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
There is some internal logic to the format the numbers should be in. So in many cases a typo of one digit would make the number invalid.
Old computer games could also do that offline where the CD key had to follow a certain format and rules.
Someone else can probably explain in detail how the check actually works.
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