## 4 IoMT Authentication Taxonomy
### Authentication Attacks  
The main purpose of adopting authentication schemes in IoMT is to ensure that only authorized users and devices are enabled to use system resources and services. Therefore, it is necessary to test the authentication schemes against attacks that succeed in getting unauthorized access to IoMT systems. Figure 18 shows the widespread attacks used in IoMT authentication. Those attacks are shown as being prevented in different ways in the literature, according to the proposed authentication scheme.  
[PERSON] et al. [42] combined long and short term parameters to securely create and share a key of the communication session.  
That combination avoids compromising the session key information. [PERSON] et al. [90] developed an authentication scheme that resists guessing and privileged insider attacks. To pass the authentication process successfully, the adversary must guess three double-hashed values. Therefore, guessing an attack is impossible. Furthermore, to avoid privileged insider attacks, the network entities cannot access the security parameters and make even slight changes due to the time stamp technique. [PERSON] et al. [68] utilized a dynamic pseudo-identity for authenticating entities to each other. The technique prevents forging messages between entities and improves security for tracking and sensor capture attacks.  
DoS is common within IoT environments due to the use of resource-constrained sensors. The method proposed by [PERSON] et al. [69] proves its resistance to Denial of Service attack (DoS). The scheme is based on request-response communication and timestamp techniques; the user needs to obtain a time-stamped confirmation from the sensor to be connected. That means that the DoS attack is not simply accomplished. The scheme of [PERSON] et al. [123] provides anonymous authentication of entities based on the Elliptic Curve Computational Diffie-Hellman schema (ECCDHP). That implies that impersonating entities is not possible, and impersonation attacks can be identified easily. Furthermore, using random session numbers and timestamps ensures synchronized communication and avoids de-synchronization attacks. [PERSON] and [PERSON] [43] designed and improved a scheme that leverages a transaction-based blockchain. The verified transactions are stamped by time and identifier; therefore, replay attempts are unlikely to succeed.  
[PERSON]'s scheme [88] provides tolerance against the man-in-middle (MIM) Attack. The scheme uses a temporal credential so that the adversary cannot exploit the communication even if he knows other private credentials for both communicating entities. Moreover, the proposed scheme does not rely on credentials provided during the authentication process to derive the verifier. The scheme has been successfully tested against verifier stolen attacks. In the same direction, [PERSON] et al. [57] developed a blockchain-enabled technique for IoMT authentication using cryptographic random values to avoid MIMA  
Figure 18: Authentication attacks.  
and eavesdropping attacks. For eavesdropping attack, the adversary attempts to build a weak connection between entities. [PERSON] et al.'s scheme [71] is based on smart card authentication. Although using a smart card is vulnerable to theft, the proposed scheme also requires the users' biometric information to verify their authentication.