## 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 and advantages of the SRHC structure to the authentication in VMF-based CNR networks will achieve the following:* The central system will automatically mitigate the problem of separate re-initialization owing to the exhaustion of the root keys used for node confirmation, identification, and updation in VMF-based closed CNRs when conducting independent operations for long periods of time [51];
* When applying this hash chain-based authentication in CNR networks, issues such as the potentially limited operational time and resources, unnecessary network transactions associated with checking the remaining hash values, and securing independence owing to specificity in the military environment can be overcome. [PERSON] et al. [24] proposed \"OTP-based two-factor authentication,\" an infinitely superimposed hash chain structure that applies a two-factor authentication-based multi-hash function to address the cost, time, and performance limitations of the authentication process. In this study, the hash chain generation and usage processes are applied in the forward direction, as opposed to the existing Lamport method. In addition, when the server side sends a specific index value to a client based on the hash chain generated in the forward direction, the client responds with an OTP. As such, because the cryptographic hash function operates in the forward direction rather than in the reverse direction, as in other hash chain structures, a re-registration owing to the exhaustion of the hash values is not required. However, specific strategies to improve the two-factor authentication-based infinitely superimposed forward hash chain structure are required owing to various limitations, such as the existing simple-password problem owing to the seed value being secret information immediately shared by the client and server, as well as the inability to realize the advantages of the OTP. Despite these limitations, applying the features of a two-factor authentication-based infinitely superimposed forward hash chain structure to the authentication in VMF-based CNRs will achieve the following:  
* The re-initialization problem occurring from the exhaustion of keys used for authentication and updation when performing independent operations over a long period of time will be simply resolved. The performance cost owing to the participation of multiple new squad and member nodes and the establishment of a joint operation with other squads will also be minimized. Similar to other advanced studies, to solve the problem of hash chain re-registration, an infinite hash chain technique based on the double independent hash function proposed by [PERSON] et al. [25] was designed such that a client had to apply a specific cryptographic hash function to a random seed value and another hash function shared by only the client and server to the output value, which was used as an OTP. Compared to the techniques used in previous studies, this technique enabled the realization of robust security at higher levels of OTP. However, this technique had a limitation in that the client and server had to share secret information, such as seed, in advance. Thus, prior studies on improving the hash chain-based OTP assumed that the secret key, cryptographic hash function, and digital signature, among other factors, were shared between the client and the server in advance. Owing to this limitation, they could not be efficiently established compared to other cryptographic primitives, which were fundamental advantages of a hash chain structure. Specific strategies to address this issue are necessary.