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Luhn Algorithm

The Luhn algorithm is a simple mod 10 checksum that validates identification numbers.

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Jan 23, 2026
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How does the Luhn algorithm work and why does it catch about one digit in ten errors?

The algorithm is a simple "mod 10" checksum that validates identification numbers by doubling every second digit from the right, summing digit-values (subtracting 9 for doubled digits >9), and checkin...

Jan 22, 2026
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Can a random 16-digit number pass as a valid credit card using format checks only?

can sometimes pass common format-only credit‑card checks—regular-expression rules, issuer prefix (IIN/BIN) patterns, and the checksum—without being an actually . Passing those format tests does not pr...