Card Validator: What It Checks and What It Cannot Do

A card validator checks whether a card number is well formed. It runs format rules and a checksum, and it may look up the issuer from the leading digits. It does not confirm that an account is open, funded, or authorized for a charge. Only the issuing bank, through an authorization request, can answer that.

What a card validator can check

• Total length and the allowed lengths for the detected network.
• Whether the number passes the Luhn checksum.
• Which issuer range the leading digits belong to.
• Whether the expiration date is a real calendar date.
• Whether the security code has the expected digit count for that network.

A Luhn check catches single-digit typos and most transpositions. It has no knowledge of the account behind the number.

What a card validator cannot check

• Whether the account exists.
• Whether the account is open or has available credit.
• Whether the cardholder authorized the charge.
• Whether the billing address or cardholder name matches issuer records.

A number that passes every local check can still be declined. Processors run authorization with the issuer, and that step is the only source of truth about whether a card can be charged.

Validating card data in a payment form

1. Detect the network from the leading digits as the customer types.
2. Apply the length rule for that network.
3. Run the Luhn checksum on the full number.
4. Check the expiration month and year against the current date.
5. Check the security code length for the detected network.
6. Send the data to your processor for authorization.
7. Show a generic decline message and log the response code.

Client-side checks reduce typos and cut down on needless authorization attempts. They are not a security control. A submitted form can be scripted, so the server must repeat every check the client performed.

Rules that apply to card data

Card security codes are sensitive authentication data under PCI DSS. They may be used to process a transaction and must not be stored after authorization. Full card numbers require encryption at rest and are out of scope only when a compliant provider handles them on your behalf.

Using card numbers you do not own is a crime in the United States under wire fraud and access device statutes. A validator cannot make invalid data usable, and no check on your side changes that.

Common mistakes

• Treating a passed checksum as proof that a card is live.
• Hardcoding one length or prefix list instead of updating it.
• Rejecting valid numbers because the customer typed spaces or hyphens.
• Storing the security code in logs or a database.