An improvement of a dynamic cryptographic key assignment scheme in a tree hierarchy (Q1962972)

Recently, \textit{H. T. Liaw}, \textit{S. J. Wang} and \textit{C. L. Lei} [Comput. Math. Appl. 25, No. 6, 109-114 (1993; Zbl 0796.68085)], proposed an efficient cryptographic key assignment scheme for solving the access control problem in a tree structure. Basically, the scheme is based on Newton's interpolating method and a one-way function. The scheme not only achieves dynamic key assignment, but is also simple and efficient for generating and derivating keys. However, the author shows that several security classes can collaborate to derive the secret key of their immediate ancestor in this article. He also proposes two schemes for modifying Liaw-Wang-Lei's scheme slightly so that the security will be greatly improved.