I came across this while surfing. I knew credit card numbers had checksums in them but never knew exactly how they worked. It's known as the Luhn Algorithm after it's creator Hans Peter Luhn. A simple checksum wouldn't catch swapped adjacent digits, one of the most common errors. So Luhn uses a weighted sum where alternate digits are multiplied by 2 and if that's greater than 10 it's digits are summed. It will detect swapped digits 97.8% and garbled or forged numbers 90% of the time. Simple and elegant.
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