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Nowadays digital images are playing an important role in people’s daily life, due to the popularity of low-cost and high resolution digital cameras, also the rapid expansion of high speed Internet. When people enjoy the entertainment brought by digital images, they also suffer from forgery images which has become a potential outbreak, because of the more and more powerful image editing software, with which digital images can be easily manipulated and altered without leaving visible footprints. The forged image may lead to serious social problem by rapid spread through Internet and mobile networks (Sugumaran, 2009), this has arouse wide academic interest, many forensic method have been developed to judge the authenticity and credibility of digital image.
The image forensic techniques includes two categories: proactive and passive forensics. For proactive forensics, special information such as watermark was embedded into original image in advance. Such information will be extracted and compared with original embedded information when checking the authenticity of an image. The main technique of proactive forensics is watermarking (Reyc, 2003; Potdar, 2005; Weng, 2013; Liu, 2010), which takes advantage of redundant information in digital images. Nowadays, some digital cameras support embedding watermark into digital image automatically when taking pictures. The main limitation of proactive forensic is that people usually can’t know which image should be checked in advance when facing large amounts of digital images. On the contrary, the passive forensics can judge the authenticity of an image without any embedded information in advance. The wide application of this kind of technique motivated many researchers focus on this domain.
As a mature standard for image compression, JPEG has been widely used. Most digital image is saved and transferred in JEPG form. Therefore, the intrinsic footprints left by the JPEG compression may give useful information for the inspector to trace the clues left by previous JPEG history. Some forensic algorithm have been proposed based on statistical feature caused by JPEG compression process: (Popescu, 2005) gave the evidence of if an image underwent double JPEG compression; (Lukas, 2003) proposed method which can estimate the primary quantization matrix; (Li, 2009) and (Sun, 2007) extracted block artifact grids (BAG) by employing the quantization artifact during JPEG processing. The mismatch between these grids can be used as footprints of tampering. We can conclude that if the original image and the corresponding tampered image are all coded in JPEG form, there must be obvious clues which can be used as forensic purposes.