Luhn Algorithm

The Luhn algorithm is a simple checksum formula used to validate a variety of identification numbers, including credit card numbers and government identifiers. It was developed by IBM scientist Hans Peter Luhn and patented on August 23, 1960; the formula is now in the public domain. The algorithm works by removing the existing check digit, doubling every second digit from right to left (subtracting 9 from any result over 9), summing all the resulting digits, and choosing a check digit that makes the total divisible by 10. Because it is designed to catch accidental transcription errors rather than deliberate fraud, it is best understood as an error-detection tool for people keying in numbers rather than a security measure. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/