  Previous work on misbehavior detection and trust management for
Vehicle-to-Everything (V2X) communication can identify falsified and malicious
messages, enabling witness vehicles to report observations about
high-criticality traffic events. However, there may not exist enough "benign"
vehicles with V2X connectivity or vehicle owners who are willing to opt-in in
the early stages of connected-vehicle deployment. In this paper, we propose a
security protocol for the communication between vehicles and infrastructure,
titled Proof-of-Travel (POT), to answer the research question: How can we
transform the power of cryptography techniques embedded within the protocol
into social and economic mechanisms to simultaneously incentivize
Vehicle-to-Infrastructure (V2I) data sharing activities and validate the data?
  The key idea is to determine the reputation of and the contribution made by a
vehicle based on its distance traveled and the information it shared through
V2I channels. In particular, the total vehicle miles traveled for a vehicle
must be testified by digital signatures signed by each infrastructure component
along the path of its movement. While building a chain of proofs of spatial
movement creates burdens for malicious vehicles, acquiring proofs does not
result in extra cost for normal vehicles, which naturally want to move from the
origin to the destination. The proof of travel for a vehicle can then be used
to determine the contribution and reward by its altruistic behaviors. We
propose short-term and long-term incentive designs based on the POT protocol
and evaluate their security and performance through theoretical analysis and
simulations.
