Secure Outsourced Attribute-based Encryption

Attribute-Based Encryption (ABE) is a promising cryptographic primitive which significantly enhances the versatility of access control mechanisms. Due to the high expressiveness of ABE policies, the computational complexities of ABE key-issuing (by Attribute Authorities (AAs)) and decryption (by eligible users) are getting prohibitively high. Despite that the existing Outsourced ABE solutions are able to offload some intensive computing tasks to a third party, for example, a cloud, so to relieve the local burden of eligible users during decryption, the high computational complexity of the key-issuing at the AAs has yet to be addressed, while an ABE system will continue to grow with more users being included, and with the user revocation being considered in practice which will trigger more key (re-)issuing. Aiming at tackling the challenges above, for the first time, we propose a Secure Outsourced ABE system, which not only supports secure outsourced decryption, but also provides secure outsourced key-issuing. Unlike the current outsourced ABE systems, our new method offloads all access policy and attribute related operations in the key-issuing process or decryption to a Key Generation Service Provider (KGSP) and a Decryption Service Provider (DSP), respectively, leaving only a constant number of simple operations for the AAs and eligible users to perform locally. Furthermore, we show that both outsourcing processes (to KGSP and to DSP) are secure, namely, the KGSP and the DSP would not be able to recover the keys or decrypt the ciphertexts, respectively. In addition, we consider the scenario that a KGSP or DSP may be dishonest and could maliciously generate some incorrect returning values rather than following the outsourced operations. Therefore, in this paper, we also propose another ABE construction which allows the AAs and eligible users to check the correctness of outsourced operations in an efficient way. The security of the construction is analyzed under a recently formalized model called Refereed Delegation of Computation (RDoC).