Security Analysis of a Time-Bound Hierarchical Key Assignment Scheme

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.