## 2 Background and Related Studies
### Related Research and Improvements for Introduction of Hash Chain-Based Authentication in VMF  
To overcome the limitations related to the authentication and integrity of VMF-based CNR networks, a hash chain-based lightweight authentication scheme including T-OTP should be introduced. The primary considerations of hash chain-based authentication are the security extension protocol (SEP)-related security requirements, computation and network transmission overhead, latency issues, maximum tactical security, and communication integrity. Moreover, to establish a hypothesis for these issues and provide a clear basis for further studies, the potential possibilities of applying a hash chain-based authentication structure, including T-OTP steps, proactive re-initialization for efficient network transactions, specialization of existing military ad hoc networks in cell planning areas, and the configuration of processes related to misbehaving node detection and anomaly auditing, are considered. Therefore, the unique features, advantages, and limitations described in previous related studies based on hash chain and ad hoc must be compared and analyzed for the construction of lightweight authentication in VMF-based CNR networks. A hash chain-based one-time password, as proposed by [PERSON], uses the hash values of a hash chain by applying the same cryptographic hash function in reverse order, thereby preventing the calculation of the hash value used in the next authentication session and addressing the vulnerability of a simple password [7; 8; 9; 10; 11]. The S/Key standard proposed by [PERSON] et al. addresses the difficulty of reducing the calculation weight when generating and re-registering the root hash values in a hash chain structure. In addition to enhancing the efficiency in a limited network environment, this standard also prevents reuse attacks [12; 13; 14]. The timed efficient stream loss-tolerant authentication (TESLA) protocol proposed by [PERSON] et al. used a multi-factor method, involving message authentication code chaining and the concepts of time-delayed key disclosure and loose time synchronization. This would mitigate any vulnerabilities to theft or abuse that might arise from a non-combination with other authentication schemes, as well as overhead problems related to the hash chain initialization that were not addressed by S/Key [15; 16; 17; 18]. [PERSON] et al. proposed a lightweight hop-by-hop authentication protocol (LHAP) specialized for MANET networks and related authentication based on TESLA [19; 20]. [PERSON] et al. proposed the hop-by-hop efficient authentication protocol (HEAP) to be employed in wireless networks [21; 22], thereby ensuring scalability. In addition, [PERSON] et al. proposed a self-renewable hash chain (SRHC) [23], [PERSON] et al. proposed an OTP-based two-factor authentication [24], and [PERSON] et al. proposed an efficient construction of infinite-length hash chains with perfect forward secrecy using two independent cryptographic hash functions [25], further alleviating problems related to initialization, root re-registration, and overhead in existing hash chains. Subsequently, in a T/Key study, a two-factor authentication based on T-OTP and S/Key structure was proposed that further reduced the computational overhead, compared to those in previous studies, while avoiding the storage of the client's secret key in the server [26]. [PERSON] et al. proposed a binary hash tree-based Merkle tree structure to solve potential issues, such as the