## 2 Background and Related Work
### Background  
#### 2.1.1 V2X Standards  
Internet of Things (IoT) is used in various ways to solve the problems of smart cities, and Cooperative Intelligent Transport Systems (C-ITS) can be applied as smart city IoT applications. IoT can provide C-ITS services that operate through inter-working with the cloud [5,6]. Data generated in the C-ITS environment can be used in various ways, and the collected data can be used to estimate travel times within the city [7]. V2X means vehicle to everything and is similar to IoT in that it can communicate with vehicles and other objects. However, V2X is a communication technology that focuses more on safety. Recently, various studies related to road safety and traffic efficiency using V2X based on 5G New Radio (NR) with high data rate, low latency, and a wide coverage are being conducted [8]. Figure 3 is a comparison of ITS stacks according to the V2X standard protocols. The V2X communication protocols used in the U.S. can be broadly divided into IEEE 802.11p-based Wireless Access in Vehicular Environment (WAVE) and Cellular V2X (C-V2X), which is based on cellulars, such as Long Term Evolution (LTE) and 5G. WAVE is a technology for short-range, high-speed communication defined by the IEEE 1609 standard family [9, 10, 11, 12] and has a shorter communication range than the cellular-based C-V2X. C-V2X uses specifications defined by Third Generation Partnership Project (3 GPP) in the lower-level physical layer, and Rel 14 [13,14] is used for the LTE-bases V2X, while Rel 16 [15] is used for the 5G-based V2X. On the other hand, WAVE and C-V2X use the WAVE Short Message Protocol (WSMP) defined by IEEE 1609.3 [11] in the higher-level network/transport layer and use V2X message sets defined by the SAE J2735 standard in the application layer. In Europe, ETSI ITS-G5 [16] corresponds to WAVE, and message sets, such as Cooperative Awareness Message (CAM) [17] and Decentralized Environmental Notification Message (DENM) [18], defined by ETSI EN 302 637, are used. Of course, the higher-level ETSI protocol can also be applied to C-V2X. This study provides descriptions focusing on V2X defined in the U.S., and this is the same for the proposed simulation-based V2X security evaluation method. SAE J2735 is a standard that defines the data frame and date element of message sets used in V2X communication [19]. Messages used for vehicle-to-vehicle (V2V) communication for basic driving status, for vehicle speed, global positioning system (GPS) information, heading, angle, brake, and route information are defined as Basic Safety Messages (BSM). Signal Phase and Timing (SPaT) for transmitting traffic signal information from the signal controller, MAP for transmitting road information such as intersections, RSA for roadside alerts, and Probe Vehicle Data (PVD) for collecting a vehicle's information via V2I are defined in the SAE J2735 standard. In the recently revised version, additional message sets have been defined for various autonomous driving-related services, such as Toll Advertisement Message (TAM) for tolling and Sensor Data Sharing Message (SDSM) for sensor data sharing. Table 1 describes the main message set of SAE J2735 standard. BSM is the most basic message in V2X. It broadcasts the surroundings through V2V to inform about the status of surrounding vehicles. Through this, the surrounding vehicles collect information about the pertinent vehicle and can use it in the Advanced Driver Assistance System (ADAS) or autonomous cooperative driving. BSM is divided into Part I data (information that must be essentially included) and Part II data (optional extension information). Part I data include essential information to inform about a vehicle's current status, such as the speed, GPS position, steering, and transmission information, to its surroundings. Part II data include supplementary information, such as a vehicle's route information, event information occurring in the vehicle, and the vehicle's size or type.