An access control system is described in which a primary credential device has a master key and a secondary credential device has a key derived from the master key. Both the master key and the derivative key are required to gain access to the resource protected by the access control system. If the secondary credential device is lost, misplaced, or stolen, it cannot be used to gain illicit access to the protected resource, and it can be easily replaced by providing a different secondary credential device with another key derived from the master key.