# Improved Chaff-Based CMIX for Solving Location Privacy Issues in VANETs
## 3 Related Work  
Several types of research have been published on pseudo-identity unlinkability and vehicle privacy in ITS. This section includes our review of the most recent works on location privacy, which is also a significant element that needs to be considered [17], as well as pseudo-identity change management in VANETs. Due to the data transmission in VANETs for safety applications and the amplification in broadcasting technology, messages can be made open to adversaries; thus, it is easy to listen to the correspondence channels via the ITS infrastructure [18; 19; 20]. In this context, researchers have attempted to prevent the attacks related to linking the communications' information. Various studies have focused on unlinkability accomplishment by optimising the pseudo-identity change management mechanism to retain vital details, such as position and trajectory. However, we are now seeing that various literature schemes suffer from a vulnerability to linkability attacks that breach privacy. Note that the following parameters are listed in the ETSI ITS technical report [8] on pseudo-identity change management as not adequate for different reasons, namely: (1) fixed parameters, (2) silent period, (3) randomness, and (4) CMIX. [PERSON] et al. [21] suggested allowing a pool of pseudo-identities for each vehicle to use within one week, whereby each pseudo-identity will be valid for ten minutes without overlapping, which is a fixed parameters scheme. It will deter attacks, such as Sybil attacks, and address the trade-off between privacy and safety. However, the use of fixed parameters is straightforward, and this allows the adversary to know the parameter values of a given vehicle, making it easier to trace them. Choosing randomness dependent on fixed parameters, such as randomly changing the pseudo-identity every five minutes plus or minus one minute, helps deter the attacker from detecting a change in the pseudo-identity. However, it is still possible to link attacks when a few vehicles change their pseudo-identities while others retain their old ones. Furthermore, an attacker can track vehicles effortlessly by using trajectory predictability algorithms, such as Kalman filters [22], especially when the density is not high or when a few vehicles change their pseudo-identities while others maintain theirs [8]. Some researchers propose shutting off the wireless transmitter at an unspecified point and changing pseudo-identities during the silent period [10; 23; 24; 25; 26; 27]. Vehicles would have adequate protection during this time, but this would dramatically limit protection due to the non-broadcasting series of CAMs, which would lead to an increase in vehicle accidents. In addition to the silent period, [PERSON] et al. propose changing the pseudo-identity when the vehicle's speed is less than 30 km/h [26]; however, this does not take into account low-density situations that lead to linking attacks. On the other side, [PERSON] et al. in [10] suggested a Vehicular Location Privacy Zone (VLPZ) to regulate service stations (e.g., diesel, fuel and charging stations or toll booths). However, syntactic linking attacks can easily occur due to the lack of coordination caused by the silent period [28], and the attackers may track the pseudo-identity change [22]. Additionally, the link attack's impact worsens in low-density situations due to the simplicity of analysing a low number of vehicles. Conversely, the mix-zones concept does not need to limit the feasibility of safety applications. Initially, the mix-zone approach was proposed by [PERSON] et al. [12], leading to the use of a pre-defined geographical region to change pseudo-identities. These zones have a CA hierarchy, and the semi-honest _RSUs_ dominate the mix-zone. [PERSON] et al. enhanced the scheme by inserting a symmetric key to encrypt messages among vehicleswithin the mix-zone and called it a CMIX [13].