  The wireless medium contains domain-specific information that can be used to
complement and enhance traditional security mechanisms. In this paper we
propose ways to exploit the fact that, in a typically rich scattering
environment, the radio channel response decorrelates quite rapidly in space.
Specifically, we describe a physical-layer algorithm that combines channel
probing (M complex frequency response samples over a bandwidth W) with
hypothesis testing to determine whether current and prior communication
attempts are made by the same user (same channel response). In this way,
legitimate users can be reliably authenticated and false users can be reliably
detected. To evaluate the feasibility of our algorithm, we simulate spatially
variable channel responses in real environments using the WiSE ray-tracing
tool; and we analyze the ability of a receiver to discriminate between
transmitters (users) based on their channel frequency responses in a given
office environment. For several rooms in the extremities of the building we
considered, we have confirmed the efficacy of our approach under static channel
conditions. For example, measuring five frequency response samples over a
bandwidth of 100 MHz and using a transmit power of 100 mW, valid users can be
verified with 99% confidence while rejecting false users with greater than 95%
confidence.
