Card numbers turn up in support tickets, chat logs, and email threads more often than anyone would like, and pasting one into an AI assistant is a needless risk. This credit card masker scans your text, validates each candidate with the Luhn checksum, identifies the network, and replaces full card numbers with a masked form that keeps only the last four digits.
How it works
The tool first finds every run of 13 to 19 digits, allowing spaces and hyphens as separators since people write cards in groups. Each candidate is stripped to digits and run through the Luhn algorithm, the same checksum issuers use, so random numeric strings that happen to be the right length are rejected. Surviving numbers are matched against standard issuer prefixes to name the network — Visa, Mastercard, Amex, Discover, JCB, or Diners — and then replaced with a masked label such as Visa ****1111. Repeats of the same card reuse the same mask.
Because the Luhn check is mathematical rather than a lookup, it works offline and produces very few false positives. Everything runs in the browser with no network calls, so the original full numbers never leave your device.
Tips and examples
The last-four-digits format is intentional: it is enough to reconcile a record or confirm which card a customer means, without exposing the full primary account number. If you need even less exposure, you can treat the masked output as the only version you keep.
Remember that masking text before pasting into an AI is one safeguard, not a complete PCI programme. Keep following your organisation’s rules for how card data is stored and transmitted elsewhere — this tool simply makes sure a full number never ends up in a chat window or an issue tracker by accident.