  Autonomous vehicle platooning promises many benefits such as fuel efficiency,
road safety, reduced traffic congestion, and passenger comfort. Platooning
vehicles travel in a single file, in close distance, and at the same velocity.
The platoon formation is autonomously maintained by a Cooperative Adaptive
Cruise Control (CACC) system which relies on sensory data and
vehicle-to-vehicle (V2V) communications. In fact, V2V messages play a critical
role in shortening the platooning distance while maintaining safety. Whereas
V2V message integrity and source authentication can be verified via
cryptographic methods, establishing the truthfulness of the message contents is
a much harder task.
  This work establishes a physical access control mechanism to restrict V2V
messages to platooning members. Specifically, we aim at tying the digital
identity of a candidate requesting to join a platoon to its physical trajectory
relative to the platoon. We propose the {\em Wiggle} protocol that employs a
physical challenge-response exchange to prove that a candidate requesting to be
admitted into a platoon actually follows it. The protocol name is inspired by
the random longitudinal movements that the candidate is challenged to execute.
{\em Wiggle} prevents any remote adversary from joining the platoon and
injecting fake CACC messages. Compared to prior works, {\em Wiggle} is
resistant to pre-recording attacks and can verify that the candidate is
directly behind the verifier at the same lane.
