Hierarchical access control is an important and traditional problem in
information security. In 2001, Wu et.al. proposed an elegant solution for
hierarchical access control by the secure-filter. Jeng and Wang presented an
improvement of Wu et. al.'s method by the ECC cryptosystem. However,
secure-filter method is insecure in dynaminc access control. Lie, Hsu and
Tripathy, Paul pointed out some secure leaks on the secure-filter and presented
some improvements to eliminate these secure flaws.