*9.1. Protection against GPA*

The GPA can cover a large part of the network to capture beacon messages broadcast by vehicles. GPA analyzes the beacon messages for pseudo-identities of vehicles and matches them with old pseudonyms. If the adversary successfully matches these pseudonyms, then it can identify vehicles at various visited locations. Here, we examine the strength of GPA to extract the identity of a target vehicle. We investigate the strength of GPA with and without additional knowledge about a target vehicle. The additional information about the

target vehicle may be collected at some road intersections or frequently visited locations. The information may be vehicle frequently visited roads, old pseudonyms, and location of interest. Based on this information, the adversary tries to match the pseudonyms of a target vehicle at the earlier locations with pseudonyms changed at the new visited locations. This knowledge improves an adversary's strength to identify a target, which may be used for matching vehicle pseudonyms. Figure 18 shows the average confusion per trace of GPA with and without additional knowledge. The GPA with additional knowledge has lower confusion in identifying a vehicle, and without additional knowledge, confusion is increasing at a higher rate. Our proposed scheme CMC increases an adversary's confusion with additional information because it efficiently mixes the vehicle context under diverse traffic conditions. Similarly, Figure 19 shows the confusion for both GPA with and without additional knowledge under different vehicle traffic densities. The increasing number of vehicles improves the average confusion rate for the GPA. Eventually, it increases the protection level of the location privacy of a target vehicle.

**Figure 18.** Adversary confusion for vehicle traces.

**Figure 19.** Adversary confusion with a different number of vehicles.
