Revocable hierarchical identity-based encryption

In practice, revocation functionality is indispensable to the public key cryptosystems since there are threats of leaking a secret key by hacking or legal situation of expiration of contract for using system. In the public key infrastructure setting, numerous solutions have been proposed, and in the Identity Based Encryption (IBE) setting, a recent series of papers proposed revocable IBE schemes. Delegation of key generation is also an important functionality in cryptography from a practical standpoint since it allows reduction of excessive workload for a single key generation authority. Although efficient solutions for either revocation or delegation of key generation in IBE systems have been proposed, an important open problem is efficiently delegating both the key generation and revocation functionalities in IBE systems. Even if the goal is very natural, there are some obstacles to achieve both functionalities at the same time. Libert and Vergnaud, for instance, left this as an open problem in their CT-RSA 2009 paper. In this paper, we propose the first efficient solution for this problem. We prove the selective-ID security of our proposal under the Decisional Bilinear Diffie-Hellman assumption in the standard model.