Communication Method and Communication System Using Decentralized Key Management Scheme

A communications method for used in a communication network, wherein a group to which a plurality of members can join is organized, 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,
• the communication method using a decentralized key management scheme including:
• when each member detects that a member leaves by joining/leaving detection means,
• a captainship determination step in which each member causes captainship determination means to determine whether or not the member will be a captain in a subtree from the tree structure data excluding the leaving member according to a predetermined rule;
• a new key generation and distribution step in which the captain causes new key generation and distribution means to generate and distribute new keys among at least its subtree members and other captains;
• and
• a tree structure data updating step in which each member causes tree structure data updating means to re-assign members of the subtree to which the leaving member belongs and updates the tree structure data stored in the member according to a predetermined rule.

A communications method for used in a communication network, wherein a group to which a plurality of members can join is organized, 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 a terminal unit that is each member comprises:
• storage means for storing tree structure data of an entire group, a group key, and all subgroup keys;
• joining/leaving detection means for detecting joining of a new member or leaving of a member;
• tree structure data updating means for either assigning a joining member to a leaf of a tree structure according to a predetermined rule and updating tree structure data stored therein, or re-assigning members of the subtree to which a leaving member belong as a leaf according to a predetermined rule and updating tree structure data stored therein;
• captainship determination means for determining, from the tree structure data, whether or not the member will be a captain of the subtree according to a predetermined rule;
• and
• new key generation and distribution means for generating and distributing a new key at least among members of its subtree, when it becomes a captain.