A Traceable Outsourcing CP-ABE Scheme with Attribute Revocation

Ciphertext-Policy Attribute-based Encryption (CP-ABE) is a useful cryptographic scheme and is being considered for cloud application as more and more users leverage cloud platforms to store and process their data. However, existing CP-ABE schemes still have a number of limitations that make it not effective to be used in a practical application. Firstly, the size of the ciphertext and the time for decryption grow with the complexity of the access formula. This efficiency issue becomes a problem when using a cloudplatform and mobile devices with limited processing capacity. Secondly, in reality, the attributes of users may be changed from time to time, or some users may eventually leave the system due to resignation. This practical concern requires a scheme with flexible and fine-grained revocation optionsupporting attribute-level changes. Lastly, traceability is also an important feature to track potential traitors who leak the partial decryption keys if ABE is used in a real scenario. None of the existing CP-ABE schemes are able to satisfy these three properties simultaneously. In this paper, we propose apractical CP-ABE that can achieve all three requirements. Our system adopts techniques on secure outsourcing of pairings to support efficient outsourcing computation and makes use of a subset cover algorithm to meet the requirements of revocation and traceability. Our scheme is proved to be a selectively replayable chosen-ciphertext attack (RCCA) secure in random oracle model. The result of our work could provide a feasible, reliable and practical CP-ABE scheme for the realistic application.