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Attribute-based encryption, especially for ciphertext-policy attribute-based encryption, can fulfill the functionality of fine-grained access control in cloud storage systems. Since users' attributes may be issued by multiple attribute authorities, multi-authority ciphertext-policy attribute-based encryption is an emerging cryptographic primitive for enforcing attribute-based access control on outsourced data. However, most of the existing multi-authority attribute-based systems are either insecure in attribute-level revocation or lack of efficiency in communication overhead and computation cost. In this paper, we propose an attribute-based access control scheme with two-factor protection for multi-authority cloud storage systems. In our proposed scheme, any user can recover the outsourced data if and only if this user holds sufficient attribute secret keys with respect to the access policy and authorization key in regard to the outsourced data. In addition, the proposed scheme enjoys the properties of constant-size ciphertext and small computation cost. Besides supporting the attribute-level revocation, our proposed scheme allows data owner to carry out the user-level revocation. The security analysis, performance comparisons, and experimental results indicate that our proposed scheme is not only secure but also practical.
Li et al. (2016) studied this question.