This is a small command line utility offering some functions related to credit card number validation.

• Generate valid credit card numbers
• Check which companies cards belong to
• Check if an entered number is valid

Credit card and debit card numbers are checked with Luhn's Algorithm, a checksum formula used to catch simple transcription errors in card numbers when they're entered. It's not perfect; if too many digits are changed, or switched, then it may incorrectly verify a number as correct.

The algorithm relies on doubling every even digit in the number, adding the individual results together (so 6 * 2 = 12, and 1+2 = 3), adding all of those results, and then comparing the mod of that sum with the check digit. In the case of a credit card number, that check digit is the last number in the credit card number.

Different card companies have certain lead numbers. for example, Visa card numbers all start with 4. By checking which digits a given card number begins with, the type of card can be identified. Not all card number ranges are used, as only specific numbers are associated with companies.

For a more thorough explanation of how this algorithm works, with a few examples, the Wikipedia article on the subject is quite good.

• -H -- Help/Usage
• -K -- Generate a valid card number with a valid company
• -U -- Generate a valid card number with an unknown company
• -C -- Check a given card number for validity (ex. python main.py -C 94304056966994)