When the grays of images present a strong contrast, merely encrypting the gray-level of image pixels is unable to eliminate the image outlines. The adoption of pixel shuffling can eliminate the image outlines, but cannot disrupt the characteristics of gray-level spectrum. In this paper, the chaotic system is adopted as the fundamental base and combined with pixel and bit shuffling to not only eliminate image outlines, but also disrupt the distributional characteristics of gray level, and the resulting increases of key space. The statistical method of entropy, provide analysis of the efficacy of the encryption method proposed. Our example reveals that the proposed encryption method can obtain highly secure encrypted images.
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