Method to authenticate an accessory

An apparatus (100) used by a plurality of devices to authenticate an accessory (120) is configured to operate with a device (110) of the plurality of devices. The accessory (120) applies an authentication algorithm to a key and a challenge (130) received from the device (110) and generates a response (132) thereto. A challenge and response memory (114) stores a subset of a set of challenges (232) and pre-computed responses (230). The enabling circuit transmits a challenge (130) to the accessory (120) and receives a received response (132) therefrom. The enabling circuit (112) also compares the received response (132) to the stored response (230) corresponding to the stored challenge (232) sent to the accessory (120). The enabling circuit (112) is configured to generate a normal operation enabling signal that enables normal operation of the device when the received response (132) matches the stored response and is configured not to generate the normal operation enabling signal when the received response (132) does not match the stored response.