Generic Construction of Outsourced Attribute-Based Encryption Without Key Escrow

Attribute-based encryption (ABE) has a broad developing prospect in fine-grained sharing of ciphertext in the background of cloud computing. However, most of ABE schemes have a potential security risk called key escrow problem because users’ secret keys are issued by a trusted attribute authority (AA). Moreover, the pairing and exponential operations are heavy for users who are resource limited. In this paper, we introduce a semi-trusted organization called ministrant attribute authority (MAA). Based on our proposed master-key and parameters (MAP) transform, we construct a key issuing protocol where AA and MAA not only cooperate with but also restrain each other skillfully to generate users’ secret keys. These keys can be utilized directly for outsourced decryption. Based on the MAP transform, the key issuing protocol and an ABE scheme, we propose a generic construction of outsourced ABE without key escrow (OABE-WoKE). We provide security definitions for three types of adversaries where AA and MAA are included, and we are the first to prove that the construction is CPA secure against any one of the three types of adversaries. Finally, we provide two instantiations of OABE-WoKE schemes. Analyzing the simulation of them, we can conclude that they are more efficient than their competitive schemes.