Patent ID: 7698743

Claim:
A server that detects an unauthorized terminal from among a plurality of terminals, said server comprising: a terminal information receiving unit operable to receive terminal information from each respective terminal of the plurality of terminals, the terminal information received from each respective terminal including (i) key identification information that is unique to secret key information, the secret key information and the key identification information being assigned to the respective terminal in advance, (ii) authentication information that is defined based on the secret key information, and (iii) additional identification information that is assigned to the respective terminal; a terminal information holding unit operable to hold server terminal information including (i) the key identification information, (ii) at least one of the secret key information and public key information corresponding to the secret key information, and (iii) the additional identification information; a key information selection unit operable to output at least one of the secret key information and the public key information, based on the key identification information included in the terminal information received by said terminal information receiving unit, wherein the at least one of the secret key information and the public key information output by said key information selection unit is held in said terminal information holding unit; a terminal verification unit operable to verify the authentication information received by said terminal information receiving unit, based on the at least one of the secret key information and the public key information output by said key information selection unit; a terminal information search unit operable to search the server terminal information held in said terminal information holding unit, based on the received key identification information; and an unauthorized terminal judgment unit operable to judge that a terminal, of the plurality of terminals, that corresponds to the additional identification information is an unauthorized terminal when a predetermined number or more of different pieces of additional identification information has been searched by said terminal information search unit, for identical key identification information.