# Improved Chaff-Based CMIX for Solving Location Privacy Issues in VANETs
## 7 A Comparison of Proposed and Existing Schemes  
In the previous section, we focused on three desired properties, namely mutual authentication, non-repudiation and unlinkability. Furthermore, it is also essential to show that our scheme is robust against low density. Note that previous studies have not given much attention to low density scenario. The comparison in Table 1 shows more factors related to existing VANETs' location privacy schemes. Moreover, it summarises that our scheme overcomes the vulnerabilities of [14] which are presented in Section 4. Here we illustrate a security and privacy vulnerabilities in related existing schemes:  
* Unlinkability: The fixed parameters schemes like [21] or schemes using the randomness such as [31] are vulnerable to linkability attacks. In addition, most of the existing works does not consider the traffic variability, which may help the attackers to break the unlinkability.
* Non-repudiation: Some schemes rely on identity changes after a random delay, if malicious vehicles do not change the pseudo-identity intentionally, then they will break the system. The randomness and the delay will break the non-repudiation. Moreover, this was also the case in [35] which is based on location obfuscation and hiding in the silent period. The location obfuscation means the vehicle can turn its speed to zero and obfuscate the position while changing the pseudo-identity. Hence, it affects the safety applications. Thus, if a malicious vehicle tried to harm those applications purposely, it can deny it because the location obfuscation is part of the system. The non-repudiation issues of [14] are related to the quality of \(CF\) that can breach the system. Furthermore, the scheme in [15], considers the \(RSU\) fully trusted; without a digital signature in the communication, it can deny any malicious activities.
* CMIX: The schemes tending to the CMIX are [13,14] and our scheme. The concept of cryptography in a mix-zone around \(RSU\) is efficient, but it is associated with road density. The higher the density, the better the effect against tracking.
* No interference with safety application: The necessity of updating the information via messages without any interruption, like silent period, assures the safety application quality. Scheme such as [10,35] using the silent period with different structure. However, this threatens the safety application technologies as reported in [8] technical report. Furthermore, other schemes like [14] that added filter for Chaff messages may weaken the safety application technologies if the filter has been corrupted. In addition, in [15] that relies on \(RSU\), if it is compromised, it will be easy to abuse the safety application and jeopardise the passengers' safety.  
[PERSON] et al. [14] evaluated the performance of their system using three metrics: chaff pseudonym pool size, the number of simultaneously active chaff pseudonyms, and the RSU signature generation overhead. We are consume more overhead because our system requires mutual authentication as we are using DDH. However, they did not measure the overhead of the filter's hash tables. Our scheme uses \(ACF\) which also uses hash functions while it has a lower false-positive rate compared to \(CF\). The \(ACF\) can find the elements that caused the false-positives after they occur, delete and re-insert them again in the hash table. Hence, it will help to find them in the search more efficiently than \(CF\)[36].