Research On Key Technologies Of Active-Forensics For Image Authentication Bsaed On Digital Watermarking

Digital image forensics is a technology for detecting the source, originality, authenticity and integrity of digital image. It can be divided into two types:active-forensics and passive-forensics. This thesis is concentrating on the active-forensics technology for digital image authentication based on digital watermarking. The active-forensics technology for digital image authentication aims to verify the validity and authenticity of a host image by the pre-embedded authentication information, and provides the tamper proofing by tamper localization and recovery.The research aim of this thesis is to improve the quality of recovered image especially for a larger tamper ratio by improving the performance of tamper detection and the code efficiency of recovery-data. The research achievements on image active-forensics based on digital watermarking are classified into the following five categories.Firstly, a review of active-forensics techniques based on digital watermarking for image authentication is given after studying and analyzing the existing methods. Some evaluation indexes are list to evaluate the performance of the image authentication active-forensics schemes. Based on the analysis, the forged attacks, including the collage attack and the constant feature attack, are considered as the greatest security threat. The recovery data encoding, tamper detection and tampering coincidence are key problems to be resolved for improving the quality of t recovered images.Secondly, a single-pixel active-forensics scheme based on neighborhood-threshold is proposed to address the problems such as high probability of false acceptance (PFA) or large payload in the existing methods. In the proposed scheme, the chaos system is used to generate the binary authentication data according to the content of each pixel in the original image. The improved security binary data by scramble encryption is embedded into the least significant bit of the host image, and the validity of each pixel is determined by the neighborhood-threshold comparison. The dynamic threshold selection and the relationship between the tamper ratio with the PFA and probability of false rejection (PFR) are discussed. Theoretical analysis and simulation results show that the proposed method has a superior performance of tamper detection in the different attacks without adding the embedding payload. At the same time, the neighborhood-threshold detection can be used for one dimensional audio signal by experiments.Thirdly, the existing DCT active-forensics schemes based on digital watermarking are discussed. On that basis, a DCT active-forensics scheme with variable payload based on multi-chain threshold is proposed. For each block, the recovery data with variable length are divided into three parts and embedded in the different blocks based on the secret key. This makes multi-blocks in the image dependent. Based on the adjacent-block statistical detection, the multi-chain threshold method is designed to detect the validity of each block. Analytical expressions of the PFA and PFR for the multi-chains threshold method and the threshold selection have been derived and analyzed. The variable embedding payload preserves the adequate information of image block to as few bits as possible. The multi-chains threshold method not only resists the collage and constant-feature attacks, but also makes the PFA as low as possible by selecting the threshold.Fourthly, an image active-forensic scheme combined basic-code with synchronous-recovery is proposed to improve the quality of recovered images especially for a larger tamper ratio. Two strategies are adopted to resolve the tampering coincidence problem.(1) both total-code and basic-code of each block are embedded to decrease the tampering coincidence ratio without significantly increasing embedding payload;(2) the tamper-coincidence blocks are recovered by its valid neighboring blocks and the image inpainting method, respectively. Simulation results demonstrate that the proposed scheme not only provides security against the known counterfeiting attacks, but also allows image recovery with an acceptable visual quality (PSNR>25dB) up to50%tampering for texture image and about70%tampering for smooth one.At last, an active-forensics scheme combined variable-length average-code with neighborhood prediction is proposed to take into account embedding payload, tamper localization accuracy, recovery quality and security. For each block of size2×2pixels, the average-code is extracted to generate the recovery data with different length----6bits for a plaint block and12bits for a texture block. To improve the recovery quality, the recovery method based on neighborhood prediction is designed to reconstruct the invalid blocks whose recovery watermark embedded in the other block is also destroyed. The variable-capacity recovery data contains the adequate information of image block to as few bits as possible. The recovery data is inserted in the original image only once, and used to both tamper detection and tamper recovery in the proposed scheme. These strategies make the embedding payload as low as possible and the ability against the known forged attacks to be improved. Meanwhile, the proposed scheme has the better quality of recovered images due to the neighborhood prediction recovery.