According to one embodiment, a management device includes a management tree storage and one or more processors. The management tree storage stores therein a binary tree including a plurality of nodes that are assigned with respective node keys. The processors update at least one of the node keys. The processors selects 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. The processors transmit a group key encrypted using a node key assigned to a root node of the selected subtree.