DFE-IP: Delegatable functional encryption for inner product

Functional encryption for inner product (FE-IP) allows an authorized user to obtain the inner product of the vectors embedded in his/her secret key and a ciphertext, respectively. Therefore, it is an elegant tool to implement secure computation over encrypted data. Existing FE-IP schemes mainly considered security and function privacy issues, but did not address the decryption delegation problem. However, in practice, delegating decryption is desirable when an authorized user is unavailable to access the system due to leaving, network problems, etc. In this paper, a delegatable FE-IP (DFE-IP) scheme is proposed to support decryption delegation. Our scheme provides the following important features: (1) a delegator can delegate his/her decryption power to a delegatee; (2) a proxy is not required to re-encrypt ciphertexts; (3) it supports temporary delegation and flexible revocation. We first formalize our DFE-IP scheme including definition and security model, and then describe a concrete construction. Furthermore, the implementation and evaluation are provided to show its efficiency. Finally, its security is formally proven. The novelty is to implement temporary decryption delegation in FE-IP when authorized users are unavailable to access the system.