Authentication system and authentication method

A portable terminal possessed by a user of a client personal computer is capable of being connected to a server independently. The server has a function of transmitting identifier-including challenge data to the client personal computer for an authentication process, receiving identifier-including response data from the client personal computer, extracting corresponding challenge data and authenticating the client personal computer based on the extracted challenge data and the received response data. The client personal computer has a function of receiving the identifier-including challenge data from the server and displaying the identifier-including challenge data. The portable terminal has a function of acquiring the identifier-including challenge data received by the client personal computer, generating identifier-including response data from the acquired identifier-including challenge data and transmitting the identifier-including response data to the server.