WINLAB Technical Report Using Public RF Sources to Build Secure Associations between Wireless Devices in Proximity

Forming new secure associations between wireless devices that meet for the first time and do not share a prior trust relationship is an important but challenging problem. We present ProxiMate, a system that allows wireless devices in proximity to securely pair with one another autonomously by generating a common secret key. ProxiMate works by making wireless devices in proximity tap into a surprisingly ambient source of randomness – namely, the fading wireless channel relative to a public source of airwaves, such as an FM or TV broadcast tower. Shared randomness is available only to the legitimate terminals in physical proximity, but not to a sufficiently distant adversary. ProxiMate enables the generation of a shared secret key, which can be used to communicate confidentially and/or to authenticate each others’ physical proximity. Key generation in ProxiMate is unconditionally secure, and does not require the adversary to be computationally bounded. We evaluate ProxiMate using analytical and experimental methods, using ambient FM and TV signals captured with a software defined radio. ProxiMate can speed up the time needed to generate a common key by monitoring multiple RF sources concurrently. Finally, we consider the case of an active adversary that has arbitrary control over the signal transmitted by the public source itself, and show that with a suitable modification to the system, ProxiMate can still extract a secret key securely.