Table 3.

Blockchain-based Authorization Mechanisms in IoT networks.

Reference	Guarantees	Drawbacks
[53]	Decentralization	Limited in Protected Health Information Storage systems
	Availability
	Confidentiality
	Integrity
	Immutability
[45]	Lightweight, scalable, decentralized, and fine-grained access control solution for large-scale IoT systems.	Every domain involves a domain owner, which is a centralized entity; this might cause issues such as single point of failure, bottleneck, performance degradation, etc.
		A token is stored on the Blockchain which is visible in every participant; this will raise privacy issues.
[47]	More fine-grained access control and more flexible token management compared with existing capability-based AC schemes.	No results on the feasibility of the proposed scheme under a IoT healthcare system model, which involves several subjects such as users, doctors, nurses, etc.
	Experiments based on a local Ethereum blockchain demonstrated the feasibility of the scheme in large-scale IoT systems.
	Promising to achieve dynamic and fine-grained access control as ABAC introduces context information and the attributes of subjects and objects into its access control policies. More accurate access control in sensitive applications such as Healthcare by including sufficient attributes. Reduces the burden of maintenance, as access policies can be changed by simply changing the attribute values without the need to change the underlying subject–object relationships.	Although the prototype demonstrates the feasibility of the proposed framework, it can hardly reflect the performance of the framework in large-scale IoT applications such as Healthcare applications. The authors consider as future work the implementation of the proposed framework in environments with larger scales.