An apparatus and method for controlling the feature set of a programmable device by assigning each programmable device a unique serial number, correlating a secret key with each serial number, and keeping that information in a secure location, such as at the manufacturer. Each optional...http://www.google.co.uk/patents/US5771287?utm_source=gb-gplus-sharePatent US5771287 - Apparatus and method for secured control of feature set of a programmable device 
