A conventional looking payment card comprises a plastic card with a legacy card reader compatible magnetic stripe for dynamic user account data. Internal to the plastic card, and behind the magnetic stripe, a number of fixed-position magnetic write heads allow the user account data to be modified autonomously. Electronics within the card are pre-loaded with many unique numbers that are selected for one-time use in financial transactions. A payment processing center keeps track of the unique numbers used, and knows which numbers to expect in future transactions. It will not authorize transaction requests if the unique number read during a magnetic card swipe is not as expected. A card-swipe detector embedded in the plastic card detects each use in a scanner, so changes can be made to the data bits sent to the write heads.