## 5 Discussion
### Limitations in Satisfying Security Requirements for Offline Payment Function
#### 5.1.1 Applying Blockchain Technology
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Security** & **Research** & **Security Requirements** & **Detail** \\ \hline \multirow{6}{*}{Blockchain technology} & \multirow{2}{*}{[PERSON] and [PERSON] [45]} & \multirow{2}{*}{No double spending} & Prevention of double spending through proof of work (A method for users to check whether the transaction is true or false). \\  & & & * Not applicable when an attacker has over 50\% of hashing power. \\ \cline{2-4}  & & & \\ \cline{2-4}  & [PERSON] et al. [55] & Unforgeability & Transparency to the supply chain through BlockVerify using blockchain’s distributed ledger technology and security. \\ \cline{2-4}  & & & \\ \cline{2-4}  & & & \\ \cline{2-4}  & & & \\ \cline{2-4}  & & & \\ \cline{2-4}  & & & \\ \cline{2-4}  & & & \\ \cline{2-4}  & & & \\ \cline{2-4}  & & & \\ \hline \multirow{6}{*}{Trusted third party} & \multirow{2}{*}{[PERSON] et al. [46]} & \multirow{2}{*}{No double spending} & Monitoring double spending by adopting the “inserting observers” \\  & & & & method to solve problems caused by Bitcoin’s fast payment. \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \hline \multirow{6}{*}{Certification of the transaction members} & \multirow{2}{*}{[PERSON] et al. [43]} & \multirow{2}{*}{No double spending} & \multirow{2}{*}{Proposal of the transaction system, tolerating the transactions for the buyer and seller with only the serial numbers matching.} \\  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \hline \multirow{6}{*}{Tracking of transaction records} & \multirow{2}{*}{Brands [42]} & \multirow{2}{*}{No double spending} & Improvement of the security system for the traceability history of e-cash trade, examining one-show blind signatures and wallets with the observer’s method. \\  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \hline \multirow{6}{*}{Tracking of transaction records} & \multirow{2}{*}{Brands [42]} & \multirow{2}{*}{No double spending} & \multirow{2}{*}{Proposal of the cryptographic technique for the users to use a blind RSA signature to prevent double spending.} \\  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \hline \multirow{6}{*}{Crypographic techniques} & \multirow{2}{*}{[PERSON] [64]} & \multirow{2}{*}{Non-repudiation} & \multirow{2}{*}{Proposal of the security system with Kececak-256 and a hash value to prevent malicious changes during the transaction process in block contents.} \\  & & & & block contents. \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \hline \multirow{6}{*}{[PERSON] [69]} & \multirow{2}{*}{Verifiability} & Improvement of the security system for the transaction process with a combination of MPC protocol and ZKP to guarantee verifiability without revealing personal information. \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \hline \multirow{6}{*}{[PERSON] [73]} & \multirow{2}{*}{Anonymity} & Proposal of the system creating an environment with a pseudonym via Ethereum’s hash value address that hides personal information in the pseudonymity. \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \cline{2-4}  & & & & \\ \hline \end{tabular}
\end{table} 
Table 3: A study on various types of security reinforcement plans for digital payment methods. situations [103; 104; 105]. However, since it is difficult to proceed with transactions under the surveillance among entities in offline payments, a consensus algorithm, such as PoW, cannot be introduced. For these reasons, there are still technical barriers to be resolved for blockchain technology to be adopted in offline situations.