## 5 Review Findings  
This section presents some findings of the literature review analysis based on the taxonomy perspectives. Figure 19 reveals the articles distribution based on scientific sources. The pie chart emphasizes that most of the reviewed articles published in Springer link with 35%, followed by IEEE with 31% of articles. Nineteen percent of the articles were published in Science Direct, and about 14% were published in different scientific databases. As illustrated in Figure 20, there is a focus on user-level authentication. That focus implies that the essential role of the authentication scheme is to secure the end-user applications since such applications are the first gateway to access IoMT systems. Beginning in 2016, the research efforts started focusing on the network level, and these have gradually increased. The researchers realized the need for authenticating all of the communicating entities on the network, whatever their computation and storage capacities. The device-level authentication is of concern in 20% of the reviewed articles. The device authentication is strongly related to resource-constrained IoMT devices. Consequently, the development of device authentication methods requires particular considerations. The article distribution showed that a centralized authentication architecture is mostly adopted for authentication schemes. That can be explained by the nature of the healthcare field, where patient privacy is highly prioritized. Privacy is fully achieved by keeping the information at centralized entities. The centralized architecture ensures that the patient credentials are kept under the control of a centralized authority responsible for the patient's  
Figure 19: Article distribution based on scientific sources. Figure 20: Article distribution based on the authentication level. privacy. Moreover, the centralized architecture ensures high governing of healthcare standards.However, the distributed authentication architecture has become more common in the last three years, see Figure 21.That refers to the maturity of distributed security solutions such as blockchain and edge computing. Blockchain technology offers high anonymity, transparency, and privacy, encouraging researchers to adopt this distributed architecture. The patient's privacy is highly supported through an anonymous blockchain network. In 2021, the need to address scalability, coordination, and data management of the distributed architecture became the main issue of the reviewed articles. The frequency distribution of reviewed articles confirms that the symmetric key approach was not commonly used for authentication in the IoMT context. That referred to the low-security level of using symmetric keys within highly distributed systems. In contrast, the asymmetric key approach was mainly used for IoMT authentication systems. Asymmetric key-based authentication schemes offer high security since the communicating entities do not need to share the same keys to authenticate each other. From 2019 on, almost all articles suggested the asymmetric key approach for securing their authentication schemes as shown in Figure 22.  
The analysis of articles reveals that about 40% of them adopted the public-key cryptography approach, as shown in Figure 23. That implies that public-key cryptography provides good security and robust authentication in a distributed IoMT environment. In addition, hash cryptography is primarily integrated with the public key approach for the authentication schemes. About one-third of the articles discussed using hash cryptography to secure the sharing of authentication credentials. The hybrid cryptography approach became popular in articles published from 2019 to 2021. The hybrid approach integrates more than one cryptography approach, which increases the robustness of the authentication schemes. However, the adoption of the hybrid approach is not straightforward in the IoMT context, because it requires a high computation and storage capacity. Figure 21: Article distribution based on authentication architecture.