eli5 how do EFTPOS machines instantly know a card number is invalid? for instance if there are 16 card numbers and I put in the 10th digit in wrong, machine beeps and does not let me put that digit it in, surely there are cards with one number difference

Question

eli5 how do EFTPOS machines instantly know a card number is invalid? for instance if there are 16 card numbers and I put in the 10th digit in wrong, machine beeps and does not let me put that digit it in, surely there are cards with one number difference

In: 8

Answer 1

Here’s another way it might work, other than a checksum:

The card company only issues card numbers that are more than one typo away from all other existing card numbers. This why there are 16 digits in the card number, but only 10 digits (7 billion) worth of people in the work – most of the possible combinations of digits in the 16 digit card number are ruled out using this rule.