## 2 Background and Related Work
### Background
#### 2.1.2 V2X Security Evaluation Method  
In V2X communication, the latency requirement is less than 100 ms because communication is performed between fast-moving vehicles [23; 24]. As a major accident can occur if the incorrect data are transmitted, data have to be sent and received quickly. Efficient security is provided using certificate-based digital signatures for V2X communication security. In V2X communication, encryption is used when the vehicle updates the certificate key. Encryption is not provided when sending general SAE J2735-based message sets; instead, the authentication, integrity, and non-repudiation properties are provided through the digital signatures. Since a BSM is a message broadcasted to the surroundings, it is transmitted by applying a digital signature. When a V2X certificate is sent, it includes a signature based on the Security Credential Management System (SCMS) [25] or the IEEE 1609.2.1 [26] standard. Figure 5 shows two cases of hacking that can occur during V2X communication in a C-ITS environment. Attackers can send compromised BSM messages to other vehicles via V2X while driving, or attack the C-ITS infrastructure, such as RSU, to occur events to near vehicles. In this situation, a digital signature can be applied effectively.  
The basic V2X security is provided through digital signatures, as described above, but V2X security standards are still under revision, and related research is underway to apply them to actual infrastructures of Cooperative Intelligent Transport Systems (C-ITS).  
Figure 4: Structure of the BSM message Part I; Part II defined by SAE J2735.  
Figure 5: Examples of an attacks using V2X. Attacker 1 is the attacker’s vehicle and causes an accident by sending a compromised BSM to near vehicles. Attacker 2 hacks the C-ITS infrastructure or RSU to send false information to the vehicle or force events, such as hard braking.  
In particular, the application and verification of security technology development in the C-ITS environment require sufficient simulations in a virtual environment before they can be performed on real roads. This is because if a security threat is created for technology verification in the real-world environment, it can lead to an accident.  
[PERSON] et al. introduce various testing methods to be performed in the development of V2X technology, as well as security tests [27]. [PERSON] et al. introduce methods of conformance testing, function testing, performance testing, vehicle gateway testing, penetration testing, accelerated testing, and field testing for a device under test (DuT), such as a vehicle or onboard unit (OBU). They also explain the HIL (hardware in the loop)-based parallel testing method for reducing the cost and risk of field testing. Figure 6 describes the parallel testing method proposed by [PERSON] et al. The parallel testing method performs simulations and verification by mapping a vehicle object of the real-world environment to a vehicle object of the virtual environment and implementing multiple vehicles running in the virtual environment. Furthermore, [PERSON] et al. define security threat attributes for V2X communication by classifying them into authentication, availability, data integrity, confidentiality, non-repudiation, and real-time constraints. For security evaluation in a virtual environment, it is necessary to build the HIL environment for parallel testing, create threats to the security attributes, and perform tests at the application level to process messages to determine whether the DuT is robust against security threats. It means the ability to process misbehaviors in driving situations. These simulations must be performed to implement and verify the function that can filter misbehavior messages when the vehicle receives them.