Patent ID: 7774598

Claim:
A method of a broadcast apparatus for managing a key of a user for a broadcast encryption, the method comprising: forming a tree including m hierarchies by repeating a process of setting an i th level including groups into each of which at least one node is grouped in a unit of n i , and setting a i+1 th level comprising groups into each of which the groups of the i th level are grouped in a unit of n i+1 , where i is from 1 to m−1, and users are mapped on nodes of the 1 st level and a message provider is mapped on the m th level; detecting a revoked user among the users; mapping a key encryption key with respect to at least one group of the groups of the m th level, the at least one group not including the revoked user; going down from the (m−1) th level to the 1 st level for mapping key encryption keys with respect to a group of each level from the (m−1) th level to the 1 st level, the group of each level including the revoked user; and transmitting, by the broadcast apparatus, a message using the key encryption keys, wherein the mapping of the key encryption keys comprises: forming a matrix having columns corresponding to groups of a level of the hierarchies and rows corresponding to a number of keys; mapping virtual users and key encryption keys in the matrix; detecting a virtual user comprising a unauthorized user on a lower level to remove key encryption keys of the detected virtual user from the matrix; and selecting key encryption keys of the users remaining on the matrix after the key encryption keys of the detected virtual user are removed from the matrix.