Abstract: A device authentication system, for example for consumer electronic products, uses a portable authenticator or key fob (16) to respond to periodic broadcast challenges from protected devices (10,12). Public key cryptosystem technology is used, with the owner's public key being stored within each of the protected devices, and the corresponding private key within the key fob. Each challenge issued by a protected device is encrypted using the public key, and on receipt decrypted using the private key. If decryption is successful, a verification message is sent from the key fob to the protected device, authorising the protected device to continue operation. If a verification message is not received by the device, the device ceases to operate.