Cryptanalysis of an identity-based public auditing protocol for cloud storage

Public verification of data integrity is crucial for promoting the serviceability of cloud storage systems. Recently, Tan and Jia (2014) proposed an identity-based public verification (NaEPASC) protocol for cloud data to simplify key management and alleviate the burden of check tasks. They claimed that NaEPASC enables a third-party auditor (TPA) to verify the integrity of outsourced data with high efficiency and security in a cloud computing environment. However, in this paper, we pinpoint that NaEPASC is vulnerable to the signature forgery attack in the setup phase; i.e., a malicious cloud server can forge a valid signature for an arbitrary data block by using two correct signatures. Moreover, we demonstrate that NaEPASC is subject to data privacy threats in the challenge phase; i.e., an external attacker acting as a TPA can reveal the content of outsourced data. The analysis shows that NaEPASC is not secure in the data verification process. Therefore, our work is helpful for cryptographers and engineers to design and implement more secure and efficient identity-based public auditing schemes for cloud storage.