The present application provides methods for downloading a key, a client, a password device, and a terminal device, in which, the client sends a request for downloading an initial key to a backend server, and receives a server identity certificate delivered by the backend server and forwards the server identity certificate to the password device. The client acquires a device identity ciphertext returned by the password device and sends the device identity ciphertext to the backend server. The client acquires a server identity ciphertext and an initial key ciphertext generated by the backend server, and sends the server identity ciphertext to the password device. After the password device successfully verifies an identity of the backend server, the client sends the initial key ciphertext to the password device.