Now, it is common for patients and medical institutions to outsource their data to cloud storage. This can greatly reduce the burden of medical information management and storage and improve the efficiency of the entire medical industry. In some cases, the group-based cloud storage system is also very common to be used. For example, in an medical enterprise, the employees outsource the working documents to the cloud storage and share them to the colleagues. However, when the working documents are outsourced to the cloud servers, how to ensure their security is a challenge problem for they are not controlled physically by the data owners. In particular, the integrity of the outsourced data should be guaranteed. And the secure cloud auditing protocol is designed to solve this issue. Recently, a lightweight secure auditing scheme for shared data in cloud storage is proposed. Unfortunately, we find this proposal not secure in this paper. It’s easy for the cloud server to forge the authentication label, and thus they can delete all the outsourced data when the cloud server still provide a correct data possession proof, which invalidates the security of the cloud audit protocol. On the basis of the original security auditing protocol, we provide an improved one for the shared data, roughly analysis its security, and the results show our new protocol is secure.
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