## 4 Proposed Lightweight Authentication Based on Multi-Factor Hash Chain with T-OTP in VMF
### Lightweight Authentication Processes Based on Hash Chain
#### 4.2.2 Authentication and Verification Phases  
After the initialization and registration phases, the user utilized the T-OTP in an LSH-based multi-factor hash chain structure that used the hash values generated by applying the same  
Figure 2: Initialization and registration phases.  
cryptographic hash function in reverse order to prevent the calculation of the hash value used in the next authentication session, thereby addressing the vulnerability of a simple password.  
The objective of this phase was to deliver to the administrator the seed hash value of the LSH-based hash chain that the user would apply as the T-OTP. During the previous phase, authentication was performed using the time-based multi-factor method, utilizing the seed during the clock synchronization process. In the S/Key, because all hash values in the hash chain were sequential, they were indefinitely valid and vulnerable to theft and abuse. To mitigate this vulnerability, prior studies on improving a hash chain-based OTP assumed that a secret key, cryptographic hash function, and digital signature, among other factors, were shared, in advance, between the client and the server, resulting in limitations compared to other cryptographic primitives. However, through time-stamp-based clock synchronization, our proposed authentication process utilized hash values and related T-OTP values by applying a predefined time slot length between nodes participating in the VMF message exchange in the operation execution in CNRs. As such, the lifetimes of the hash values and T-OTPs of valid authentication keys that have not been used for long periods of time were limited to prevent, identify, and attenuate the continuous collection of information by specialized hostile nodes. The related phases are illustrated in Figure 3.  
1. Definition of initial seed hash: The initial seed hash of T-OTP, in which clock synchronization is executed and defined based on static transmission rules in military corps scenarios.
2. Clock synchronization: The real-time authentication-based squad interoperability between the user and administrator within a predetermined time-error range is confirmed from the perspective of the operation start time in CNRs in cell-planning areas.
3. Sharing and calculating based on time slot length: The calculated hash values between the administrator and user within the range of the dynamic time slot length is verified. Next, authentication is performed based on T-OTP in the LSH-based hash chain structure.  
The significant advantage of the LSH-based hash chain with multi-factor seed management was the independence of the T-OTP table between the administrator and the user in CNRs. Security was ensured because the administrator did not know the user's secret key. In terms of the feature securing availability in a mobile environment, such as ad hoc networks, by applying a finite dynamic time slot length, the validity of a specific hash value was limited up to the moment a predetermined period of time had passed. Thus, the proposed hash chain-based T-OTP yielded better computational and utilization efficiency compared to other cryptographic primitives. In addition, an independent salt value was assigned to each cryptographic hash function in the hash chain to verify the lower limit of the hash value of each function, thereby establishing the level of security. Here, the administrator could change the predetermined period for each operation group; as such, the clock synchronization process did not cause any issues owing to a low influence on real-time operation.  
Figure 3: Authentication and verification phases.