Private Signaling

We introduce the problem of private signaling. In this problem, a sender posts a message on a certain location of a public bulletin board, and then posts a signal that allows only the intended recipient (and no one else) to learn that it is the recipient of the message posted at that location. Besides privacy, two efficiency requirements must be met. First, the sender and recipient do not participate in any out-of-band communication. Second, the overhead of the recipient must be (much) better than scanning the entire board. Existing techniques, such as server-aided fuzzy message detection (Beck et al., CCS'21), could be employed to solve the private signaling problem. However, this solution leads to a trade-off between privacy and efficiency, where the complexity of the recipient grows with the required privacy. Specifically, this would require a scan of the entire board to obtain full privacy for the recipient. In this work, we present a server-aided solution to the private signaling problem that guarantees full privacy for all recipients while requiring only constant amount of work for both the recipient and the sender. Specifically, we provide three contributions: First, we provide a formal definition of private signaling in the Universal Composability (UC) framework and show that it captures several real-world settings where recipient anonymity is desired. Second, we present two server-aided protocols that UC-realize our definitions: one using a single server equipped with a trusted execution environment, and one based on two servers that employ garbled circuits. Third, we provide an open-source implementation of both of our protocols, evaluate their performance, and identify for which sets of parameters they can be practical.