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

The [Luhn algorithm] (https://en.m.wikipedia.org/wiki/Luhn_algorithm). It’s a simple checksum formula used to validate a variety of identification numbers