RevCast: Fast, Private Certificate Revocation over FM Radio

The ability to revoke certificates is a fundamental feature of a public key infrastructure. However, certificate revocation systems are generally regarded as ine↵ective and potentially insecure: Some browsers bundle revocation updates with more general software updates, and may go hours, days, or indefinitely between updates; moreover, some operating sys-tems make it dicult for users to demand recent revocation data. This paper argues that this sad state of a↵airs is an inexorable consequence of relying on unicast communication to distribute revocation information. We present RevCast, a broadcast system that dissemi-nates revocation data in a timely and private manner. Rev-Cast is not emulated broadcast over traditional Internet links, but rather a separate metropolitan-area wireless broad-cast link; specifically, we have designed RevCast to oper-ate over existing FM radio, although the principles apply to alternative implementations. We present the design, imple-mentation, and initial deployment of RevCast on a 3 kW commercial radio station using the FM RDS protocol. With the use of two types of receivers (an RDS-to-LAN bridge that we have prototyped and an RDS-enabled smartphone), we show that, even at a low bitrate, RevCast is able to deliver complete and timely revocation information, anonymously, even for receivers who do not receive all packets all the time