How do websites know when you input a fake credit card number?
In: Technology
Valid card numbers follow a set of rules.
Some of the numbers in certain positions are reserved for the different providers like Visa or Mastercard. If you say you have a Visa but type in certain numbers, it’ll “look like” a Mastercard thus it’s clear you made a mistake.
There is also a “checksum”. This means for valid credit card numbers, if you do certain math with the digits, the result should match the last digit. If you’re just guessing randomly, it’s likely this check fails.
But that stuff is widely known. People can write programs that generate valid credit card numbers. So how does a site know if you just guessed?
Well, you have to put in information about the cardholder like a name and address. Then the website connects to the card’s issuer and says, “Hey, I got this card number with this name and this address. Can I charge their card this much money?”
The issuer knows who they have given credit cards. If you manage to guess a number of one of their cardholders, they’ll note the name and address don’t match. “No, you can’t charge this card.”
If you manage to guess a number AND a name AND an address, well, lucky you. The problem now is that cardholder is going to report a fraudulent charge. So if you got the item shipped to your house… well, someone’s going to have a talk and it’s not going to be a good one.
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