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

The last digit of the card number is a checksum digit: for a given combination of 15 digits, there’s only one possible value for the final digit.

If that value doesn’t match up, the card number clearly was entered incorrectly