# Improved Chaff-Based CMIX for Solving Location Privacy Issues in VANETs
## 4 Security and Privacy Issues of [PERSON] et al.'s Chaff-Based CMIX Scheme  
[PERSON] et al. [14] proposed a protocol using Chaff messages in low-density scenarios to increase the number of fake vehicles which prevents linkability attacks. However, there is a security vulnerability in their protocol which is run outside the mix-zone. More concretely, the signature and the certificate are not encrypted in the following message which allows attackers to break the unlinkability:\[C_{PS_{VID_{j}}}=Enc_{PK_{VID_{j}}}(sk_{r},CF_{r},Sign_{RSU_{i}}),t_{j},Sign_{PS_{VID_{ j}}},Cert_{PS_{VID_{j}}},\]  
where \(PS_{VID_{j}}\) is the pseudo-identity of the \(j\)-th vehicle \(VID_{j}\), \(PK_{VID_{j}}\) is the public key of \(PS_{VID_{j}}\) while \(Enc_{PK_{VID_{j}}}\) denotes message (\(msg\)) encryption with the specified key, \(sk_{r}\) is the symmetric key of the \(r\)-th mix-zone, \(CF_{r}\) is the CF of the \(r\)-th mix-zone, \(Cert_{PS_{VID_{j}}}\) is the \(VID_{j}\)'s certificate, and \(Sign_{RSU_{i}}\), \(Sign_{PS_{VID_{j}}}\) denote \(RSU_{i}\)'s and \(VID_{j}\)'s signatures. In addition, the message contains non-encrypted values, namely the timestamp \(t_{j}\), the sender's signature \(Sign_{VID_{j}}(M_{V2R})\), and the certificate \(Cert_{VID_{j}}\). Nevertheless, sending these messages outside the mix-zone leads to the following security issues:  
* **The privacy of vehicles can be compromised.** Vehicles update their pseudo-identity once they move from one mix-zone to another. However, outside mix-zones, while they communicate with other vehicles securely, they also send \(Cert_{VID_{j}}\) and \(Sign_{VID_{j}}\) separately. Therefore, it is trivial for the adversary to link the old and the new pseudo-identities, meaning they can identify the sender, which breaks the unlinkability property.
* **Unreliable CFs update.** To update the CF, vehicles forward the \(CF_{i}\) as a new version in a ciphertext; [PERSON] et al. argue that the signature of \(RSU_{i}\) is attached to prevent forgery attacks. It is possible for malicious vehicles to inject real pseudo-identities and send them to the \(RSU_{i}\) inside the mix-zone or other vehicles, subsequently denying this malicious activity. Moreover, if an adversary compromises the \(RSU_{i}\) and tries to tamper with the CF, the vehicles will discard messages from these fingerprinted pseudo-identities, which leads to accidents by affecting safety applications.
* **The secret key of the mix-zones can cause leakage.** The ciphertext \(C\) contains the encryption key \(sk_{r}\), which must only be used in the specified area. The receiving of this key outside the mix-zone by an external unauthorised vehicle (i.e., the mix-zone is compromised) threatens the security of the communication. Hence, an attacker would be able to communicate maliciously with honest vehicles outside the specified area, which would break the system's safety.  
Moreover, we should note here that [PERSON] et al. [36] specified a drawback that may affect the CF's performance. In particular, they suggested that the false positives that can occur in a CF may affect the search for an element inside these hash tables. To fix this, they proposed a technique that identified the element that caused false positives, then removed it and re-inserted it again differently. Then, if they searched for that element, it would be found. Undoubtedly, the performance of the filter in a Chaff-based scheme plays a vital role. Thus, we are using ACF rather than CF in our scheme.