A personal blog by Khürt Williams, with imagery, and inchoate ramblings on coffee, beer, and geekery.
Add all other digits, with the exception of the last digit (the checksum), to this total.
The difference between the sum and the next multiple of 10 is the check digit.
For example, the correct hash digit for credit card number 5012 3456 7890 123X is 6.
Note: Until recently all credit card companies were using the same algorithm. Recent advances in credit card fraud have caused the credit card companies to pursue further measures of validating credit cards such as the 3-digit number found on the back of your credit card.” — From CISSP course
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