A Robust Time-Bound Hierarchical Key Assignment Scheme

Abstract

Recently, Chien proposed a time-bound hierarchical key assignment scheme based on tamper-resistant devices, which improves the performance in terms of the computational efforts and the implementation costs. Later, Santis et al. demonstrated a collusion attack on Chien's scheme to show that collusive malicious insiders can cooperatively derive some encryption keys and further proposed an improvement to eliminate the security flaws inherent in Chien's scheme. In this paper, we will prove that Santis et al.'s key derivation is incorrect and their claimed security requirements cannot be achieved. On the other hand, we will further propose a new key derivation to improve the weakness in Santis et al., scheme.