Color image encryption combining a reality-preserving fractional DCT with chaotic mapping in HSI space

A color image encryption algorithm by combining the reality-preserving fractional DCT (RPFrDCT) with chaotic mapping in HSI space is presented, in which the generating sequence (GS) is introduced and produced by 2D chaotic mapping to ensure the uniqueness of the transform matrix and enlarge the cipher key space of the encryption system. In addition, the color image is converted from standard RGB space into HSI space and the nonlinearity of the spatial transform makes the proposed encryption algorithm more secure than linear ones. Three components, namely H, S and I, are encrypted by the RPFrDCT with fractional orders and GS as cipher keys. Three-dimensional (3D) scrambling is adopted to further enhance the security of the encryption algorithm. The nonlinearity of the spatial transform from RGB to HSI, the real-valued output and the high cipher key-sensitivity guarantee the security and the feasibility of the proposed encryption algorithm. Experimental results demonstrate that the proposed encryption algorithm is sensitive to cipher keys and, to some extent, robust to noise and occlusion attacks.