## 1 Introduction
### Importance of the Offline Payment Function of CBDCs and Security Requirements  
Regardless of type, all CBDCs have one important feature in common--CBDCs are the national currency that is to be used as legal tender replacing the physical cash. Like cash, it should be accepted irrespective of all circumstances. However, the CBDC system is basically an electronic financial system where users are required to be connected to the internet, and for this reason, so far, CBDCs have not truly worked as a national legal tender.  
CBDCs, which can be used only with the internet, can exclude many people who cannot use the internet from financial services. Further, if the system is shut down due to power outages and cyberattacks, financial services could turn out to be unavailable, which could cause a lot of confusion. Therefore, China and the Bahamas, where the CBDC system is already introduced, are reviewing the offline payment function of CBDCs, and the number of countries that are considering the offline payment function of CBDCs is expected to increase in the future [38].  
The offline payment function of electronic financial systems is defined as a transaction function that does not require internet or telecom connectivity [39]. Under the electronic financial system with an offline payment function, people can use digital money stored in portable devices regardless of the communication environment, and therefore, the problems of financial exclusion or system malfunction, mentioned previously, are expected to be  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**CBDCs in Use Worldwide** & **Types** & **Reasons** \\ \hline \multirow{3}{*}{Sand Dollar (The Bahamas)} & Single ledger & No external organizations are involved in managing the ledger. \\ \cline{2-3}  & Direct & The central bank provides financial services directly. \\ \hline \multirow{3}{*}{SOV (Republic of the Marshall Islands)} & Single ledger & No external organizations are involved in managing the ledger. \\ \cline{2-3}  & Direct & The central bank provides financial services directly. \\ \hline \multirow{3}{*}{eNaira (Nigeria)} & Single ledger & The central bank directly manages the ledger. \\ \cline{2-3}  & Direct & The central bank provides financial services directly. \\ \hline \multirow{3}{*}{E-CNY (China)} & Single ledger & The central bank directly manages the ledger. \\ \cline{2-3}  & Indirect & Commercial and private banks are designated as operating \\ \cline{2-3}  & & institutions to provide financial services. \\ \hline \multirow{3}{*}{Bakong (Cambodia)} & Distributed ledger & Operated using the Iroha blockchain, which is based \\ \cline{2-3}  & & on Hyperledger. \\ \cline{2-3}  & Permissioned & Iroha uses a permissioned blockchain. \\ \cline{2-3}  & Indirect & Various digital banks provide financial services. \\ \hline \hline \end{tabular}
\end{table}
Table 1: CBDCs in use worldwide.  
Figure 1: Different forms of the CBDC systems (classification standards from Ref [26]).  
solved. However, the electronic financial system with an offline payment function is in a blind spot of security problems--not only portable devices but also transactions in an offline situation are isolated from the main system. Malicious behaviors, such as hacking attempts on portable devices or fraudulent transactions that exploit the system, cannot be detected in real time.