Adaptive HEVC Steganography Based on Steganographic Compression Efficiency Degradation Model

High Efficiency Video Coding (HEVC) places great emphasis on optimizing compression efficiency, where compression efficiency denotes file size ratio before and after compression. The current HEVC steganography is prone to cause degradation in compression efficiency. To analyze and avoid this problem, a Steganographic Compression Efficiency Degradation Model (SCEDM) is first proposed, which leverages the area ratio of different types of Coding Units (CU) as the distribution of block partitioning structure and combines with the K-L divergence to describe the compression efficiency degradation. By minimizing the output of the SCEDM, the degradation of compression efficiency caused by steganographies can be minimized. Besides, it is also proved that this minimizing process will not increase extra visual quality distortion. Based on this model, a novel adaptive steganography using HEVC intra block partitioning structure is proposed. This steganography consists of three parts: the CU Depth based Hierarchical Coding (CDHC) method, the structure merging strategy and the adaptive matching method. The CDHC method can convert secret binary bits to different block structures. The structure merging strategy improves the capacity, and the adaptive matching method minimizes the compression efficiency degradation according to the proposed SCEDM. The proposed steganography is further compared with state-of-the-art steganographies to confirm the effectiveness and advantages of the proposed model and steganography in compression efficiency, capacity, visual quality and resistance to video steganalysis.