Decentralized Access Control on Data in the Cloud with Fast Encryption and Outsourced Decryption

Fine grained access control is a requirement for data stored in untrusted servers like clouds. Owing to the large volume of data, decentralized key management schemes are preferred over centralized ones. Often encryption and decryption are quite expensive and not practical when users access data from resource constrained devices. We propose a decentralized attribute based encryption (ABE) scheme with fast encryption and outsourced decryption. The main idea is to divide the encryption into two phases, offline preprocessing phase which is done when the device is otherwise not in use and an online phase when the data is actually encrypted with the policy. This makes encryption faster and more efficient than existing decentralized ABE schemes. For decryption outsourcing, data users need to generate a transformed version of the decryption key allowing an untrusted proxy server to partially decrypt the ciphertext without gaining any information about the plaintext. Data users can then fully decrypt the partially decrypted ciphertext without performing any costly pairing operations. Comparison with other CP-ABE schemes shows that our scheme significantly reduces computation times for both data owners and data users.