Key points are not available for this paper at this time.
Tzeng (2002) proposed a time-bound cryptographic key assignment scheme for access control in a partial-order hierarchy. In this paper, we show that Tzeng's scheme is insecure against the collusion attack whereby three users conspire to access some secret class keys that they should not know according to Tzeng's scheme.
Yi et al. (Tue,) studied this question.