Fine-Grained Access Control for Cloud Data Sharing by Secure and Efficient Attribute-Revocable Ciphertext-Policy Attribute-Based Encryption

Nowadays, more and more users outsource their data to third party cloud storage servers for the purpose of sharing, so cloud data sharing becomes one of the popular services offered by cloud service providers. However, the third party storage servers in cloud data sharing systems, which are not fully trusted by data owners, make access control to the shared data a challenging issue. Although Ciphertext-Policy Attribute-Based Encryption (CP-ABE) is an emerging cryptographic solution for this issue, dealing with dynamic changes to users' access privileges (attribute revocation) in its practical applications as cloud data sharing systems is a real challenge. To overcome this challenge, we propose a fine-grained access control scheme for cloud data sharing systems by designing secure and efficient attribute-revocable CP-ABE scheme. Our scheme only allows non-revoked users in the attribute group to update their secret key by themselves using their unique key-update keys and the ciphertexts are updated by minimally trusted cloud server using a ciphertext-update key. Compared with the existing access controls achieved by attribute-revocable CP-ABE schemes, our proposed access control scheme reduces the trust degree of the cloud server in the attribute revocation mechanism. Furthermore, the analysis indicates that our access control scheme is more secure and efficient to apply to practical scenarios.