Abstract: The conventional generation camouflage method combined with block rotation and photo mosaic expresses secret information by placing the circular images related to secret information in the embedding position, thus leading to a poor visual quality of resulted mosaic image and the leakage of hidden secret information. To address this, an improved generation camouflage method combined with block rotation and photo mosaic is proposed in this paper. In the method, firstly several grayscale images are converted into circular images and arranged as encoded images according to the ascending order of pixel mean value, and the cover image is directly transformed into the multiple halftone image by error diffusion. Secondly, the hidden positions of secret information are determined by random coordinate sequences, and the secret information was encrypted and mapped by generating random encryption mapping matrix. Finally, the coded images with the corresponding pixel values are selected by traversing the cover image. For the hidden and unhidden positions, the encoded images are rotated by a determined angle relative to secret information and a random angle respectively to generate a photo mosaic image. In order to improve the accuracy of secret information authentication, secret information is authenticated by the user key combined with centroid rotation matching strategy and interval authentication strategy. Experiments show that the proposed method directly uses the rotation angle of the encoded circular image to express secret information and it always selects the encoded circular image corresponding to the pixel value in the cover image to express secret information, so it neither brings any deviation nor causes any visual quality degradation. The proposed method performs strong anti-attack robustness and high security due to the entire key-dependency in extraction process.

Key words: photo mosaic, error diffusion, encryption mapping, interval extension, generating information hiding