Double digital signature: When we make a purchase with a credit card, as a cardholder, we give the merchant ordering information and the bank payment information, thus authorizing the bank to pay. However, we don't want the merchants to know the information of their accounts, and we don't want the banks to know the specific consumption content, so we just need to credit or debit the accounts according to the amount. This is actually a double digital signature, which sends two related messages to the receiver, and the receiver can only open one, while the other only needs to forward, but can't open it to see its content. This effectively protects the privacy of consumers and the business secrets of businesses.