Luhn mod N check character

This capability computes that character for a payload, or validates a full string that already ends with one, and it fails fast when the input contains symbols outside the alphabet so you never silently accept garbage. Run it free in the browser or call the KIT API at the published price; both paths use the same pure arithmetic with no network and no stored values.

What Luhn mod N is and how it extends classic Luhn

Wikipedia’s formulation starts the generate pass with factor two on the rightmost payload character and the validate pass with factor one on the trailing check character, which is exactly what this tool implements so goldens match the public reference. The response always names algorithm luhn_mod_n so logs and partner contracts can record which scheme protected the value.

How to compute a check character or validate a full string

Empty text, missing alphabet, alphabets shorter than two distinct characters, and unknown mode values fail before any arithmetic runs and are not billed. Provide text as the payload without a check character for mode compute (the default), and alphabet as the ordered set of valid symbols. The engine maps each distinct alphabet character to successive code points, walks the payload right to left, and returns check_character plus with_check (payload plus that character). Mode validate expects a full string of at least two characters; the response reports valid, the trailing check_character, expected_check_character recomputed from the prefix, and the payload substring without the last character so UIs can show a clear correction. Every symbol in text must appear in alphabet: if any character is outside, the call fails with invalid_input and the message lists those symbols so you can fix the alphabet or strip noise before retrying. Because the path is pure parse with no clock, random source, or network, the same input always yields the same JSON on the free browser tool and on the API. At $0.002 per successful request you can protect bulk imports, label printers, and onboarding forms without a custom tier. Use digits 0123456789 to reproduce classic mod-10 Luhn-style generation for pure digit payloads; use 0-9A-Z for Base36-style product codes; use a custom house set when your inventory already forbids ambiguous glyphs such as O and 0.

Where generalized Luhn check characters fit real systems

Warehouse and asset tags often mix letters and digits so operators can read them aloud; appending a Luhn mod N character catches adjacent swaps and single substitutions before a handheld scan creates the wrong shipment. Software license keys and coupon codes that already exclude confusing characters still need a cheap integrity digit that partners can reimplement without a shared secret; publishing the alphabet and this algorithm is enough. Do not treat a passing check as proof of authorization or ownership — it only proves the string is well formed under Luhn mod N for that alphabet, the same honesty bar you apply to any checksum. Membership and loyalty programs that outgrew pure numeric IDs can keep one mint path: generate a sequential or random payload in the house alphabet, call compute, store with_check, and validate on every form and partner file. Data-quality jobs recompute expected_check_character for every row in an export and quarantine mismatches, which catches both human edits and off-by-one bugs in generators. Integration tests freeze goldens against known Wikipedia-style examples such as payload abcdef over alphabet abcdef yielding check character e, so upgrades never silently change the scheme.