## 6 Open Issues and Future Directions  
IoMT systems are subject to many vulnerabilities because of their reliance on the distributed resource-constrained devices that cannot handle heavy security solutions. Moreover, IoMT devices are accessible online, which can be easily breached. Therefore, the need for considering those challenges in a robust and lightweight authentication scheme is persistent. Although the researchers may prove their secure and lightweight authentication schemes within the IoMT environment, there remain open issues. This section highlights some of them as follows:  
* Cross-domain authentication: It implies that the authentication scheme can authenticate entities from their trusted domain. Cross-domain authentication has become an urgent need since integrating multiple healthcare applications has become popular.
* Cross-layer authentication: The concept of cross-layer authentication must be supported for improving the security of the authentication process within IoMT networks. Cross-layer authentication ensures that all entities involved in communication at different layers are authenticated to each other. This issue requires a comprehensive authentication scheme to support layered IoMT architecture.
* Scalability: Since IoMT is the building block of today's smart healthcare applications, it has become massive and highly distributed. Authentication solutions need to cope with such improvements by supporting the scalable schemes. A scalable scheme refers to adding IoMT entities as required without sacrificing the system's performance. Many researchers attempted to handle scalability issues through a decentralized authentication scheme. However, those schemes require high governance and privacy standards, especially for healthcare applications.
* Adjustability: It is the ability of an authentication scheme to be adjusted in response to changes in the IoMT network. It is a challenging task for a static authentication scheme. Therefore, a continuously developing authentication scheme will gain more research attention in the next few years. Furthermore, context-aware authentication schemes need a research focus to improve the response to the changing performance of IoMT networks.
* Anonymity: This issue relates to the cryptographic-based scheme where entities are authenticated anonymously. This scenario implies full-anonymity where it is easy for an exploited entity to access the system's resources without discovering its identification. For the majority of IoMT use cases, partial anonymity supported by some authentication schemes is adequate for keeping privacy acceptable.
* The number of exchanged messages: The authentication scheme performance is affected by the number of messages required to achieve the authentication task. Consequently, it is necessary to reduce the number of exchanged messages to support real-time authentication for entities within time-sensitive healthcare applications such as remote monitoring healthcare services. This issue seems easy to accomplish, but the trade-off here is to reduce the number of messages and keep a high level of security.
* Re-authentication: It is more efficient to authenticate IoMT entities only once to reduce communication overhead. However, this can affect the security level of the overall system. Many research efforts intended to balance the communication overhead and the re-authentication need by delegating the re-authentication process to the edge entities instead of relying on a centralized authentication server.
* User-friendly authentication: This issue is subject to quality assurance in smart healthcare applications where their stakeholders are not interested in the latest technological techniques for security. The authentication scheme for such an application requires considering a straightforward process, and keeping the same level of security for the IoMT systems.