## 3 Security Issues Facing CVs and Solutions
### Cellular Network  
Cellular networks are considered a potential source that can guarantee the mobility and seamless connection in V2V and V2I. Existing solutions to connect vehicles together through widely deployed cellular infrastructure can be divided into two modes: brought-in and built-in [49]. The brought-in connectivity refers to that users in a vehicle tether their own smart phone to the vehicle's infotainment system such that the vehicle gains immediate access to the internet and some duplicate functions of a smartphone. Incidentally, built-in connectivity integrates the cellular module into a vehicle's on-board infotainment system, and the internet connection relies on a built-in module. C-V2X is a part of the overall 3 GPP process to advance cellular systems from 4G to 5G technologies. Starting from Release 14 published by 3 GPP, the LTE Direct and LTE broadcast laid the foundation for C-V2X, while after Release 16+, the 5G technology built new capabilities for C-V2X networks and augmented C-V2X direct communications over time [51; 52]. As [PERSON], [PERSON] et al. highlighted in [53], the power of 5G enables cellular and mobile communication networks to connect to hundreds of billions of devices with extreme-high throughput and extreme-low latency, including IoT, smart-connected vehicles, smart cities, smart agriculture, smart retail, and intelligent transportation systems. Built on many existing cellular infrastructures, the C-V2X service covers large areas, and has a high penetration rate and low cost to potentially support the high-bandwidth demands and QoS-sensitive requirements of vehicular applications. C-V2X defines multi-types of services, including V2V, V2P and V2I, and V2N, and two modes of transmission, including network-based communication and direct communication [54]. It is also designed for both in-coverage and out-of-coverage services. C-V2X direct communications can support active safety and enhance situational awareness by detecting and exchanging information using low-latency transmission in the 5.9 GHz ITS band for vehicle-to-vehicle (V2V) as well as V2I and V2P scenarios. In practice, C-V2X needs to address some technical challenges, such as a high Doppler effect, resource scheduling, and synchronization [55]. \begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline  & **Application** & **Attacks** & **Existing Enhancement** & **Open Issues** \\ \hline Bluetooth & V2P and specific scenarios with low density and low speed of vehicles (e.g., rural roads). & Pin interception injection of fake pin. Traceability attack. Infection attack. & Strong PIN authentication. Frequency hopping. Pre-shared key for authentication and encryption. & Security risks caused by different pairing modes. \\ \hline Wi-Fi & Built-in or through-in. Scenarios: V2V, V2I, V2I, V2P, etc. & DoS, cracking, rekeying, karma attack, etc. & MIMO to improve transmission. Passpoint/Hospot 2.0 provide Wi-Fi with security, WPA 2. & Long establishing time, including association, authentication, etc. Unsecure mode (e.g., WPA 2 is cracked). \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of Bluetooth and Wi-Fi in connected vehicles. Concerning security, C-V2X imposes some specific requirements on current cellular networks [56; 57]. Currently, the security key in cellular networks is priori-configured. Future direct communication between two vehicles should communicate without having to be provisioned with a shared key; D2D communications cannot rely on pre-shared pairwise keys. More importantly, pre-shared keys alone cannot provide a non-repudiation service. Therefore, the receiver needs a way to verify whether the received message was transmitted from a trusted entity. Secondly, no integrity protection on application layer messages is provided in current communication between the user and network or D2D communication. Confidentiality for safety messages can be ignored, but strong integrity is paramount.