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Recently, Chien proposed a time-bound hierarchical key assignment scheme based on tamper-resistant devices. Without public key cryptography, Chien's scheme greatly reduces computation load and implementation cost. In this paper, we show that Chien's scheme is insecure against a collusion attack whereby three users conspire to access some secret class keys that they should not know according to Chien's scheme.
Index Terms- Cryptography, hierarchical key assignment.

X. Yi, "Security of Chien's Efficient Time-Bound Hierarchical Key Assignment Scheme," in IEEE Transactions on Knowledge & Data Engineering, vol. 17, no. , pp. 1298-1299, 2005.
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