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, wherein 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 which is acquired by encrypting the request information using a common key method while using the first authentication information as a key; and a transmission unit that transmits the request information and the encryption information to the server via a communication network, and wherein the server includes: a reception unit that receives the request information and the encryption information via the communication network; a second authentication information generation unit that generates second authentication information that is synchronized with the first authentication information and kept from being transmitted via the communication network; a decoding unit that generates reference information which is acquired by decoding the encryption information using the 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. 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 which is acquired by encrypting the request information using a common key method while using the first authentication information as a key; and a transmission unit that transmits the request information and the encryption information to the server via a communication network, and wherein the server includes: a reception unit that receives the request information and the encryption information via the communication network; a second authentication information generation unit that generates second authentication information that is synchronized with the first authentication information and kept from being transmitted via the communication network; a decoding unit that generates reference information which is acquired by decoding the encryption information using the 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. a reception unit that receives the request information and the encryption information via the communication network; a second authentication information generation unit that generates second authentication information that is synchronized with the first authentication information and kept from being transmitted via the communication network; a decoding unit that generates reference information which is acquired by decoding the encryption information using the 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.