Recently Bu and Wang [Bu S, Wang B-H. Chaos, Solitons & Fractals 2004;19(4):919–24] proposed a simple modulation method aiming to improve the security of chaos-based secure communications against return-map-based attacks. Soon this modulation method was independently cryptanalyzed by Chee et al. [Chee CY, Xu D, Bishop SR. Chaos, Solitons & Fractals 2004;21(5):1129–34], Wu et al. [Wu X, Hu H, Zhang B. Chaos, Solitons & Fractals 2004;22(2):367–73], and Álvarez et al. [Álvarez G, Montoya F, Romera M, Pastor G. Chaos, Solitons & Fractals, in press, arXiv:nlin/0406065] via different attacks. As an enhancement to the Bu–Wang method, an improving scheme was suggested by Wu et al. by removing the relationship between the modulating function and the zero-points. The present paper points out that the improved scheme proposed by Wu et al. is still insecure against a new attack. Compared with the existing attacks, the proposed attack is more powerful and can also break the original Bu–Wang scheme. Furthermore, it is pointed out that the security of the modulation-based schemes proposed by Wu et al. is not so satisfactory from a pure cryptographical point of view. The synchronization performance of this class of modulation-based schemes is also discussed.
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