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Techy: How secure are credit card numbers

Last updated on September 4, 2012

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