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

It’s a basic error check called a luhn check. There’s a standard equation the machine runs against the 1st 15 (or 17) digits in a card, the answer to this has to exactly match the last number on the long card number (PAN).

It’s not really a security feature but it will almost always prevent an incorrect card number being entered by mistake for manual entry, and if course fraud.