For security, a credit card number is coded in the following way, so that it can be sent as a message. "Subtract each digit from 9" code the credit card number 3201 2342 3458 0931 I dont understand the subtract each digit from 9 is that to code or uncode? Also for the above code the credit card what do I do to each number? do I add 9 or subtract 9? I have another credit card number that is already coded, to find the original card number do i subtract each number from 9?