# Improved Chaff-Based CMIX for Solving Location Privacy Issues in VANETs
## 1 Introduction  
Intelligent transportation systems (ITS), particularly Vehicular Ad Hoc Networks (VANETs), are constantly growing in importance. Efficiency and security are achieved in VANETs mainly through safety applications and non-safety applications such as entertainment and internet access. In safety applications, beaconing services are necessary because they are essential for ITS effectiveness; otherwise, accidents can occur. Open networks that are accessible by any node are characteristic of VANETs. In general, the two types of communication performed by VANETs are Vehicle to Vehicle (V2V) and Vehicle to Infrastructure (V2I), which communicate through the latest Radio Access Technology (RAT) IEEE 802.11 bd for Dedicated Short Range Communications (DSRC) and New RadioNR-V2X for Cellular-V2X (C-V2X). They reduce the packet collisions [1] and they can work in tunnels and confined areas [2]. The On-Board Units (OBUs) of vehicles must transmit Cooperative Awareness Messages (CAMs) as safety messages due to their high mobility, providing real-time information on velocity, location, and heading [3]. In compliance with international standards (i.e., IEEE 1609.2 WG [4] and the European Telecommunications Standards Institute (ETSI) ITS [5]), to ensure the integrity, non-repudiation and authenticity of messages, vehicles and Roadside Units (_RSUs_) are formed with public and private key pairs, in addition to digital signatures. Furthermore, established safety requirements based on Vehicle Public Key Infrastructure (VPKI) require multiple Certificate Authorities (CAs) to administer certificates for the underlying bodies [6; 7]. These CAs permit long-term certificates for vehicles and _RSUs_ after registration. Later, they grant certificates based on pseudo-identities and \"anonymous credentials\" to deter some types of road attacks. However, the standard ETSI [8] body also suggests changing pseudo-identities in combination with modifying all communication stack layer identifiers, such as the Network Access Control (MAC) and the Internet Protocol (IP) addresses [9]. Nevertheless, a passive attacker who gathers information from these CAMs can comfortably perform location tracking by _syntactic linking attack_, referring to the old and new vehicle pseudo-identities. Additionally, adversaries can link pseudo-identities by analysing signed messages' contents, and the adversary can accurately determine the next location of the car. This is defined as a _Semantic linking attack_[10]. We should note that the semantic link attack is more powerful than the syntactic link attack because the adversary produces an attack based on the details included in the security messages used to link the pseudo-identities, which produces better results [11]. Various research works have focused on managing the pseudo-identity change to solve these linking attacks over the last few years. For example, some techniques recommend that vehicles set up a silent period, i.e., their transmitters stay off (do not send messages) for a specific duration after changing their pseudo-identities, although they can still accept and process incoming messages [8]. Nonetheless, while this tends to make tracking very difficult, safety applications may be impaired because vehicles cannot send safety messages during this time. As a consequence, the probability of collision rises for such techniques. An alternative is the idea of a mix-zone, which is pre-defined as a geographic region (bound to the _RSU_ coverage) in which vehicles exchange messages, except for position-related messages since they are in the same place, and they change pseudo-identities within that region. Researchers have suggested improving the privacy strategy for pseudo-identity transition techniques in Cooperative-ITS (C-ITS) [12]. [PERSON] et al. [13] suggested encrypting the exchanged messages inside the mix-zone and called it the CMIX strategy.