1. An authentication system including a plurality of user terminals that are used by the same user, and a server capable of communicating with the plurality of user terminals, wherein a user terminal transmits, to the server, an identification ID of the user terminal, a first code that is used in common in the plurality of user terminals, and a second code that was encrypted using the first code, the server is configured to decrypt the encrypted second code using the first code, and perform authentication of the user terminal using the identification ID and the decrypted second code, and the server, upon receiving a second code that is not encrypted from one of the plurality of user terminals, transmits the second code that was encrypted using the first code to the one of the user terminals.