Public auditing protocol is crucial for the success of cloud computing, as it can ensure the outsourced data in cloud server are not tampered by attackers. Due to its importance, public auditing protocol has received considerable attention in the past years. In 2015, Liu et al. proposed a privacy-preserving public auditing protocol for regenerating-code-based cloud storage and claimed it is secure under the considered security model. However, in this article, we will show that their protocol is not as secure as they claimed, i.e., the proxy delegated by the data owner can forge an authenticator for any data block, which obviously invalidates their protocol’s security. We hope that by identifying the design flaw, similar weaknesses can be avoided in future protocol design

Key words : Public auditing, Cloud computing, Privacy-preserving, Regenerating code, Proxy delegation, Authenticator, Security flaw