Abstract
Recently, Wu et al. proposed a time-bound hierarchical key assignment (TBHKA) scheme which supports discrete time periods. In their scheme, it fused RSA and pairing-based cryptosystems and is provably secure. In this paper, we demonstrate a weakness of Wu et al.'s TBHKA scheme, ie. a collusion attack. Any two users can collude to compute an encryption key to access some class in an un-subcribing time period.
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