# Virtual Pseudonym-Changing and Dynamic Grouping Policy for Privacy Preservation in VANETs
## 2 Related Work
Based on this concept, a vehicle should keep silent and not broadcast beacon messages, to hamper pseudonyms linkage. A safe distance metric is introduced in [33] that searches radius for obfuscation in which the values of velocity, direction, and position are considered to hide vehicles' identity. If a vehicle does not find any neighbor at a safe distance, then it remains silent for a random period. In [34], the local parameters of speed and direction are taken as input to update the pseudonym autonomously. The vehicle will change pseudonyms in silent mode to meet a certain traffic threshold; otherwise, it will wait for another cooperative silent period. A context-based scheme is introduced in [17] in which the vehicle entry and exit from a silent period is based on the number of neighboring vehicles and pseudonyms are changed  
Figure 1: Location privacy protection techniques taxonomy. cooperatively. Similarly, in [18], the vehicle waits for at least \(k\) silent neighbors before changing the pseudonym. Further, misbehaving vehicles are detected in the network and considered before changing the pseudonym. Another silent period scheme uses the concept of scheme permutation [16], in which vehicles exchange pseudonyms. This increases the confusion of an adversary about the actual location of a vehicle. Limitations of silent period location privacy schemes include that they affect road safety applications; the use of a short silence period provides a way for pseudonyms to be linked, while for a long silent period the adversary may find temporal and spatial relationships to track the location of a vehicle in the network [15]. A path confusion scheme is proposed in [35] that slightly changes the informed positions of two users in proximity. This decreases the probability of vehicle tracking for fixed adversary strength. In [36], to increase adversary confusion in linking pseudonyms of a vehicle, inaccurate beacons are added in between the accurate beacon messages. For using location services, the concept of decoy vehicles is used in [37], in an attempt to protect the location of other vehicles in the network. The decoy vehicle communicates with LBS on behalf of network vehicles. To break the continuous path with LBS in [38], vehicles cooperate to generate plausible locations for each other to conceal/obfuscate the actual location information. Similarly, in [39], the target vehicle takes the surrounding neighboring vehicle's virtual location dynamically to mislead an adversary about the actual driver route on the road. In [40], a location privacy method based on blockchain is presented that satisfies the requirements of k-anonymity and does not rely on third party server. A route discovery protocol is presented in [41] that helps the source node to find an efficient path to a destination dynamically, and provides privacy protection to a node through a privacy-preserving verification process. A cache-based user location privacy protection scheme is introduced in [42] during communication with LBS. Limitations of using inaccurate data, introduced in these techniques is the impact on road network applications. In addition, extra overhead is introduced into the network due to duplicated data and redundant data. In the literature, much attention is given to the mix zone or silent period location privacy schemes; however, the mix zone concept only covers a limited area, and the vehicle privacy is protected only in the zone. In silent period location privacy techniques, vehicles do not broadcast beacon messages, and this can impact on road network applications, and sometimes vehicle change pseudonyms individually in particular road network cases (lower-traffic condition), which provides a way for an adversary to link the old pseudonym