Scalable, Server-Passive, User-Anonymous Timed Release Cryptography

We consider the problem of sending messages into the future, commonly known as timed release cryptography. Existing schemes for this task either solve the relative time problem with uncontrollable, coarse-grained release time (time-lock puzzle approach) or do not provide anonymity to senders and/or receivers and are not scalable (server-based approach). Using a bilinear pairing on any Gap Diffie-Hellman group, we solve this problem by giving scalable, server-passive and user-anonymous timed release public-key encryption schemes allowing precise absolute release time specifications. Unlike the existing server-based schemes, the trusted time server in our scheme is completely passive - no interaction between it and the sender or receiver is needed; it is even not aware of the existence of a user, thus assuring the privacy of a message and the anonymity of both its sender and receiver. Besides, our scheme also has a number of desirable properties including a single form of update for all users, self-authenticated time-bound key updates, and key insulation, making it a scalable and appealing solution. It could also be easily generalized to a more general policy lock mechanism