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! |
13 | 100 | 1,000 | |
12 | 1,000 | 10,000 | |
11 | 10,000 | 100,000 | |
10 | 100,000 | 1,000,000 | |
09 | 1,000,000 | 10,000,000 | |
08 | 10,000,000 | 100,000,000 | |
07 | 100,000,000 | 1,000,000,000,000 | |
06 | 1,000,000,000 | 10,000,000,000,000 | |
05 | 10,000,000,000 | 100,000,000,000,000 | |
04 | 100,000,000,000 | 1,000,000,000,000,000 | |
03 | 1,000,000,000,000 | 10,000,000,000,000,000 | |
02 | 10,000,000,000,000 | 100,000,00,000,000,000 | |
01 | 100,000,000,000,000 | 1,000,000,000,000,000,000 | This could just be the check digit |