Relay-Proof Channels Using UWB Lasers

Consider the following situation: Alice is a hand-held device, such as a PDA. Bob is a device providing a service, such as an ATM, an automatic door, or an anti-aircraft gun pointing at the gyro-copter in which Alice is travelling. Bob and Alice have never directly met before, but share a key as a result of secure hand-offs. Alice has used this key to request a service from Bob (dispense cash, open door, don’t shoot). Before complying, Bob needs to be sure that it really is Alice that he can see in front of him, and not Mort. Mort and her accomplice Cove are planning a wormhole attack along classic man-in-the-middle lines. They wait until Alice is in a situation where she expects to be challenged by Bob, and then Mort pretends to Bob that she is Alice, while Cove pretends to Alice that he is Bob. Mort and Cove relay the appropriate challenges and responses to one another over a channel hidden from Alice and Bob, in order to allow the dual masquerade to succeed, following which Alice waits impatiently in front of a different ATM, or the wrong door, or another gun. How can such an attack be prevented? Obviously it suffices if Alice and Bob both have a secure way of identifying their (relative) location to sufficient accuracy. But how can they do this? Suppose that we could enclose Bob inside a Platonic Faraday cage, which blocks all information-bearing signals, not just the RF ones. At the end of the protocol run, Bob could be confident that Alice was also inside the Faraday cage. Provided the Platonic Faraday cage was of sufficiently small size and shape to satisfy the appropriate proximity requirement, Bob could proceed. One way of providing such a Platonic Faraday cage is to use the laws of physics: for example distance-bounding protocols use the fact that the speed of light is finite. In this position paper, we argue that the laws of information theory can be used instead. The key insight is that it doesn’t matter if the Platonic Faraday cage leaks some information, provided the amount of information that Mort and Cove need to exchange is greater. Previous work in this direction has relied upon restricting the rate at which Mort and Cove can communicate, essentially using a variant of covert channel analysis to reduce the rate of leakage. We propose the converse approach, using Ultra Wide Band (UWB) lasers to transfer so much information from Bob to