Credit Card Security Code (English: card security code) is a security feature of credit cards in online and telephone transactions. It is usually a 3-or 4-digit number printed on a credit card to prove that the payer has a credit card at the time of transaction, thus preventing credit card fraud. For the cardholder, there is no specific meaning.
The security code of China UnionPay is called CVN2 (card verification number 2), which has 3 digits and is printed after the 4 digits of the card number on the back of the credit card.
Visa's security code is called CVV2 (card verification value 2), which has 3 digits and is printed on the last 4 digits of the card number on the back of the credit card. A security code is generated when a credit card is issued. The issuing bank uses the credit card verification key to encrypt the card number, expiration date and service number of the credit card. Theoretically, only the issuing bank (and cardholder) knows the final generated security code.