Secure Steganographic Methods for Palette Images

In this paper, we study non-adaptive and adaptive steganographic techniques for images with low number of colors in palette image formats. We have introduced the concept of optimal parity assignment for the color palette and designed an efficient algorithm that finds the optimal parity assignment. The optimal parity is independent of the image histogram and depends only on the image palette. Thus, it can be used for increasing the security of steganographic techniques that embed message bits into the parity of palette colors. We have further developed two adaptive steganographic methods designed to avoid areas of uniform color and embed message bits into texturerich portions of the cover image. Both techniques were tested on computer generated images with large areas of uniform color and with fonts on uniform background. No obvious artifacts were introduced by either technique. The last, embedding-while-dithering, technique has been designed for palette images obtained from true color images using color quantization and dithering. In this technique, both the color quantization error and the error due to message embedding are diffused through the image to avoid introducing artifacts inconsistent with the dithering algorithm.