Secure Group Sharing in Dynamic Public Cloud Environment

Data integrity of cloud data outsourced is been analysed and secured using Symmetric key cryptosystem with the rise in demand for data outsourcing. In this paper, we propose a symmetric key cryptosystem for data sharing to the users in the cloud storage against unexpected privilege escalation by introducing a special type of public-key encryption which we call Key-Aggregate Cryptosystem (KAC). In KAC, users encrypt a message not only under a public-key, but also under an identifier of cipher text called class. That means the cipher texts are further categorized into different classes. The key owner holds a master-secret called master-secret key, which can be used to extract secret keys for different classes. More importantly, the extracted key have can be an aggregate key which is as compact as a secret key for a single class, but aggregates the power of many such keys, i.e., the decryption power for any subset of cipher text classes.  Extensive security and performance analysis shows that our proposed scheme is highly efficient and satisfies the security requirements for public cloud based secure group sharing.