From my “over a year ago” post about test credit and debit card numbers, I did wonder how unique/secure a credit or debit card number is. For example: If I knew “x” digits of your card number, how many possible cards would I have to guess at to hit your credit card number.
I’m basing these findings on a the most common length of credit card numbers – 16 digits.
|Number of digits known||Number of card possibilities if you have the “check digit” at the end||Number of card possibilities if you do not have the “check digit”||Observations|
|16||1||1||You’ve got the whole number anyway!|
|15||1||1||You can either calculate the missing check digit if that’s the only thing you are missing, or calculate the missing number from the check digit|
|14||10||100||By covering one number and the final number, you’ve just made it 10 times harder to work out your credit card number!|
|01||100,000,000,000,000||1,000,000,000,000,000,000||This could just be the check digit|