Fun Luhn fact: you can transpose any two odd or even indexed digits except the final check digit in a credit card number, and still get a card number that passes a Luhn checksum. You can use this to give cybercriminal scammers card numbers that probably aren't connected to any actual account.
They updated the title. The original was something like "the math behind secure credit card numbers". Note that this is not for security, just to detect typos and give a friendly error message.
