Study On Ciphertext-Policy Attribute-Based Encryption

The concept of attribute-based encryption(ABE) was firstly put forward on EUROCRYPT by Sahai et.al, which drew large attention extensively. ABE can be divided into Ciphertext-Policy Attributed Based Encryption(CP-ABE) and Key-Policy Attributed Based Encryption(KP-ABE). In CP-ABE scheme, the private key of a user associated with an attribute set, while the ciphertext associated with a specific access structure, the user can decrypt the ciphertext successfully only when the attribute set of the user satisfy the access structure; however, KP-ABE scheme is opposite, the private key of a user associated with a specific access structure, while the ciphertext associated with an attribute set, the user can decrypt the ciphertext successfully only when the attribute set of ciphertext satisfy the access structure. With better data confidentiality and a more flexible access control, ABE has a broad prospect for applications. In practical applications, ABE will face different application requirements. Contraposing the updating requirement of access structure, Lai et.al put forward the concept of Adaptable Ciphertext-Policy Attribute-Based Encryption,(Adaptable CP-ABE), and then designed a specific scheme, which allowed a semi-trusted proxy to modify a ciphertext under one access policy into ciphertexts of the same plaintext under any other access policies. We firstly study on the scheme designed by Lai et.al, and then point out the disadvantage of the scheme that the calculated amount of cloud server is too heavy. To solve this problem, we proposed a modified adaptable CP-ABE scheme based on the original one, and then proved the security of our scheme. We also implements the scheme and analyzes its efficiency. It show that the computational efficiency of cloud server of the modified scheme is better than Lai et.al’ s. The modified scheme is appropriate for the cloud system whose access policy updates frequently or the system that has higher real-time demand.