Patent ID: 8750511

Claim:
A root node in a wireless mesh network including a plurality of nodes to which a device key is respectively assigned, each of the device keys decrypting a cipher text and belonging to one of a plurality of groups, wherein a number of the device keys belonging to each group is a finite number, the root node being realized in a computer by executing a program with a processor, the root node comprising: a device key assignment database configured to store a correspondence relationship between the each of the plurality of nodes and its assigned device key, and correspondence relationships between past join nodes joined in the past in the wireless mesh network and a device key assigned to past join nodes; a join node control unit configured to detect a new node newly joining in the wireless mesh network; a device key assignment unit configured to: when the new node is one of the past join node, assign a past device key that was assigned to the past join node to the new node again, based on the correspondence relationships, when the new node is not one of the past join node, assign a new device key to the new node, encrypt the device key assigned to the new node, and notify the new node of the encrypted device key; and a cipher text generation unit configured to, when a message is sent to the nodes and the new node, generate a cipher text by encrypting the message using the device keys assigned to the nodes and the new node, wherein the cipher text has a fixed length if the message is encrypted using device keys belonging to the same group, and a size of the cipher text is reduced if a number of groups to which the device keys assigned to the nodes and the new node belong is reduced.