Multiple And Color Images Encryption Algorithms Based On Fractional Fourier Transform And Fractional Cosine Transform

With the rapid popularization of the computer and Internet, the exchange of information plays an important role in the modern society and images are widely used as a good information carrier in many fields. Thus, image security has become an important topic in the field of information security. At present, various image encryption algorithms have been proposed, including the well known image encryption algorithm based on fractional Fourier transform which is a hot research topic and attracts the attention of many researchers. In addition, in order to meet the real-time requirement of image data transmission, the encryption algorithm should not add to the transmission load too much. In recent years, the algorithms of multiple images encryption and color image encryption become a focus. The research work in this dissertation is summarized as follows:1. With the help of image compression and spectrum truncation, the encryption algorithm for multiple images is proposed based on the multiple-order discrete fractional Fourier transform (MODFrFT). The spectra of images gotten by the discrete cosine transform (DCT) are truncated and combined into a single spectrum sequentially encrypted by the MODFrFT. Further, the complex mode is introduced to reduce the data loss in the compression process. The images to be encrypted are transformed with MODFrFT two times and chaos is introduced to scramble the phase information both in the DCT plane and in the MODFrFT domain.2. By taking full advantage of the multiple images encryption algorithm, a single-channel color image encryption based on the MODFrFT and chaos scrambling is proposed. A color image can be decomposed into three primary components and each one can be seen as a gray image. Human vision is more sensitive to the Y component than to other two components in YCbCr color space and this color format is used for encrypting the color image. Chaos is also introduced to scramble the position of the elements in the MODFrFT domain.3. The encryption algorithms for multiple images and color image are proposed based on the multiple-order discrete fractional Cosine transform (MODFrCT). and the technology of rate-distortion control is utilized during the spectrum cutting to balance the qualities of the multiple decrypted images. The zigzag scanning is used in the process of spectrum cutting and this scanning way makes the low frequency cutting precisely in every coefficient. The numerical simulations demonstrate the validity and efficiency of these algorithms. The robustness of the schemes against occlusion attack and noise attack is also examined.