Improved Subset Difference Method with Ternary Tree.

This paper proposes a ternary subset difference method (SD method) that is resistant to coalition attacks In order to realize a secure ternary SD method we design a new cover finding algorithm label assignment algorithm and encryption algorithm These algorithms are required to revoke one or two subtrees simultaneously while maintaining resistance against coalition attacks We realize this two way revocation mechanism by creatively using labels and hashed labels Then we evaluate the efficiency and security of the ternary SD method We show that the number of labels on each client device can be reduced by about 204 percent The simulation results show that the proposed scheme reduces the average header length by up to 15 0 percent in case where the total number of devices is 65 536 On the other hand the computational cost imposed on a client de vice stays within O(log n) Finally we prove that the ternary SD method is secure against coalition attacks