Authentication based on cryptographic protocols is a key technology for recent security systems. This paper proposes a new authentication method that utilizes the side channel that already exists in many authentication systems. Side-channel analysis has been studied intensively from the attacker viewpoint and is best known for key-recovery attacks against cryptographic implementations using physical information. In this paper, reversing the traditional thought, we propose to use the key-dependent side-channel information constructively to enhance, or as an alternate to, existing cryptographic protocols. Using Advanced Encryption Standard (AES)-based authentication as an example, we demonstrate, based on experiments using an Field Programmable Gate Array (FPGA), that the side-channel information leaked from cryptographic devices is sufficiently unique for authentication.