A Robust BPCS-Steganography against the Visual Attack

This paper discusses a visual attack to BPCS-Steganography (Bit-Plane Complexity Segmentation-Steganography) and presents a countermeasure. BPCS categorizes each small block within cover images into either a "noise-like region" or an "informative region" by means of the binary-image feature called complexity and replaces noisy blocks with the binary patterns mapped from the bit sequences of secret data. The binary patterns must be complex because it is replacing complex regions. If the binary patterns to be inserted are simple, then the logical operation called conjugate operation is applied that transforms a simple pattern to a complex pattern. We need to keep, for each block, the flag called a conjugation flag representing whether or not conjugation had been applied to correctly extract the embedded information. In the case where the conjugation flags must be embedded within cover images, we are unable to use the complexity thresholding for the embedding of the flags. Thus the flags must be embedded into a fixed area. To this effect it is easy to recognize unnatural patterns on the LSB plane of stego-images. It can be used as a signature or a distinguishing mark between natural images and images with information embedded by BPCS. We propose secure BPCS which is robust against the visual attack. It is realized by assigning the meaning of the conjugation flag to a pixel of each block and making the specific rule to embed and extract it