Patent ID: 7441117

Claim:
A group management device that manages a group, comprising: a reception unit operable to receive, from a member device, a request for registration in the group; a judging unit operable, (i) upon receiving the request, if the member device is authenticated as being a legitimate device, to judge whether a registered number of member devices is less than a maximum number of member devices registerable in the group, (ii) when judged in the affirmative, to issue valid period information showing a valid period of use of common secret information, the valid period being unique to the member device, and the common secret information being unique to the group and common among a plurality of member devices registered in the group, and to increase the registered number, and (iii) to monitor an elapse of the valid period and reduce the registered number when the valid period ends; and a communication unit operable, when judged in the affirmative, to output to the member device, the common secret information and the valid period information.