## 3 Literature Review
### Smart Home Devices  
This section describes the survey conducted for the literature that has researched the implementation of zero trust for smart home devices. To address security risks, access control solutions based on zero trust network architecture in smart home environments are researched by [39]. To meet zero trust principle of \"never trust, always verify\" for any new device introduced into the start home environment, the authors use mutual authentication between devices. They achieve this with the new device, generating a random number that is encrypted using quadratic residues and sending it to multiple devices for verification. The verification result and a hash value are sent by the verifying device. The authors claim that the basis of their device access solution is based on zero trust architecture. However, they do not include any zero trust architecture components like PEP, PDP PA. The authors conclude that their solution provides higher efficiency and better security than traditional methods. The authors of [40] survey various techniques and approaches to implementing zero trust architecture based on NIST SP 800-207 [5] for smart home devices. The authors discuss communication layer encryption, software defined perimeter and micro-segmentation as essential components of zero trust architecture (ZTA). The logical components of their study include Policy Engine (PE), Policy Administrator (PA), PEP and trust algorithm. The ZTA implementation techniques include identity management, access control, logging and monitoring, consuming threat intelligence, continuous monitoring and maintenance, and compliance with various applicable and industry acknowledged standards. The authors study issues with traditional access control methods and the fulfilment of ZTA implementation techniques to address issues in conventional access control methods. They conclude that ZTA cannot be implemented with any technology or any defined architecture. They also conclude that successful realisation of ZTA can be achieved with a constant evaluation of trust in access control techniques by considering an organisation's context, threats and behavioural analytics. [PERSON] et. al. [41] apply zero trust principles for access control like contextual awareness and user behaviour to smart home systems (SHS). Their system, Zero-Aware Smart Home (ZASH), uses zero trust architecture in the form of continuous authentication for smart home devices. Their paper is an extension of [42], written for the Department of Computer Science of the Federal University of Minas Gerais, Brazil, by the same set of authors and an additional author, [PERSON]. ZASH considers active and passive devices in SHS that collect user behaviour data. The authors consider user categories (like admin user and normal user) and state machine representation for server authorisation and client authorisation. An ontology manager contains user categories mapped to devices, and any request by a user category for a device is verified by a context manager. The authors conclude that ZASH protects users' privacy, blocks most attacks and ensures that all interactions are access controlled. Challenges of privacy protection in smart homes that use IoT devices are a subject of research by [43]. The authors designed the Smart Home Privacy Protection (SHPP) standard framework that includes terminology, a reference framework and criteria for guidelines. The relationship between various parts of the SHPP is explained with the reference framework to data managers. They discuss challenges related to privacy risk evaluation, demand diversity and privacy protection methodology. They propose that zero trust offers data security to address internal and external threats. The data manager monitors the ingress and egress of data and locates sensitive data and uses the zero-trust model to check for data compliance and identify any privacy issues. They conclude that the development of the SHPP has provided support for data security governance.