text and ss t is the signature of the trusted third party tp furthermore the second message sending unit 43 is further configured if the first token includes the token tokenta to verify whether the signature of the trusted third party tp is valid and to determine whether the random number r u2032 a generated by the entity a in the message 2 is consistent with the random number r u2032 a included in the signature of the trusted third party tp before sending the message 4 to the entity b and to send the message 4 to the entity b when the signature of the trusted third party tp is verified as being valid and the random number r u2032 a generated by the entity a in the message 2 is determined as being consistent with the random number r u2032 a included in the signature of the trusted third party tp a if the group gb is determined from res gb as being legal and valid to perform the step b otherwise to determine the entity b as being illegal and end the flow and b to obtain a public key of the group gb by i gb to verify against the public key whether the signature of the entity b is valid to determine whether a distinguishing identifier of the group ga is consistent with a group distinguishing identifier ga included in the signature of the entity b and to determine whether a random number r a generated by the entity a in the message 4 is consistent with a random number r a included in the signature of the entity b and if all are positive to determine the entity b as being legal otherwise to determine.