Patent ID: 8005225

Claim:
A hierarchical threshold tree-based broadcast encryption method comprising: subscribing users by a server; distributing by the server, a message to enable a privileged user to decrypt a group key; and decrypting, by a privileged device corresponding to the privileged user, the message using the group key, wherein the subscribing the users comprises: constructing, by the server, a tree with h levels, the tree including all users; dividing, by the server, each of nodes at all levels except a node at level “0” of the tree into corresponding one or more sub-groups, each of the one or more sub-groups including w nodes; randomly creating, by the server, node keys corresponding to the respective nodes of the tree except the node at the level “0”; creating, by the server, random polynomials of a d-th degree for corresponding sub-groups at all the levels except the level “0”; creating, by the server, shared values which restores secret values of the d-th degree polynomials by using identifiers (IDs) of the respective nodes in the created random polynomials of the d-th degree; and transmitting, by the server, the created node keys and the shared values to the respective users, wherein the identifiers inform a member of a shared value among the shared values retained by the member, and wherein the shared value is used to recover a corresponding secret value, and wherein the distributing the message comprises selecting one of two encryption methods in accordance with a number of nodes belonging to a Steiner tree constructed by linking all nodes corresponding to given revocators, creating broadcast messages using the selected encryption method, and transmitting the created broadcast messages.