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 ineffective 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 systems make it difficult for users to demand recent revocation data. This paper argues that this sad state of affairs is an inexorable consequence of relying on unicast communication to distribute revocation information.We present REVCAST, a broadcast system that disseminates 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 broadcast link; specifically, we have designed REVCAST to operate over existing FM radio, although the principles apply to alternative implementations. We present the design, implementation, 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.