Patent ID: 8249258

Claim:
A communications method comprising: organizing a group to which a plurality of members can join or leave, a group key for use in encryption or authentication of communication data is not only shared in the group, but also assigned to a most significant root, a subgroup key is assigned to nodes that are branching points of branches, each member is assigned to a leaf at the end of a least significant subtree, and each member communicates by retaining the group key and all subgroup keys from the group key to itself, wherein each member belonging to the group stores in advance tree structure data of the entire group, the group key, and all the subgroup keys, when each member detects that a new member joins, a tree structure data updating step in which each member causes tree structure data updating means to assign a joining member to a leaf of the tree structure according to a predetermined rule, and to update the tree structure data stored by each member; a captainship determination step in which each member causes captainship determination means to determine whether or not the member will be a captain of a subtree according to a predetermined rule from new tree structure data; a new key generation and distribution step in which the captain causes new key generation and distribution means to generate and distribute new keys at least among respective members of its subtree, wherein the captain is a member assigned to a leaf and the captain generates and distributes a subgroup key to each member of the subgroup; and wherein, in the captainship determination step, a predetermined rule for determining whether to be a captain or not selects a captain candidate member in a subtree from members of leaves of a branch opposite to a branch where a joining member lies, when viewed from the high level of the subtree.