Clock-Based Proxy Re-encryption Scheme in Unreliable Clouds

In this paper, we propose a clock-based proxy re-encryption (C-PRE) scheme to achieve fine-grained access control and scalable user revocation in unreliable clouds. Our scheme, which is built on top of cipher text-policy attribute-based encryption (CP-ABE) and proxy re-encryption (PRE), allows the data owner and the cloud to share a secret key in advance, with which the cloud can be delegated to re-encrypt data on behalf of the data owner. The main merit of our scheme is that the cloud can automatically re-encrypt data based on its internal clock without receiving any command.