Bridging the UI gap for authentication in smart environments

In this paper we describe a common problem when it comes to mutual authentication of devices in smart environments. Basically, there exist two approaches. In the brokered approach, two parties authenticate with the help of a common trusted party. However, this often introduces a single point of failure. The alternative approach lets the devices authenticate with each other directly, usually by exchanging a shared secret out of band. But this is only possible if both devices have the necessary user interface peripherals to establish an out-of-band channel. We will describe an abstract basic protocol to bridge the possible peripherals´ gaps by authenticating indirectly with the help of nowadays basically omnipresent multimedia devices such as smart phones. After extending the proposed protocol for more complex use cases, we refine it to a practical authenticated key establishment protocol for indirect authentication. Finally we describe our freely available prototype implementation consisting of an internet-of-things-enabled light bulb and light switch and an Android app that comprise our proof of concept.