# Blockchain-Based Decentralised Authentication in Closed Environments
## 2 Background Knowledge and Related Work
### Blockchain for Authentication  
Owing to the advantages of blockchain, many researchers have attempted to apply this technology to create a new paradigm for authentication systems. Several studies have proposed the use of blockchain for authentication in Internet of Things (IoT) systems [11; 12; 13]. However, these studies consider a hierarchical operation of authentication systems. In other words, IoT devices are divided into groups, and each group has a cluster head or edge server response for the authentication of IoT devices. The problem with these studies is that, if the cluster head or edge server fails, the devices within the group cannot be authenticated. Blockchain is used only to verify the identity of the cluster heads or store the transactions sent from the edge servers. As seen above, the application of blockchain technology in decentralised authentication has gained significant attention in the IoT domain. Another notable contribution to decentralised authentication in IoT is called BCTrust and was proposed in [5]. This study presented an approach to device authentication in IoT environments by utilising blockchain's decentralisation architecture, which means that devices in an IoT network could authenticate one another without relying on a central authority. The concept of authenticating devices without a central authority was crucial, claimed the authors, because in IoT environments, secure and efficient communication was needed. The authors, therefore,proposed an authentication framework based on the principle that 'the friend of my friend is my friend', which allowed authenticated devices to extend trust across the IoT network. BCTrust employed blockchain as a distributed ledger to store authentication data to ensure that the data were accessible to all nodes and remained tamper-proof. While BCTrust contributed to the mechanism of decentralised authentication, it seems that its primary focus was on securing device identities and authentication data rather than fully decentralising the authentication process itself. This, therefore, differs from our proposed blockchain-based decentralised authentication protocol, which aims to utilise a consensus mechanism among participating nodes for truly decentralising the authentication process. This implies that our proposed protocol attempts to utilise the strengths of blockchain technology for both authentication data storage and authentication process. Other studies have proposed methods for applying blockchain for authentication in the medical domain [14,15]. One study used blockchain to ensure patient data security and privacy, whereas another one used blockchain to identify therapeutic options for patients. Both studies focused on the use of blockchain as a distributed storage system. In addition, the study in [14] applied the practical byzantine fault tolerance consensus mechanism, in which a leader node among all nodes was chosen to verify transactions. While these studies utilised blockchain technology, it is important to note that they do not achieve truly decentralised authentication. They appeared focus on ensuring patient data security through a distributed storage system. Although these applications applied blockchain's strengths to protect sensitive information, they aimed to secure sensitive data rather than to fully decentralise authentication processes. Furthermore, several studies on vehicular ad hoc networks [16,17] have designed authentication schemes based on blockchain. However, these studies focus on using blockchain to store vehicle data, rather than providing details on how authentication is decentralised. Similar to the blockchain applications, in the medical domain discussed above, these studies emphasised securing vehicle data. They did not necessarily implement fully decentralised authentication mechanisms. By focusing on data storage and security, these studies overlooked critical aspects of decentralised authentication and some processes still remained decentralised or relied on specific nodes for identity verification.