Achieving an effective, scalable and privacy-preserving data sharing service in cloud computing.

Data sharing in the cloud, fueled by favorable trends in cloud technology, is emerging as a promising technique for allowing users to conveniently access data. However, the growing number of enterprises and customers who stores their data in cloud servers is increasingly challenging users' privacy and the security of data. This paper focuses on providing a dependable and secure cloud data sharing service that allows users dynamic access to their data. In order to achieve this, we propose an effective, scalable and flexible privacy-preserving data policy with semantic security, by utilizing ciphertext policy attribute-based encryption (CP-ABE) combined with identity-based encryption (IBE) techniques. In addition to ensuring robust data sharing security, our policy succeeds in preserving the privacy of cloud users and supports efficient and secure dynamic operations including, but not limited to, file creation, user revocation and modification of user attributes. Security analysis indicates that the proposed policy is secure under the generic bilinear group model in the random oracle model and enforces fine-grained access control, full collusion resistance and backward secrecy. Furthermore, performance analysis and experimental results show that the overheads are as light as possible.