Hi,
I also don´t recommend to use credit cards for a security system.
I´ve heard (not sure if this is ture) that nowadays credit cards don´t use the magnetic strip anymore. They use just the chip and/or NFC.
Magnetic strip is not reliable and is easily "deleted" by a nearby magnet.
My cell phone case has slots for credit cards and a magnetic close mechanism....not a good combination for the magnet strip.
Klaus
Added:
Then NFC may be a solution for you. But for sure it most probably will read out the credit card number.
Same security problem as with the magnet strip (data).
--> there are cheap credit card size RFID cards... and many other RFID tags. Read only or R/W.. I recommend to use read only.
I assume there are NFC cards available, too.