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 |