## 2 Background of Analysis  
### Authentication  
Authentication to confirm the person's identity is carried out in various ways. Authentication can be classified into three types: Physical authentication (PA), Cyber authentication (CA), and Cyber-Physical authentication (CPA). PA is the most common method to authenticate identity. For example, when people enter the theater to watch a movie, they should buy tickets and show them to a clerk. The clerk identifies the people to check whether they have a valid identity on site. The characteristic of PA is that authentication is carried out on site and in person. This makes it difficult for remote malicious users to interfere with authentication. However, PA must require physical contact with an unidentified person. When the user contacts the unidentified person to check their identity, the user does not know whether the unidentified person is a criminal, a patient with an infectious disease, or a normal person. This leads to a situation where the user's safety is at risk from physical contact.  
Cyber authentication (CA) is another type of authentication. CA is carried out in the digital world. For example, the digital signature is used to prove the author of a digital document [12; 13]. Digital ID is a common method to prove the user's identity to the system [14; 15]. Blockchain is also used to enhance the integrity of data in the authentication process [16; 17]. The characteristic of CA is that authentication does not require physical contact. So, there is no risk of problems from physical contact. However, there is a risk that a remote malicious user can interfere with the authentication.  
Cyber-physical authentication (CPA) is a novel authentication that combines PA and CA. It authenticates the identity with physical and digital information. Amazon Key is an example of the CPA [18]. It provides authentication for a delivery man. When the delivery person arrives at a customer's house, they request the Amazon system to open the door. Then, the delivery person's identity is authenticated by Amazon. If authentication is successful, the door opens. This service uses the physical environment of the delivery person's location and the digital process using the Amazon system. There is another CPA [19]. The authors propose a delivery authentication system using a bar-code and Kerberos authentication. In this system, the users must meet to check the bar code. After that, the authentication using Kerberos is performed. In [20], users authenticate their identity using digital information such as a username and password. Then, continuous authentication is performed using RSSI obtained from the physical world.  
The proposed system uses a digital twin to provide safety interaction. The system takes physical information, such as a proximity level between users, and digital information, such as a dynamic ID. Also, the dynamic ID, which is used for authentication, is generated based on the user's location. Because physical information and digital information are used for authentication, it can be seen that CPA is applied to the system. Table 1 shows the authentication types described.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Authentication Type** & **Technology** & **Description** \\ \hline \multirow{2}{*}{Physical Authentication (PA)} & Tickets & The authentication process takes place only in the physical world. \\ \hline \multirow{2}{*}{Cyber Authentication (CA)} & Digital Signature [12; 13] & The authentication process takes place only in the digital world. \\  & Blockchain [16; 17] & The authentication process takes place only in the digital world. \\ \hline \multirow{2}{*}{Cyber-Physical Authentication (CPA)} & Amazon Key [18] & The authentication process takes place in the physical world \\  & Research in [19; 20] & and the digital world. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Three authentication classifications depending on the environment.