrate that our proposed scheme is provably secure and highly efficient.