## 1 Introduction  
### Background  
In recent years, the economy has become more and more prosperous, but the security of the city has not improved along with economic growth. To strengthen the security of the city, except for the police placed by the government in the city, the general company or the community will sign a contract with a private security company, and that company provides the security service to ensure the property of the client. [PERSON] [1] wrote that \"There has been a steady increase in security guard employment in the United States in recent decades. Data estimates for 2010 suggest that there are over 10,000 security companies in the United States, employing 1,046,760 guards.\" The above description indicates that the demand to hire private security services is increasing and the social needs of the security industry have become very important. Furthermore, the Federal Bureau of Investigation (FBI) in the United States analyzes offense statistics every year [2]. We sorted out the annual statistics from 2015 to 2019 regarding cases of burglary and larceny-theft in buildings. Table 1 shows the number of the two types of cases and the average US dollar value of the property loss. Except for the United States, the burglary record in England and Wales has come to 417,416 cases [3]. Therefore, private security services are important to prevent the occurrence of criminals. When crime continues to exist, people need the service to avoid it. There are several types of services of private security companies: community security, patrol security, bodyguards, armored cars, etc. In this research, we focus on community security. The operation of community security comprises several cars with security guards patrol in a specific area; if a client's house or company sends an alarm to the security control center, the control center needs to notify the security guards and solve the problem to protect the property of the client. Most security companies include security systems with their services, such as video surveillance cameras, access control keys, and virtual fencing systems [4]. Nowadays, many Internet of Things (IoTs)-related products have been developed, e.g., door/window sensors, smoke detectors, motion sensors, and alarm sirens. Those devices can also be integrated into the security system which makes the system automated. Although the ability to automatically alert has been achieved, there are still some problems. Firstly, the security of the Internet of Things is constantly being challenged [5; 6]. Secondly, the security company is unable to keep trackable records; it is insufficient to clarify the responsibility between the security company and the client. However, if it needs real-time processing, records can be verified and traceable. Blockchain is a technology that can solve issues. Blockchain technology is widely applied to the cryptocurrency Bitcoin. Bitcoin was developed and deployed by [PERSON] in 2008 [7]; it is a digital virtual currency that is secured by cryptography [8]. The notion of blockchain is decentralized networks, and every data chained in the network is irreversible. Blockchains are also authentic, consistent, able to provide consensus, and traceable. Because of the characters and advantages, blockchain technology has been affected and applied in various fields in recent years; for example, in healthcare [9; 10], payment [12; 13; 14], digital content [15; 16; 17], etc. For all the benefits mentioned above, blockchain technology is very suitable to be involved in security systems. There are at least three types of blockchains: public blockchains, private blockchains, and hybrid blockchains. In our scheme, private blockchains are more suitable for protecting customer privacy, and it is maintainable by the security service provider. Those security events are stored in an immutable ledger, and the ledger will be mainly stored with the security company, official agency, and client, to ensure that the information will not be arbitrarily altered.