1. An authentication system, comprising: a user terminal comprising a processor and memory; and a server for communicating with the user terminal, wherein the user terminal transmits, to the server, an identification ID of the user terminal, a first code that was entered, and a second code that was encrypted using the first code, the server decrypts the encrypted second code using the first code, performs authentication of the user terminal using the identification ID of the user terminal and the decrypted second code, a data volume of the first code is smaller than that of the second code, the first code includes characters of the same type as the characters that constitute the second code, and the first code is configured to be used in common in a plurality of user terminals that are used by the same user, wherein if the plurality of user terminals used by the same user include: a first user terminal that is registered in association with the identification ID and the second code; and a second user terminal that is not yet registered in association with the identification ID and the second code, the server, upon accepting the second code transmitted from the second user terminal, registers the identification ID and the second code of the first user terminal, in association with the second user terminal, and transmits the encrypted second code of the first user terminal to the second user terminal.