Registered network-based identification corresponding to any of networks is stored in a key device 12. A terminal device 11 sends ciphertext and network-based identification to the key device 12, using a network. The key device 12 receives the sent ciphertext and network-based identification and outputs response information corresponding to a decrypted value of the ciphertext when the network-based identification corresponds to the registered network-based identification. The terminal device 11 obtains the decrypted value from the response information.