# A Distributed Mix-Context-Based Method for Location Privacy in Road Networks
## 2 Related Work  
As mentioned, a pseudonym can be used in beacon messages, rather than the real identity of a vehicle, as a means to provide privacy. However, some limited knowledge of the whereabouts of a vehicle means that the pseudonym of a vehicle can be identified, and all journeys of that vehicle can be recovered. For this reason, pseudonyms are changed periodically. However, simple pseudonym-exchange schemes may suffer from pseudonym linkability. That is, an adversary can discover the relationship between pseudonyms and hence recover vehicle journeys. Several schemes of pseudonym changing are proposed and tested in the literature; here, we review some of these. A taxonomy of location privacy schemes is shown in Figure 1. Table 1 contains details of a comparative analysis of existing schemes in a vehicular network. A group navigation, in combination with a random silence period, is proposed in [21]. The vehicle remains silent, not broadcasting beacons in the network for a random period to avoid linkability. The vehicles are restricted to forming groups on the road, and the group leader will broadcast messages while other members of the group remain silent. Similarly, an advanced version is presented in [27] again using the combination of silent periods with the group formation concept. The vehicles remain silent if a certain low-speed threshold is met (below 30 km/h) and should change pseudonyms during this period as given in [28]. This means that a vehicle will not broadcast heartbeat messages at slow speeds, with the justification that the possibility of an accident during lower speeds is low. In [29], a safe-distance metric is used to find an obfuscation radius in which the value of velocity, position, and direction is perturbed to enhance the privacy of vehicles. In addition, if a vehicle did not find any other vehicle within a safe distance, it remains silent to preclude tracking. An autonomous pseudonym update mechanism is presented in [23], which takes the speed and direction as parameters. If a certain traffic weight threshold is met, the vehicle will update its pseudonym in a silent mode, otherwise the vehicle waits for one more silent period. In the context-based scheme, the vehicle adaptively enters and exits from a silent period, changing the pseudonym based on the number of silent neighboring vehicles [30]. A similar scheme is introduced in [31] in which vehicle changes its pseudonym based on the context of silent neighbors. It also assesses the presence of misbehaving vehicles in the network and checks the success of the pseudonym changing process. Another scheme is based on a silent period that uses the concept of permutation to exchange pseudonyms between vehicles to create confusion for an adversary attempting location identification [32]. Silent period-based schemes have certain limitations, i.e., it impacts road network applications, the management of silent period duration for a vehicle trip is difficult to utilize for location protection: the use of a short silent period can provide one way to measure the effectiveness of a pseudonym linkability attack, while, for a long silence period, the knowledge of spatial and temporal relationship makes it possible for an adversary to track the vehicle [33]. A group communication method with a random encryption period is introduced in [20; 34] for location privacy in VANETs. The scheme increases the confusion of the external attacker by creating an encryption zone around a vehicle's OBU. Another synchronized pseudonym changing protocol is proposed in [35] to provide unlinkability of the vehicle location tracks. The main aim is to break the spatial and temporal relationship of the vehicle pseudonyms. A data forwarding protocol is used in [36] for location privacy based on the social behavior of a vehicle driver. The social behavior of vehicles is collected from visiting social spots, i.e., shopping malls, intersections, and museums. If a vehicle visits a social spot, it can retrieve messages from RSU anonymously.