  In this paper, we propose a Sybil attack detection scheme using proofs of
work and location. The idea is that each road side unit (RSU) issues a signed
time-stamped tag as a proof for the vehicle's anonymous location. Proofs sent
from multiple consecutive RSUs is used to create vehicle trajectory which is
used as vehicle anonymous identity. Also, one RSU is not able to issue
trajectories for vehicles, rather the contributions of several RSUs are needed.
By this way, attackers need to compromise an infeasible number of RSUs to
create fake trajectories. Moreover, upon receiving the proof of location from
an RSU, the vehicle should solve a computational puzzle by running proof of
work (PoW) algorithm. So, it should provide a valid solution (proof of work) to
the next RSU before it can obtain a proof of location. Using the PoW can
prevent the vehicles from creating multiple trajectories in case of low-dense
RSUs. Then, during any reported event, e.g., road congestion, the event manager
uses a matching technique to identify the trajectories sent from Sybil
vehicles. The scheme depends on the fact that the Sybil trajectories are
bounded physically to one vehicle; therefore, their trajectories should
overlap. Extensive experiments and simulations demonstrate that our scheme
achieves high detection rate to Sybil attacks with low false negative and
acceptable communication and computation overhead.
