## 1 Introduction
### Methodology and Limitation  
The security problem of the offline payment function may act as an obstacle to elevating CBDCs as the national legal tender that can be used regardless of all circumstances. In this respect, this paper focuses on reviewing security requirements for offline payment functions to utilize CBDC as a national legal tender, based on research studies considering various electronic financial systems, which are predecessors of CBDC. In Section 2, security requirements that should be fulfilled by electronic financial systems are defined, and solutions for security requirements are reviewed. Research studies dealing with the security requirements of offline payment functions as well as online payment functions of electronic financial systems were examined, considering the CBDC's character as a national legal tender. To utilize the CBDC system as a national legal tender, it is obvious that a thorough discussion on its security matters is needed. However, studies on the offline payment function of electronic financial systems tend to rely on one method, cryptographic techniques, by which the security requirements that can be fulfilled are limited, unlike various studies related to the online payment function, which are suggesting various security enhancement methods. Therefore, in this section, the scope of the review is extended to diversify the security requirements and find various solutions. In Sections 3 and 4, studies related to the offline payment function of electronic financial systems are reviewed while examining whether the security enhancement methods suggested by these studies meet the security requirements examined in Section 2. In particular, studies dealing with not only the CBDC system but also the electronic cash (e-cash) and the cryptocurrency among electronic financial systems, were selected, since the CBDC system can be largely divided into a single-ledger-based system and distributed-ledger-based system, as covered in Section 1.2. In terms of system operation, e-cash, which is usually operated by a single entity, such as a bank or a credit card company, has similarities to single-ledger-based CBDC, and a cryptocurrency based on blockchain technology has similarities to distributed-ledger-based CBDC. Therefore, research related to the security matters of each electronic financial system can be referred to in dealing with security matters of the CBDC system. Finally, Section 5 reviews the limitations that the solutions in the previous research study on CBDC regarding offline payment function have. In the previous research study on the offline payment function of the CBDC system, security enhancement solutions that utilize electronic devices equipped with the trusted execution environment (TEE) technique are suggested. Since those solutions have weak spots for DDoS attacks and counterfeit, improvement methods for the solutions are proposed, while still unresolved fundamental problems and countermeasures are emphasized. The major research questions discussed in each section can be summarized as follows:  
* What are the security requirements that the offline payment function of electronic financial systems should meet? * What are the methods that can be applied to electronic financial systems to meet the security requirements? * How well do the methods proposed in the research studies of offline payment functions of electronic financial systems meet the security requirements? * What solutions can be suggested for improving TEE-based methods for the offline payment functions of electronic financial systems?