Attribute-Based Encryption Supporting Direct/Indirect Revocation Modes

Attribute-based encryption (ABE) enables an access control mechanism over encrypted data by specifying access policies among private keys and ciphertexts. In this paper, we focus on ABE that supports revocation. Currently, there are two available revocable ABE schemes in the literature. Their revocation mechanisms, however, differ in the sense that they can be considered as direct and indirect methods. Direct revocation enforces revocation directly by the sender who specifies the revocation list while encrypting. Indirect revocation enforces revocation by the key authority who releases a key update material periodically in such a way that only non-revoked users can update their keys (hence, revoked users' keys are implicitly rendered useless). An advantage of the indirect method over the direct one is that it does not require senders to know the revocation list. In contrast, an advantage of the direct method over the other is that it does not involve key update phase for all non-revoked users interacting with the key authority. In this paper, we present the first Hybrid Revocable ABE scheme that allows senders to select on-the-fly when encrypting whether to use either direct or indirect revocation mode; therefore, it combines best advantages from both methods.