Key escrow-free attribute based encryption with user revocation

There exists a key escrow issue in ciphertext-policy attribute-based encryption (CP-ABE). The key generator center issues all users’ secret keys and can decrypt each ciphertext by calculating the corresponding secret key. Besides, applying CP-ABE in data sharing environment also brings another challenging problem which is users’ revocation. To resolve the above issues, we provide a key escrow-free CP-ABE scheme with the user revocation, which withstands collusion attack between malicious users and revoked users. In our scheme, a secret key is calculated utilizing a secure key issuing protocol between key authority (KA) and data user (DU). KA is unable to obtain DU’s secret value and generate the complete secret key independently, which solves the key escrow issue. When a user revokes from the system, the secret keys of the unrevoked users require to be updated. We introduce a group manager (GM) to update the unrevoked users’ group secret keys and generate a re-encryption key. The re-encryption technology is applied to prevent the revoked users from decrypting ciphertexts. Moreover, the decryption cloud server provider (D-CSP) executes most of decryption operations to decrease computation costs. The performance analysis indicates that our scheme is practical and efficient. The security of the presented scheme is reduced to divisible computable Diffie-Hellman (DCDH) assumption.