Compact CCA2-secure Hierarchical Identity-Based Broadcast Encryption for Fuzzy-entity Data Sharing

With the advances of cloud computing, data sharing becomes easier for large-scale enterprises. When deploying privacy and security schemes in data sharing systems, fuzzy-entity data sharing, entity management, and efficiency must take into account, especially when the system is asked to share data with a large number of users in a tree-like structure. (Hierarchical) Identity-Based Encryption is a promising candidate to ensure fuzzy-entity data sharing functionalities while meeting the security requirement, but encounters efficiency difficulty in multiuser settings. This paper proposes a new primitive called Hierarchical Identity-Based Broadcast Encryption (HIBBE) to support multi-user data sharing mechanism. Similar to HIBE, HIBBE organizes users in a tree-like structure and users can delegate their decryption capability to their subordinates. Unlike HIBE merely allowing a single decryption path, HIBBE enables encryption to any subset of the users and only the intended users (and their supervisors) can decrypt. We define Ciphertext Indistinguishability against Adaptively Chosen-Identity-Vector-Set and Chosen-Ciphertext Attack (IND-CIVS-CCA2) for HIBBE, which capture the most powerful attacks in the real world. We achieve this goal in the standard model in two steps. We first construct an efficient HIBBE Scheme (HIBBES) against Adaptively Chosen-Identity-Vector-Set and Chosen-Plaintext Attack (IND-CIVS-CPA) in which the attacker is not allowed to query the decryption oracle. Then we convert it into an IND-CIVS-CCA2 scheme at only a marginal cost, i.e., merely adding one on-the-fly dummy user at the first depth of hierarchy in the basic scheme without requiring any other cryptographic primitives. Our CCA2-secure scheme natively allows public ciphertext validity test, which is a useful property when a CCA2-secure HIBBES is used to design advanced protocols and auditing mechanisms for HIBBE-based data sharing.