Patent ID: 7225342

Claim:
A terminal apparatus that communicates with another terminal apparatus on a peer to peer network, said terminal apparatus possessing a public key of a group formed on the peer to peer network, said terminal apparatus comprising: an inquiry information sending unit operable to send inquiry information to the other terminal apparatus, the inquiry information indicating an inquiry about whether or not the other terminal apparatus is a terminal apparatus of an authorized member of the group formed on the peer to peer network; an encrypted information receiving unit operable to receive predetermined encrypted information from the other terminal apparatus in response to the inquiry information sent by said inquiry information sending unit; a decryption trial unit operable to try decrypting the received encrypted information by using the group public key; an information judgment unit operable to make a judgment as to whether or not decrypted information is appropriate, only when said decryption trial unit successfully decrypts the received encrypted information, and the decrypted information includes a group participation certificate whose expiration date does not exceed a predetermined expiration date; and a terminal judgment unit operable to judge that the other terminal apparatus is a terminal apparatus of an authorized member of the group, when said information judgment unit judges that the decrypted information is appropriate.