The credit card usage process is as follows: The cardholder uses the card to shop or consume and signs the purchase form. Merchants provide goods or services to cardholders. The merchant submits the purchase order form to the card issuer. The card issuer pays the merchant. The card issuer issues a payment notification to the cardholder. The cardholder repays the loan to the card issuer. Using credit cards as a payment method is efficient and convenient, can reduce the amount of cash in circulation, simplify payment procedures, and can be used to deposit and withdraw cash, which is very flexible and convenient. However, credit cards also have some disadvantages: Higher transaction fees. Credit cards have a certain validity period and become invalid upon expiration. It may be lost and cause risk and trouble to the cardholder. Bills and credit cards are closed payments. Generally speaking, open payment is more convenient because the payment instrument does not need to be re-confirmed by the issuing entity; while closed payment is obviously not as good as open payment in this regard, because re-issuance increases the cost of the payment instrument itself. In this way, instruments such as endorsement and transfer can be used to increase their liquidity. However, due to technical limitations, traditional open payment has great risks and inconveniences. It is this trade-off that made payment methods such as bills and credit cards frequently used in large-value transactions before the emergence of electronic payments.