Single-Channel Color Image Encryption Using the Reality-Preserving Fractional Discrete Cosine Transform in YCbCr Space

A novel single-channel color image encryption algorithm is proposed, which utilizes the reality-preserving fractional discrete cosine transform in YCbCr space. The color image to be encrypted is decomposed into Y, Cb, and Cr components, which are then separately transformed by Discrete Cosine Transform (DCT). The resulting three spectra sequences, obtained by zig-zag scanning the spectra matrices, are truncated and the lower frequency coefficients of the three components are scrambled up into a single matrix of the same size with the original color image. Then the obtained single matrix is encrypted by the fractional discrete cosine transform, which is a kind of encryption with secrecy of pixel value and pixel position simultaneously. The encrypted image is convenient for display, transmission and storage, thanks to the reality-preserving property of the fractional discrete cosine transform. Additionally, the proposed algorithm enlarges the key space by employing the generating sequence as an extra key in addition to the fractional orders. Simulation results and security analysis demonstrate the proposed algorithm is feasible, effective and secure. The robustness to noise attack is also guaranteed to some extent.