  Cooperative vehicle platooning significantly improves highway safety, fuel
efficiency, and traffic flow. In this model, a set of vehicles move in line
formation and coordinate acceleration, braking, and steering using a
combination of physical sensing and vehicle-to-vehicle (V2V) messaging. The
authenticity and integrity of the V2V messages are paramount to safety. For
this reason, recent V2V and V2X standards support the integration of a PKI.
However, a PKI cannot bind a vehicle's digital identity to the vehicle's
physical state (location, velocity, etc.). As a result, a vehicle with valid
cryptographic credentials can impact platoons from a remote location.
  In this paper, we seek to provide the missing link between the physical and
the digital world in the context of vehicle platooning. We propose a new access
control protocol we call Proof-of-Following (PoF) that verifies the following
distance between a candidate and a verifier. The main idea is to draw security
from the common, but constantly changing environment experienced by the closely
traveling vehicles. We use the large-scale fading effect of ambient RF signals
as a common source of randomness to construct a {\em PoF} primitive. The
correlation of large-scale fading is an ideal candidate for the mobile outdoor
environment because it exponentially decays with distance and time. We evaluate
our PoF protocol on an experimental platoon of two vehicles in freeway,
highway, and urban driving conditions. We demonstrate that the PoF withstands
both the pre-recording and following attacks with overwhelming probability.
