# Blockchain Technology Application in Security: A Systematic Review
## 4 Discussion  
The main goal of this paper is to provide a systematic review of practical applications of blockchain usage as a security solution with an in-depth analysis of their blockchain architectures. Hence, the selected papers are investigated based on this goal. One of the important notes about blockchain solutions to be applied is the use of some well-established, ready-to-use, and open-source platforms such as Ethereum and Hyperledger Fabric (HLF). Ethereum is a decentralized peer-to-peer blockchain platform that executes application codes called smart contracts securely. Paper [23] uses its decentralized authentication technique for IoT devices in the Ethereum platform with a smart contract that is automatically run in the Ethereum. Authors [38] provide a secure sharing protocol with blockchain technology for electronic health records written into smart contracts using solidity language and run on the Ethereum blockchain. HLF platform supports multiple blockchain ledgers, and only some verified users can store the data in the corresponding blockchain ledgers. Paper [20] provides a lightweight blockchain with HLF for IoT networks. Some of the most practical and implement-ready blockchain solutions were those implemented on Ethereum or HLF platforms. Next, we are focusing on providing rigid solutions for each research question. * What prominent domains employ blockchain security applications and their specific use cases? * IoT: IoT networks are in the first stage of blockchain application usage in security. The most use cases of blockchain security solutions for IoT networks are for authentication and authorization of devices [24, 26] and for providing a secure network for data confidentiality [28, 30]. * Healthcare: blockchain is mostly used to guarantee the privacy of personal healthcare data storage and management [38, 43]. * Wireless network: securing data transmission [69] and user authentication are the most common blockchain usage in wireless networks [24]. * Energy: securing energy trading in energy markets [50, 51] and enhancing the security of smart grids [54] is accomplished with blockchain networks. * What are the prevailing consensus mechanisms in use? From the results, it is obvious that PoW is still the favorite consensus algorithm among researchers. Although PoW is the most powerful computing algorithm, since it is simpler  
Figure 6: Smart contract usage. Figure 7: Integration with other technologies. to deploy and more mature, it is widely used in different use cases. However, for some use cases, such as IoT networks, the PoW mechanism is not a good idea since IoT devices have limitations in computing power. Based on the above studies, only four IoT articles use PoW, and others use PBFT [20, 26, 27] or some context-based consensus mechanisms [25, 34]. The Proof of Authority (PoA) algorithm aims to set up an authorized node to build all the other blocks to reduce the building block time [41]. It is faster and highly secure compared to PoW; consequently, it is preferred to be used in some networks [28, 38]. What influences the selection of blockchain types? There is no popular type within considered articles in our systematic review. It can be concluded that authors select blockchain types by considering their requirements and users' permission to access networks before researchers select their appropriate type. For example, [18, 19, 25, 26] latency is an important subject in [17, 24]. Hence, the authors use private and consortium blockchains instead of public, where the process in public is slower than in others. Additionally, in [27], the power consumption problem is so important that the authors decided to use two kinds of nodes, full and light, with different responsibilities to decrease the power consumed in each node. Hence, the choice of private blockchain is meaningful. How do smart contracts bolster blockchain security? Smart contracts are rules that run automatically when some conditions happen.