# Virtual Pseudonym-Changing and Dynamic Grouping Policy for Privacy Preservation in VANETs
## 2 Related Work  
This section contains a review of some of the existing privacy protection schemes in the literature. A pseudonym is used in a number of existing privacy schemes instead of the actual vehicle identity in the beacon message. The vehicle broadcasts beacons, which contain a pseudonym that anonymizes the vehicle. However, when using a fixed identifier (pseudonym) in a beacon message for a vehicle trip a privacy danger arises. An adversary can link the pseudonyms of a vehicle with the various locations visited. To protect a vehicle's privacy, a pseudonym must be periodically changed to prevent an adversary from locating a particular vehicle on the road network. There are several pseudonym changing schemes in the literature. Table 1 contains a comparative analysis of existing location privacy schemes. The detailed taxonomy diagram of location privacy techniques is shown in Figure 1. Some of the existing privacy-preserving strategies are discussed below. The cryptographic mix zone is established at a road-side intersection in [19], at which vehicles change temporary identifiers. The zone is combined with a mix network to provide pseudonym unlinkability. The vehicles in fixed zones are forced to change pseudonyms. The extended cryptographic zone scheme is proposed in [20] to tackle vulnerability to internal attackers. The RSUs are involved in the pseudonym-changing process. Similarly, in [21], mix zones are created in social spots such as parking lot. The vehicles that are gathered at social places change pseudonyms collectively to hide location information. The concept of a dynamic mix zone is introduced in [22], based on vehicles' requests. Traffic statistics and privacy requirements determine the size of the zone. The messages communicated in the zone are encrypted to avoid pseudonym linkability. Similarly, in [23], the vehicles form a mix zone dynamically and motivate the neighboring vehicles to take part in the pseudonym change process. A vehicle finds neighboring vehicles a similar status in [24] and exchange pseudonyms with neighbors based on trigger information, thereby confusing an adversary about the actual identity of a vehicle. [PERSON] et al. introduced the concept of multiple mix zones [25; 26], where the vehicles change pseudonyms dynamically. The pseudonym alteration is based on vehicle direction, displacement, and acceleration. [PERSON] et al. proposed a scheme using an independent mix zone [13] where each vehicle creates some randomized version of pseudonyms to establish an anonymous zone of vehicles on the road network. In [12], fixed mixing zones are establish at road intersections. Vehicles change pseudonyms only at the fixed zone to protect location information under different traffic conditions. However, the scheme creates an extra cost of deployment and suffer from duplicated data. A new concept of creating a silent mix zone is presented in [27; 28], where vehicles remain silent and exchanging their pseudonyms under the control of RSU. A distributed pseudonym-changing scheme is proposed in [29], also based on the construction of a silent mix zone. The zones are created based on vehicle traffic density. Location privacy strategies based on the mix zone concept have certain limitations. Firstly the cost of deploying zones at a road intersection requires infrastructure for implementation and of equipment installation, which can be costly. Secondly, in low vehicle traffic density it can be challenging to achieve the necessary level of privacy protection. In [30; 31], the concept of a silent period combined with group navigation is proposed for location privacy in a vehicular network. One of the vehicles is selected as a group leader (GL), and only the GL communicates with the network, while the remaining members of the group remain silent as a technique to hamper pseudonym linkability. Similarly, in [32], the vehicle remains silent under at lower speeds. When a vehicle is traveling at low speed, the probability of road accidents is lower.