## 4 Security Requirements in Current Offline Payment Functions
### Offline Payment Function with CBDC Proposed by [PERSON] et al. [95] proposed TEE technology applicable to the devices capable of the payment method by CBDC. However, in the proposed system via CBDC offline payment function, fraudulent activities may occur when executing the withdraw protocol,protocol for offline payment function, and deposit. This section provides a review of the security requirements for the devices utilizing the offline payment function by CBDC.  
In the protocols for withdrawal and deposit, double spending can be prevented through counters \"i\" in the device and server. When the currency is transferred from the server to the offline account or from the offline account to the server, 1 is added to the device's counter \"Ti\" and the server's counter \"S.i.\". Then, it is possible to confirm whether double spending has occurred through sync, which is a process of checking whether \"Ti\" and \"S.i\" are of the same value. In the offline payment protocol, double spending can be prevented by an offline payment counter \"i\" in the device that increases by 1 whenever the transaction occurs, and the device proceeds with a transaction and a payment log that stores transaction details.  
In the presented device, non-repudiation can be prevented by using a two-tier hierarchical infrastructure. The certificate of the users with the TEE device is stored in the server storage, S.Registry, through the TA registration protocol. When the payer transfers money to the payee, the payee can verify the payer's certificate in the storage before the payment starts. After the payer's information is checked, the payee then processes payment requests to the payer, and offline payment function can proceed when the payer sends the currency. In this way, both the payer and the payee cannot repudiate that offline payment has already been processed.  
When offline payment occurs, transaction information P, including the amount of currency sent, sender's certificate, information on the recipient, and transaction counter \"i\" to prevent double spending are stored in the device's TinPaymentLog. This information makes it possible to determine the part of the transaction in which problems occurred, and the security requirement of verifiability in offline payment is satisfied.  
The device user cannot view the transactional history through the TEE device. The entity responsible for the management of the device only has access to the TEE device and verifies the transactional information. Therefore, the anonymity for average users utilizing the devices in the offline payment by CBDC can be maintained.  
On the other hand, [PERSON] et al. [95] proposed that the offline payment system does not directly deal with unforgeability and prevents DDoS attacks. Despite someone freely duplicating or creating a CBDC via offline payment, it is still not easy to detect whether CBDC is forged, and countermeasures are required. In addition, when the transactions are in the process between the server through withdraw protocol and the offline account through the deposit protocol, they are connected to the server. Thus, it is essential to prepare a countermeasure against malicious users generating traffic to the server. Table 2 represents whether research studies on existing offline payment functions satisfy the security requirements.