Patent ID: 7925697

Claim:
A group judgment device that is connected to a network and that shares common private information with a target device connected to the group judgment device via the network, the group judgment device comprising: at least one processor configured to operate as: a conversion unit operable to convert the common private information into first conversion information according to a predetermined conversion; a transmission/reception unit operable to transmit first data to the target device, and receive, from the target device, second data including second conversion information in response to the first data, the target device converting the common private information into the second conversion information according to the same conversion as the predetermined conversion, and transmitting to the group judgment device the second data including the second conversion information; a measurement unit operable to measure, as a target time, a time required between (a) transmission of the first data by the transmission/reception unit and (b) reception of the second data by the transmission/reception unit; and a judgment unit operable to (i) compare the target time measured by the measurement unit with a reference time, the reference time being a time required between (a) transmission of the first data to a device belonging to a predetermined group and (b) reception of the second data from the device belonging to the predetermined group, and (ii) compare the first conversion information generated by the conversion unit and the second conversion information included in the second data received by the transmission/reception unit, and judge that the target device belongs to the predetermined group when (i) a difference between the target time and the reference time is within a predetermined range and (ii) the first conversion information matches the second conversion information.