Management device and management method

1. A management device comprising:

a management tree storage configured to store therein a binary tree including a plurality of nodes that is assigned with respective node keys;

one or more processors configured to:

update at least one of the node keys;

select at least one of a first subtree and a second subtree, the first subtree and the second subtree being subtrees including leaf nodes of the binary tree, the leaf nodes corresponding to respective communication devices included in a group, the first subtree including only leaf nodes with the respective node keys assigned thereto not having been updated, the second subtree including only leaf nodes with the respective node keys assigned thereto having been updated; and

transmit a group key encrypted using a node key assigned to a root node of the selected subtree, wherein

the management tree storage is configured to store therein a first binary tree including a plurality of nodes with the respective node keys assigned thereto not having been updated yet, and a second binary tree including a plurality of nodes with the respective node keys assigned thereto having been updated, and

the one or more processors are configured to select at least one of the first subtree that is a subtree of the first binary tree and the second subtree that is a subtree of the second binary tree.

2. The management device according to claim 1 , wherein the one or more processors are configured to generate the second binary tree in such a manner that a plurality of leaf nodes, corresponding to a plurality of respective communication devices belonging to a same group, has a second ancestor node, a level of the second ancestor node being closer to the leaf nodes compared with a level of a first ancestor node that is an ancestor node of the plurality of leaf nodes in the first binary tree, and store the second binary tree in the management tree storage.

3. The management device according to claim 2 , wherein the one or more processors are configured to transmit the node key having been updated to a communication device corresponding to a leaf node of the second binary tree.

4. The management device according to claim 3 , wherein the one or more processors are configured to transmit a set of node keys assigned to respective nodes between a root node and a first leaf node of the second binary tree to the communication device corresponding to the first leaf node.

5. The management device according to claim 1 , wherein the one or more processors are configured to select a subtree including leaf nodes of the first binary tree, the leaf nodes corresponding to respective communication devices included in a group, based on a complete subtree (CS) method, to select the first subtree from the selected subtree, to select a subtree including leaf nodes of the second binary tree, the leaf nodes corresponding to the respective communication devices included in the group, based on the CS method, and to select the second subtree from the selected subtree.

6. A management method comprising:

updating at least one of node keys of a binary tree including a plurality of nodes that is assigned with the respective node keys;

selecting at least one of a first subtree and a second subtree, the first subtree and the second subtree being subtrees including leaf nodes of the binary tree, the leaf nodes corresponding to respective communication devices included in a group, the first subtree including only leaf nodes with the respective node keys assigned thereto not having been updated, the second subtree including only leaf nodes with the respective node keys assigned thereto having been updated; and

transmitting a group key encrypted with a node key assigned to a root node of the selected subtree, wherein

the selecting includes selecting at least one of a first subtree that is a subtree of a first binary tree and a second subtree that is a subtree of a second binary tree,

the first binary tree and the second binary tree are stored in management tree storage,

the first binary tree includes a plurality of nodes with the respective node keys assigned thereto not having been updated yet, and

the second binary tree includes a plurality of nodes with the respective node keys assigned thereto having been updated.

7. The management method according to claim 6 , further comprising:

generating the second binary tree in such a manner that a plurality of leaf nodes, corresponding to a plurality of respective communication devices belonging to a same group, has a second ancestor node, a level of the second ancestor node being closer to the leaf nodes compared with a level of a first ancestor node that is an ancestor node of the plurality of leaf nodes in the first binary tree, and

storing the second binary tree in the management tree storage.

8. The management method according to claim 6 , wherein the selecting includes selecting a subtree including leaf nodes of the first binary tree, the leaf nodes corresponding to respective communication devices included in a group, based on a complete subtree (CS) method, selecting the first subtree from the selected subtree, selecting a subtree including leaf nodes of the second binary tree, the leaf nodes corresponding to the respective communication devices included in the group, based on the CS method, and selecting the second subtree from the selected subtree.