排列與擴散為一混沌加密系統的兩個基本單元。近年新興的一改進方向為結合排列與擴散以減少影像掃描次數，進而達到速度與效益上的提升。

本論文以Wang的加密系統為基礎，提出一新的影像加密方法。我們保留了Wang將明文分割成區塊和利用NCML同時執行排列與擴散的優點，並引入一將明文轉換為新格式以加快加密的概念。新格式由8張獨立的轉換子影像堆疊而成。這些轉換子影像的上半部資訊較低，只需較低複雜度運算的加密，下半部的資訊量較高，則需較高複雜運算的加密。再者，每張轉換子影像大小只有原明文的1/8，只需計算1/8數量的擴散遮罩與新位置。基於以上理由，本論文所提加密方法可以大大節省計算量。

實驗結果顯示，所提的加密系統在達到NPCR=99.5973%及UACI=33.45785% 的安全要求下，與Wang的系統相比有較佳的速度效益。以 4096 x 4096 的影像為例，我們的速度只有Wang的43%。

In recent years, various chaos-based cryptosystems with permutation and diffusion have been proposed. An improvement direction is to combine permutation and diffusion to avoid duplicating scanning and to raise the efficiency.

Based on the Wang’s system, this thesis proposed an efficient image encryption scheme which maintains the advantages of dividing the plain image into blocks type and utilizing spatiotemporal chaos with NCML to perform permutation and diffusion simultaneously. Besides, we incorporates an idea which transforms the plain image into a new format. The new format could be seen as 8 independent transformed subimages stacking together. These transformed subimages consist of two parts. The upper part is nearly random and unrecgonizable and therefore needs encryption of lower computational effort. The lower part is highly informative which needs encryption of higher computational effort. Since the size of each transformed subimage is just 1/8 of that of the plain image and same diffusion masks and new posistions are used for every subimage, only 1/8 numbers of diffusion masks and new positions should be calculated. Based on the above observations, our proposed scheme is very efficient.

Experimental result shows that the propsed encryption scheme under the security requirement NPCR=99.5973% and UACI=33.45785% has better speed efficiency as compared with the Wang’s. For an image of 4096 x 4096, the encryption time is only 59% of the Wang’s for R = 1, and 43% for R=2.

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