Abstract: A method and apparatus for verifying that the bearer of a card (e.g., credit card, bank card, etc.) is authorized to use the card. The card bears machine-readable indicia of an account number (PAN) and the bearer of the card has memorized a personal identification number (PIN). There is associated with the PAN a check number (PCN) which is derived by (1) generating a first cipher Y1 by encrypting the PAN using the PIN in combination with a secret security number as a key, the bits of which address a data encryption process; (2) generating a second cipher by decrypting the first cipher using the secret security number as a key so that the decryption process is the reverse of the encryption process; and (3) storing the second cipher as the check number PCN in a machine-accessible location, which may be in a separate memory or recorded on the card itself.