Cryptanalysis of a simple key assignment for access control based on polynomial

In 2013, Lo et al. proposed a simple key assignment based on polynomial for access control. Access control in a large leaf class hierarchy let a superior can get the data of his/her subordinates without the approval of the subordinates. The scheme can reduce the computation of leaf class's key generation. It is more efficient and secure than the others. However, we will show an attack in Lo et al.'s scheme in this article. The attack is a compromising attack about how to use the changed polynomial to derive the member's secret key, and then the attacker can access information in the hierarchy.