Abstract: This application note examines keys for physical access control from a new perspective: information technology (IT). It compares various types of key technology (mechanical, magnetic, contact, RFID), and evaluates these keys for their strengths and weaknesses. Keys with challenge and response authentication overcome the limitations of the traditional static data keys. The challenge and response concept is discussed; suitable keys are presented and compared. The document concludes with an explanation of why challenge and response authentication is more secure than concepts that rely on static data.