  The explosive growth of vehicle amount has given rise to a series of traffic
problems, such as traffic congestion, road safety, and fuel waste. Collecting
vehicles' speed information is an effective way to monitor the traffic
condition and avoid vehicles being congested, which however may bring threats
to vehicles' location and trajectory privacy. Motivated by the fact that
traffic monitoring does not need to know each individual vehicle's speed and
the average speed would be sufficient, we propose a privacy-preserving traffic
monitoring (PPTM) scheme to aggregate vehicles' speeds at different locations.
In PPTM, the roadside unit (RSU) collects vehicles' speed information at
multiple road segments, and further cooperates with a service provider to
calculate the average speed information for every road segment. To preserve
vehicles' privacy, both homomorphic Paillier cryptosystem and super-increasing
sequence are adopted. A comprehensive security analysis indicates that the
proposed PPTM can preserve vehicles' identities, speeds, locations, and
trajectories privacy from being disclosed. In addition, extensive simulations
are conducted to validate the effectiveness and efficiency of the proposed PPTM
scheme.
