Provided is an authentication system in which a client terminal that receives input of request information is connected to a server that executes a process with regard to the request information. The client terminal includes: a first authentication information generation unit that generates first authentication information based on information which is shared with the server; an encryption unit that generates encryption information; and a transmission unit that transmits the request information and encryption information to the server. The server includes: a reception unit that receives the request information and encryption information; a second authentication information generation unit that generates second authentication information; a decoding unit that generates reference information which is acquired by decoding the encryption information using the same common key method as in the client terminal while using the second authentication information as a key; and an authentication unit that compares the request information with the reference information.
