## 3 Comparative Analysis of the Existing Hash Chain-Based Authentication Approaches
### Applicability Analysis of VMF-Based CNRs in Previous Studies on Hash Chain-Based Authentication
Despite these limitations, applying the features of a double independent hash function-based infinite hash chain to the authentication process in VMF-based CNR networks will achieve the following:  
* The security of the authentication in each closed CNR network will be further strengthened, and the scheme will be more robust than those in other methods against the issues of limited key maintenance, authentication delays from duplicate hashing, wormhole attacks, and MITM;
* Application of this approach also enables the conversion of the authentication scheme into one that is specialized for changing battlefields in which enemy forces employ active confusion tactics based on the network. [PERSON] et al. [26] proposed a \"T/Key\" based on a multi-authentication scheme that combines the S/Key and T-OTP and does not store the secret key on the server. T/Key uses independent hash functions to address any potential security instabilities that arise from deriving hash values through the same cryptographic hash function in the S/Key. In addition, regarding the vulnerability to phishing attacks owing to the indefinite validity of the hash value by applying the concept of the time interval length, the validity of a specific hash value can be limited up to only a predetermined period of time. Furthermore, to alleviate the ripple effects of preprocessing attacks, an independent salt value is assigned to each cryptographic hash function in the hash chain, thereby verifying the lower limit of the hash value for each function and establishing proper security. However, because the hash values in the T/Key structure are time-limited, the structure is significantly longer than the existing S/Key structure, thereby leading to potential structural vulnerabilities. Moreover, other potential issues, such as the management of finite hash chain length, the complexity of computation according to hash chain length, the lack of a self-reinitialization scheme, the security problems dependent on hash chain length, and the efficiency of T-OTP generation and verification, remain. Therefore, possible designs to be employed in limited-resource CNRs as well as a specific authentication formulation based on T/Key are required. Despite these limitations, applying these features of the T/Key to the authentication process in VMF-based CNR networks will achieve the following:  
* Limiting the lifetime of valid authentication keys that have not been used for long periods based on a predefined time span will enable the preparation of functional countermeasures to prevent, identify, and attenuate the continuous collection of information by specialized enemy nodes and conspirator nodes;
* Laying the foundation for military cyber agility [52; 53] in a limited-resource CNR, based on independent cryptographic hash functions that can be deployed at any given time for each node in a tactical network, will facilitate the development of an authentication scheme that proactively prevents intrusions from specialized hostile nodes in a battlefield environment;
* Establishing a multi-factor hash chain-based authentication process including T-OTP will secure high levels of robustness and responsiveness against any initial reconnaissance attempts by enemy nodes, maintain tactical covert activities, and maximize the covert activities of friendly forces based on a scheme specialized for low-spec and low-speed closed squad networks;
* Furthermore, the ability to easily establish joint operation systems with other squad members and foreign militaries by synchronizing the hash functions will help satisfy various interoperability-related scenarios. Based on these potential possibilities, introducing the T/Key concept is vital for schemes related to the multi-factor hash chain-based authentication scheme including T-OTP in VMF-based CNRs.