Provably Secure Timed-Release Proxy Conditional Reencryption

Cloud storage service is getting popular nowadays. It can help users synchronize their files stored in various devices and allows them to share their files with others. However, it has been indicated that the privacy of the stored files is not protected well in cloud service. Recently, many solutions have been proposed in literature, but most of them cannot support secret file sharing when they are deployed in a cloud environment. In addition, in some situations, a file owner may hope that the receiver cannot get the actual file content until a specified time arrives. Nevertheless, most of the existing solutions cannot achieve these goals simultaneously. In this paper, we propose a timed-release proxy conditional reencryption scheme for cloud computing. Not only is the privacy of users' files guaranteed but also each user can freely share some desired file with another user under the restriction on the time factor. In other words, a receiver cannot obtain any information about the file until a designated time arrives. It also provides flexible encryption. Finally, we demonstrate the security of our proposed scheme via formal proofs based on the random oracle model.