Unbounded Revocable Hierarchical Identity-Based Encryption with Adaptive-ID Security

Revocable hierarchical identity-based encryption (RHIBE) is a good candidate encryption for secure wireless communications. Its dynamic user management through the hierarchical key generation and revocation is efficient. Most existing RHIBEs are bounded which have to fix the maximum hierarchical depth of users at setup. In this paper, we propose the first unbounded and adaptively-ID secure RHIBE called URHIBE. Firstly, we design U-RHIBE extended from one unbounded HIBE scheme. The size of every secret key in our URHIBE is only linear with the hierarchical depth and the size of our public parameter is constant. Secondly, we design the first nested dual system encryption framework exclusively for the proof of the security of unbounded and revocable HIBE schemes. Our system model is scalable inherently to accommodate more levels of user adaptively with no adding workload or restarting the system. With a little increase of decryption cost comparing with Ryu's scheme, U-RHIBE firstly covers properties of adaptive-ID security, unbounded key delegation, efficient revocation, history-free key update, key exposure resistance and security against insiders.