Timed-release encryption with pre-open capability and its application to certified e-mail system

Abstract

We propose timed-release encryption with pre-open capability. In this model, the sender chooses a release time to open the message and a release key to pre-open, and encrypts the message using them. The receiver can decrypt the message only after the release time. When the sender wants the message to be opened before the release time, he may publish the release key. Then, the receiver can decrypt the message from his private key and the release key before the release time. However, an adversary cannot extract any information at any time even with the release key. We formalize the security model and provide an efficient construction secure under the BDH assumption in the random oracle model. In addition, we discuss the application of our schemes to efficient fair exchange systems such as a certified e-mail system.