# Improved Chaff-Based CMIX for Solving Location Privacy Issues in VANETs
## 3 Related Work
While CMIX-based systems are sensitive to linking attacks, they depend heavily on vehicle traffic, vehicle arrival, speeds, and probable vehicle movements through mixed zones. These schemes are also vulnerable to linking attacks in low-density [8] scenarios. [PERSON] et al. [29] proposed changing pseudo-identities at social spots, i.e., a public space, which gives the benefit of traffic to confuse the attacker. For example, vehicles stopped at traffic lights can change their pseudo-identities when these turn green, and in shopping mall parking, vehicles can change their pseudo-identities before they exit. However, this scheme is simplistic and inadequate to guarantee unlinkability because high density does not guarantee that certificates can be linked and traced back to identity. The authors in [30] proposed a context-based system to use credibility scores sent in order to cause synchronous pseudo-identity changes; those scores are part of the periodic safety beacons, thus increasing the anonymity set. With the same, i.e., context-based, approach [PERSON] et al. [31] presented another pseudo-identity change management technique, which is an entirely uncoordinated pseudo-identities change in distributed networks after a random delay to allow regular and unlinkable changes, whereby they suppose that the anonymity can be accomplished at trivial throughput loss is in large networks. [PERSON] et al. [32] proposed a pseudo-identity changing game that is mixed-context and was developed by examining the relationship between changing the pseudo-identity, expense, and privacy. Moreover, [PERSON] and [PERSON] [33] suggested pseudo-identity changing privacy preservation authentication based on a mixed-context. However, attackers can easily threaten these systems if the density is low, as with traffic-based techniques. Furthermore, a dynamic zone-based pseudo-identity change management [34] has recently been proposed to set up a temporary on-demand swap zone in which a vehicle will randomly pick and exchange a pseudo-identity with another vehicle without a group manager. This change adapts to reduce the contact cost of establishing pseudo-identity swap zones based on the environment. [PERSON] et al. 's [35] is proposal based on two main factors: the strategies of \"hiding inside the crowd\" and \"location obfuscation\". When the vehicle either exits a particular geographical area or the pseudo-identity reaches its expiry, it must change it. This change management holds the count of the neighbouring vehicles, and if the pre-defined neighbour threshold matches the current neighbours, then it changes pseudo-identity with other vehicles cooperatively. Otherwise, for the vehicle to change its pseudo-identity, the vehicle obfuscates its location and turns the speed to zero. The downside is that the unreliable broadcasting of speed and location information poses questions regarding safety applications. In 2018, [PERSON] et al. [14] introduced a development for CMIX by adding Chaff messages (representing vehicles on the road) to stabilise the density in low-density scenarios in CMIX. Hence, the Chaff-based CMIX protocol alleviates [PERSON] et al. 's CMIX scheme's weaknesses in low-density situations by confusing the attacker to strengthen the CMIX scheme. As ETSI ITS confirmed the importance of considering the low density as mentioned in their report \"the higher the density of vehicles, the more efficient the mix-zone is against tracking.\" [8]. Moreover, this scenario is possible in the peak and off peak hours daily. Furthermore, the lockdown of the Coronavirus Disease of 2019 (COVID-19) pandemic raises the chances of low-density scenarios in different countries. In fact, this emphasises the value of [PERSON] et al. 's scheme. However, their Chaff-based CMIX scheme has some issues that could break the system.