## 1 Introduction
### Methodology and Limitation
To figure out the research questions above while reviewing research studies, four different search engines were utilized, namely, Google Scholar ([[https://scholar.goo-gle.co.kr/](https://scholar.goo-gle.co.kr/)]([https://scholar.goo-gle.co.kr/](https://scholar.goo-gle.co.kr/)), accessed on 11 April 2022), IEEE Xplore ([[https://ieeexplore.ieee.org/Xplore/home.jsp](https://ieeexplore.ieee.org/Xplore/home.jsp)]([https://ieeexplore.ieee.org/Xplore/home.jsp](https://ieeexplore.ieee.org/Xplore/home.jsp)),accessed on 11 April 2022), Elsevier ([[https://www.sciencedirect.com/](https://www.sciencedirect.com/)]([https://www.sciencedirect.com/](https://www.sciencedirect.com/)), accessed on 11 April 2022), and Springer ([[https://link.springer.com/](https://link.springer.com/)]([https://link.springer.com/](https://link.springer.com/)), accessed on 11 April 2022). Google Scholar was used in comprehensive searching of journals, while IEEE Xplore, Elsevier, and Springer were used in advanced searching of specific topics in engineering, such as blockchain technologies, protocols, data security, and cryptology techniques. On the other hand, discussions on various electronic financial systems, including CBDC and cryptocurrency, can be found not only in research studies in the engineering and technology field but also in gray literature and research studies in the financial field, such as news, company reports, government reports, and white papers. In searching the literature, the websites used were as follows: Investopedia ([[https://www.investopedia.com/](https://www.investopedia.com/)]([https://www.investopedia.com/](https://www.investopedia.com/)), accessed on 11 April 2022), Experian ([[https://www.experian.com/](https://www.experian.com/)]([https://www.experian.com/](https://www.experian.com/)), accessed on 11 April 2022) and Ledger Insights ([[https://www.ledgerinsights.com/](https://www.ledgerinsights.com/)]([https://www.ledgerinsights.com/](https://www.ledgerinsights.com/)), accessed on 11 April 2022), and central banks' country websites. In searching the literature through the websites mentioned above, the search keywords were selected based on the first, second, and third research questions. The keywords \"electronic cash\", \"digital cash\", \"digital currency\", \"electronic financial systems\", and \"security requirements\" were mainly used for the first research question, and six keywords about security requirements were found by using these keywords: No double spending, Unforgeability, Non-repudiation, Verifiability, Anonymity, and DDoS attack prevention. The combination of these six security requirements and keywords \"electronic/digital currency\", \"e-cash\", \"cryptocurrency\", and \"CBDC\" were mainly used for the second research question. The keywords \"electronic/digital currency\", \"e-cash\", \"cryptocurrency\", \"CBDC\", and \"offline payment\" were mainly used for the third research question. Despite efforts to answer research questions based on the mentioned methodology, this study has some limitations. First, as countries do not disclose the specific realization methods of their CBDC systems, considering the importance of the CBDC systems as national legal tenders, the security measures of the offline payment function of the CBDC systems in use could not be reviewed. Second, this study focused on reviewing existing research studies related to the security issues of the offline payment function of CBDCs. The implementation of the offline payment function will be carried out in later research based on the review conducted in this study.