Don't Bump, Shake on It: the exploitation of a popular accelerometer-based smart phone exchange and its secure replacement

As the capabilities of smartphones increase, users are beginning to rely on these mobile and ubiquitous platforms to perform more tasks. In addition to traditional computing tasks, people are beginning to use smartphones to interact with people they meet. Often this interaction begins with an exchange, e.g., of cryptographic keys. Hence, a number of protocols have been developed to facilitate this exchange. Unfortunately, those protocols that provide strong security guarantees often suffer from usability problems, and easy-to-use protocols may lack the desired security guarantees. In this work, we highlight the danger of relying on usable-but-perhaps-not-secure protocols by demonstrating an easy-to-carry-out man-in-the-middle attack against Bump, the most popular exchange protocol for smartphones. We then present Shake on It (Shot), a new exchange protocol that is both usable and provides strong security properties. In Shot, the phones use vibrators and accelerometers to exchange information in a fashion that demonstratively identifies to the users that the two phones in physical contact are communicating. The vibrated information allows the phones to authenticate subsequent messages, which are exchanged using a server. Our implementation of Shot on DROID smartphones demonstrates that Shot can provide a secure exchange with a similar level of execution time and user effort as Bump.