Eli5: how does platforms like Steam or Amazon know if you’re using a correct visa card or not if you didn’t buy anything with it?

Question

When you enter your card details to add it to your account and you write 1 number wrong, it instantly tells you that the card is wrong. How do they know?

In: Technology

Answer 1

There is a method called the [Luhn algorithm](https://en.wikipedia.org/wiki/Luhn_algorithm) that can be used to check the digits of the card number. The last couple digits of the card number are the result of the Luhn algorithm on the other digits. This is built in as a simple check for errors.