A review of Content Adaptive Image Steganography methods

Steganography is the art of transferring information through secret communication. The essential aim of steganography is to minimize the distortion caused by embedding the secret message; so that the image containing the message (stego) cannot be distinguished from the original image (cover), and the existence of the hidden message cannot be detected. The distortion in content-adaptive steganography depends on the local structure of the image. The embedding changes into the areas with rich textures are less detectable than smooth areas, so the textured areas have a higher modification priority. In this regard, three main steganography approaches are proposed: model-based, cost-based, and adversarial. The model-based approach considers a statistical model for the cover image and tries to preserve this model during the embedding process. The cost-based one focuses on minimizing the distortion obtained from the sum of the heuristic costs of modified pixels. The adversarial approach uses the competition between steganography and steganalysis to improve the embedding performance. In the first section of this paper, the concept of steganography and its history is expressed. Digital steganography including three types of cover synthesis, selection, and modification is introduced in the second section. The focus of this paper is on steganography based on the cover modification. The goal is to estimate the best probability distribution of modifications, and embedding the message in the estimated places is left to existing coding algorithms. In the third section, the problem of estimating the probability distribution is formulated as an optimization problem with the aim of distortion minimization. The distortion-based methods compute the probability distribution of embedding changes using a pre-defined distortion function. In the additive distortion function, the embedding changes are assumed to be independent. Thus, the distortion function cannot capture interactions between changes caused by embedding, and it leads the performance to suboptimality. In this regard, the non-additive distortion functions are presented that consider the dependencies among the modification of adjacent pixels. The distortion-based methods include two model-based and cost-based approaches are introduced in the fourth section. Then, their most significant methods are reviewed in the fifth section. Considering the competitive nature of steganography and steganalysis, a new steganography approach is presented in the sixth section that takes advantage of adversarial learning to improve secrecy. Adversarial learning includes two strategies: Generative adversarial networks (GANs) and adversarial attacks. In the concept of steganography, the GAN-based strategy tries to train the steganographic network against a steganalysis network. This is an iterative and dynamic game between steganographic and steganalysis networks to reach the Nash equilibrium. Another strategy attempts to simulate an adversarial attack and generate stego images that deceive the steganalysis network. The adversarial-based steganography methods are reviewed in the seventh section. In the eighth section, different methods are compared from various points of view. The results of this study show that some techniques, such as smoothing the embedding changes, considering the interactions between the changes, using side-informed information, and exploring adversarial networks, can help to estimate the proper embedding probability map and improve performance and security. In the ninth section, suggestions are stated that can be considered for future research. Finally, the conclusion is expressed in the tenth section.