Research On Passive Forensics And Anti-forensics For Digital Image

Nowadays, with the widely use of the computer network、multimedia technology and low cost digital equipment, it is more convenient to access a digital image. However, when people enjoying the entertainment brought by digital images, the powerful image editing software arose that make it easier for those evil people to modify a digital image. Obviously, when the tampered image was used by the news、 judicature and military, it may mislead the public and throw doubt on the authenticity of the multimedia image, the fairness of the judicial evidence and the reliability of the military intelligence. So, the research of the identification technology for digital image is significative not only to rebuild people’s confidence in the news media, but also to maintain social fariness and justice.Based on the projects granted by the National Nature Science Foundation and Key Program of Natural Science Fund of Tianjin, we study on the identification technology for digital image and pay particular attention to the image content detection and anti-forensic technique. The main contributions of this dissertation are as following:1. A scheme for detecting the tampered regions based on the energy distribution of the DCT coefficients was proposed. Firstly, the image was transformed from spatial domain to frequency domain, then it was split to sub-blocks and each sub-block was represented by the transformed DCT coefficients, according to the nature of the Discrete Cosine Transform, we extracted only four features for each block, which significantly reduce the computational complexity. The experiments show that the proposed scheme could locate the doctored regions accurately, moreover, it was also robust to Gaussian white noise, Gaussian blur and multiple tampered operations.2. A robust scheme for identifying the doctored areas that underwent rotation was proposed. Through the analysis of the image statistical characteristics, in order to resist the rotation operation, the Pseudo-Zernike moment was used as the image feature. The image was divided into overlapped blocks, and then n-dimensional low order Pseudo-Zernike moments were extracted for each block, which ganrantee the proposed scheme is more robust to noise adding. During the feaure extraction, a recursive method is adopt, which reduce the computional complexity from O(N2p3) to O(N2p2).3. A passive detection scheme that can not only locate the tampered regions but also estimate the rotation angle and scaling factor was proposed. The image was segmented into a series of small blocks, each block was mapped to the log-polar domain, which can assure the rotated or scaled tampered areas were just represented by the translation along the log-polar coordinate. In the matching step, PCA was applied to make dimension reduction and the tampered regions were located by using the Fourier transform and phase correlation technique, moreover, by using the phase correlation spectrum, the proposed scheme could estimate the rotation or scaling parameter of the tampered areas perfectly. The experimental results show that the proposed scheme is also robust to image distortion and enhancement operation.4. A new anti-forensic scheme for the single JPEG compression image was proposed. The chaos theory was applied to the anti-forensic method for the first time. Firstly, the original JPEG image was transformed by the discrete cosine transform, and the model for each DCT sub-band was estimated according to the transformed DCT coefficients, then by using the chaos technique and the estimated model, the chaotic noise was added to each DCT sub-band. The experiments show that, by adding chaotic noise, the proposed scheme could erase the "gully" which caused by the JPEG compression perfectly, moreover, the proposed scheme could also deceive the existing forensic method that based on detecting the JPEG block artifact, compared with the competing method, under certain compression factor, the proposed method only introduce little visual distortion when adding chaotic noise.Finally, the paper makes a summary of the work, and points out the new research direction for the future work.