# Privacy-Preserving Authentication Based on PUF for VANETs
## 2 Related Works  
VANETs have become a prevalent research field in the intelligent transportation system in order to avoid traffic congestion and accidents and enhance the driving experience. Several methods have been proposed for the security and privacy issues of VANETs. Zhu et al. [12] introduced a privacy-preserving authentication and data aggregation framework for fog-based smart grid systems. Their study outlines the architecture of a fog-based smart grid, addressing its applications, security, and privacy challenges. Additionally, they propose a privacy-preserving authentication and data aggregation scheme tailored for fog-based smart grids. This scheme leverages short randomizable and blind signatures to offer anonymous authentication under specific conditions. Furthermore, the integration of fog nodes addresses billing issues subsequent to anonymous authentication. To preserve privacy, Zhang et al. [13] introduced an authentication framework that integrates fifth-generation communication technology (5G) with edge computing, diverging from the architectural conventions of previous 802.11p-based inter-vehicle communication networks. Within this proposed framework, device-to-device technology serves as the conduit for communication between vehicles, deviating from the traditional model for VANETs. However, achieving secure communication within a 5G-enabled model poses a formidable challenge. To address this challenge, the proposed framework adopts a two-step approach to security authentication. Initially, authentication and the selection of an edge computing vehicle are imperative, leveraging a fuzzy logic mathematical method during the selection process. Subsequently, mutual authentication between edge computing and ordinary vehicles is executed. The procedural sequence facilitates the exchange of security information among vehicles within a group while simultaneously safeguarding the identity privacy and traceability of each vehicle [7]. To circumvent the redundant authentication of identical messages and identify invalid messages within a batch, Cui et al. [14] innovatively integrated an edge-computing concept into the message-authentication process of VANETs. Within their framework, an RSU can adeptly authenticate messages from neighboring vehicles and subsequently disseminate the results to vehicles within its communication range. This approach effectively mitigates redundant authentication procedures and enhances the overall system efficiency. Hathal et al. [15] put forward a certificateless and lightweight authentication scheme aimed at furnishing secure communication avenues for vehicle communication systems. In their study, they introduce authentication tokens as substitutes for digital certificates, thereby alleviating the administrative load associated with certificate management for a trusted authority (TA). Furthermore, the adoption of tokens guarantees the attainment of mutual authentication for V2I communication. Cui et al. [16] introduced a lightweight message authentication framework based on a reputation system tailored for 5G-enabled VANETs. Within this authentication framework, the TA assumes responsibility for reputation management. Notably, if a vehicle's reputation score falls below the specified threshold, it becomes ineligible to receive a credit reference from the TA, thus mitigating the influx of untrusted messages within VANETs at the source. However, none of the above schemes take into account the possibility that the vehicle could be physically attacked, leading to the disclosure of the secret key. In fact, with side-channel attacks, such as power, electromagnetic, and time usage attacks on vehicles, the attacker could still access the secret key stored in TPD. To combat this issue, a physical-preserving authentication based on PUF for VANETs is proposed by us. In the proposed scheme, we use PUF to generate a secret key only when it is needed for V2I authentication without ever storing the secret key in TPD's permanent storage, which ensures the physical security of the system.