Privacy-preserving online/offline and outsourced multi-authority attribute-based encryption

Attribute based encryption(ABE) is an efficient technique that exploits attributes and access policies to achieve fine grained access control in cloud computing. Besides, outspread ABE schemes with multiple authorities(multi-authority ABE) are more suitable for practical applications than basic single authority ABE schemes. Nevertheless, existing multi-authority ABE schemes either can't preserve access policies' privacy or sustain expensive computational cost of encryption and decryption phases. In this paper, to tackle the above challenges, we propose an online/offline and outsourced multi-authority ABE scheme with policy protection. Roughly speaking, our main idea is to alleviate the online computation overhead for owners by splitting the encryption algorithm to the online encryption and offline encryption. During the decryption phase, users outsource massive decryption operations to the proxy server via the technique of transformation key. We can demonstrate that our scheme is secure and also protects the privacy of access policies according to security analysis. Furthermore, performance analysis shows that our scheme can lighten the computation burden for both owners and users and is quite appropriate for resource-limited devices in the multi-authority systems.