A store allows you to join their loyalty club by simply presenting an ID and a credit card. Your identity is somehow linked to your card and automatically registered the next time you pay, no extra actions required. This means the store is able to uniquely identify your credit card, but how? Directly using the card # is not secure, as it could be stolen in case of an incorrectly/maliciously configured cash register. The card reader and cash register are two separate devices from different manufacturers, so an identifier must be sent by the reader to the cash register. How is this done without modifying the actual card and complying with data security standards?