# Offline Payment of Central Bank Digital Currency Based on a Trusted Platform Module
## 2 Literature Review  
For cash transactions, once the sender and receiver meet and the exchange is completed, all claim-obligation relationships are immediately extinguished without the involvement of any financial institutions or payment service providers. In contrast, electronic payment methods require the presence of an intermediary, as both the sender and receiver must be connected to the internet or other networks to facilitate the transaction. In other words, when an individual makes a credit card purchase or an online e-commerce payment, he or she must be online to access the payment service provider. This connection is essential for checking the account balance, verifying the legitimacy of the transaction, and preventing the risk of double spending; this risk can also apply to blockchain [8]. These constraints similarly apply to the use of CBDC. Consequently, many organizations have been researching offline payment solutions for CBDC to overcome these limitations. For instance, the Bank for International Settlements (BIS) and the Bank of Canada have recently published the results of a study detailing the identification of security requirements for offline CBDC payments [9,10]. Additionally, the European Central Bank announced procurement details for digital euro projects in January 2024, highlighting offline payments as a challenge that needs to be addressed [11]. Since the late 1980s, studies on electronic payments have been conducted continuously. These studies have particularly focused on the application of cryptographic technology to address security and privacy concerns, such as preventing counterfeiting and double spending, while also providing anonymity. However, the primary objective of these studies has been to explore methods for secure transactions, based on the assumption that most parties involved in a transaction must be connected to the internet or other networks. Consequently, typical peer-to-peer payments using electronic devices require either both of the sender's and receiver's terminals to be connected to the internet, or at least one of the terminals (such as a Point Of Sales) to be connected to the internet or local payment networks, as illustrated in Figure 1a,b. The scenario depicted in Figure 1b can be classified as a hybrid environment. Figure 1: Online and hybrid environment. (**a**) Fund transfer (online), (**b**) Card processing (hybrid). In a recent study, Dmitrienko et al. proposed an offline payment model to prevent double spending [12]. However, it is challenging to regard it as a fully offline model, as it requires access to the blockchain network for token validation. Similarly to cash transactions, our ultimate goal is to facilitate payments in situations where an internet connection between the two parties is not reliably available. To achieve this, short-range wireless communication, such as the peer-to-peer (P2P) mode of Bluetooth or Near Field Communication (NFC), is essential between terminals. However, this study does not focus on the security of wireless communication itself but rather on transactions after the connection is established. Additionally, it is proposed that the P2P payment model discussed in this study is based on tokenized CBDC, which can be transferred between different terminals. Figure 2 illustrates an example of the complete offline P2P payment environment that serves as the foundation for this study. There have been several recent studies in this area. Aboulaiz et al. analyzed various offline CBDC payment systems, including a complete offline environment model [13]. They approached their study of offline payments as a complementary solution in the event of online network failures. Their analysis included four models, with Table 1 summarizing the key features of each. While they examined one of the models, OPERA, as a complete offline payment solution, they also highlighted several shortcomings that could hinder the widespread adoption of OPERA.