A New Rule for Cost Reassignment in Adaptive Steganography

In steganography schemes, the distortion function is used to define modification costs on cover elements, which is distinctly vital to the security of modern adaptive steganography. There are several successful rules for reassigning the costs defined by a given distortion function, which can promote the security level of the corresponding steganographic algorithm. In this paper, we propose a novel cost reassignment rule, which is applied to not one but a batch of existing distortion functions. We find that the costs assigned on some pixels by several steganographic methods may be very different even though these methods exhibit close security levels. We call such pixels “controversial pixel”. Experimental results show that steganalysis features are not sensitive to controversial pixels; therefore, these pixels are suitable to carry more payloads. We name this rule the controversial pixels prior (CPP) rule. Following the rule, we propose a cost reassignment scheme. Through extensive experiments on several kinds of stego algorithms, steganalysis features, and cover databases, we demonstrate that the CPP rule can improve the security of the state-of-the-art steganographic algorithms for spatial images.