Abstract: A process and arrangement that gives selective access to a security system, particularly in a payment system using debit cards, credit cards, or withdrawal of funds contained in a so-called smart card, and in particular on a chip card. The system comprises at least one first device and at least one second device that must be used in a correct configuration with one another in order to gain access. Access is gained by verifying that a certain coded key K2, held in the second device, is recognized as being valid by the first device, after comparing it with a renewing key K1 and previous versions of the code key K1 contained in the first device.