## 3 Literature Review
### IoT
#### 3.3.1 Addressing Security and Privacy Issues by Adopting Zero Trust  
The authors of [44] study security and privacy issues related to IoT brought about by interconnected networks which bring in accessibility from anonymous users and the Internet. The authors discuss IoT/OT alignment, as IoT devices are used in OT environments. The reasons for security challenges include the non-standard nature of IoT devices, security requirements for the large amount of data generated by IoT, vulnerabilities developed in IoT devices over a period of time due to the difficulty of patching and upgrading them, the lack of security awareness in the IoT industry, and skills shortages. They discuss the importance of resiliency in strategy in protecting IoT assets, which in turn helps achieve resiliency in network. The operational resiliency and cyber resiliency can be achieved with zero trust. The authors discuss the elimination of implicit trust by zero trust using continuous verification of all assets. They further discuss the zero trust architecture and access to assets granted only after continuous evaluation. They conclude that the security of IoT devices remains a major concern. These concerns are addressed with respect to the resilience of these devices using technology and organisational policies.  
The application of a blockchain's cross-chain to achieve value exchange or value transfer for IoT is a focus of study for [45]. The authors specifically focus on information and transactions related to the security of personal assets. Their framework is based on a virtual blockchain address generator and an authentication scheme using zero-knowledge proof. They introduce a protocol named cross-chain privacy protection, which is interactive. They further study the loyalty of each node of blockchain IoT in a zero trust environment and analyse the interaction protocol for security threats, including replay attack, man-in-the-middle attack, zero knowledge proofs and anonymity. The authors elaborate that the common blockchain addresses were replaced with virtual blockchain addresses for anonymity of the transactions. Virtual blockchain addresses provide anonymity for each of the transactions by not repeating the existing external addresses. The privacy of a user's transactions is secured with multiple layers of security, as a malicious actor can only reach a user's transaction by obtaining all block records by compromising the main chain. They conclude that their experiment shows that their proposal demonstrates efficiency in terms of performance, as the time taken by the virtual address generation to anonymise external addresses is shorter than the transaction time. In terms of storage and transmission, the proofs are smaller in size. To effectively address IoT blockchain cross-chain privacy, multiple smart contracts that are compatible with existing framework standards can be used.  
The authors of [46] state that IoT devices share information in a decentralised way which can be addressed with zero trust, as zero trust addresses authentication and information sharing in decentralised and untrusted environments and supports fairness and privacy awareness in information sharing. They propose an information sharing protocol that is blockchain based and which operates in zero trust environment. To address authentication for ad-hoc information sharers, their blockchain based authentication protocol has a mechanism that is based on smart contracts to detect and filter synthesised information, thus preventing unauthenticated users from sharing information. This approach reinforces fairness, which is one of the privacy principles, in processing information. The smart contract also removes dependencies on trusted third parties for sharing information. The authors conclude that their proposed protocol proves that it is universally composable. They additionally conclude that future research can be conducted to reduce latency in the protocol.