Halftone Image Steganography with Flippability Measurement Based on Pixel Pairs

In recent years, many state-of-the-art steganographic schemes for halftone images have been proposed. Most of them only focus on the visual quality of the marked image but ignore the statistical security. In this paper, a novel steganographic scheme for halftone images is proposed, which focuses on both visual imperceptibility and statistical security. For halftone steganography, embedding messages can only be achieved by flipping pixels. The significant distortion will occur if the pixels are flipped arbitrarily in a halftone image. To select the optimal flipping pixels, we define the flippability measurement based on pixel pairs, which can provide a flippability score for each pixel in halftone images. The higher flippability score of a pixel means that the visual distortion caused by flipping the pixel will be smaller, so the pixels with high flippability scores can be selected as carriers. To achieve high embedding capacity and minimize the embedding distortions, syndrome-trellis code (STC) is employed in the embedding process. The experimental results demonstrate that the proposed scheme can achieve high embedding capacity and realize acceptable statistical security with high visual imperceptibility.