Uloga forenzike u otkrivanju zloupotrebe digitalne slike
The Role of Forensics in Digital Image Forgery Detection

Summary/Abstract: Considering that the development of information technology has completely changed the way of accessing, processing and sharing of information, and that nowadays, thanks to the Internet, the vast amount of information is literally at our fingertips, negative consequences of this phenomenon increasingly come to the fore, especially when it comes to multimedia data. Thanks to the sophisticated softwares, it has become relatively easy to preform manipulation of the image, audio or video material in order to undermine their integrity and authenticity without leaving any trace that man can visually notice. This paper analyses the current problem of manipulation of the digital image which role may be to hide, emphasize or to completely change some specific information in the image, with consequences ranged from trivial to catastrophic ones. Moreover, the focus of this paper is the role of digital forensics in detecting and proving performed manipulations. Regarding to this, the paper briefly describes the most common manipulations that can be performed on the image, copy-move or cloning and splicing or photomontage. Copy-move manipulation is achieved by copying a specified region of the original image and pasting it to another part of the same image in order to conceal useful information, while splicing manipulation is achieved by combining parts of two different images into one, final image. This paper also presents concrete examples of copy-move manipulations done in Corel Photo Paint X5 program, as well as changes in the image histograms caused by the manipulation, which proves that the image has undergone some changes. However, every manipulation of the image leads to disrupting of certain regularities that each image owns and which originate from the natural environment, the camera and the image itself. It is exactly these regularities and their deviations that detection techniques are based on. Accordingly, we distinguish detection techniques based on statistics, geometry and physics, then detection techniques based on sensor and noise of the camera that is used, and lastly detection techniques based on the format and quality of the image, compression and so on. The paper also discusses the active and passive approach of digital forensics in accordance with ability to modify the process of generating images itself by inserting specific identification marks such as watermark or digital signature. At the end, the process of forensic technique based on the image format, so-called double JPEG, is described. It is shown how the double compression, which is almost inevitable in the process of manipulation, introduces some specific effects that do not exist in the single compressed image. The aim of this paper is to highlight the need and importance to preserve the identity and authenticity of digital images, as well as to point to the possibilities of manipulation of the image, and finally, if the manipulation has been performed, to reveal it and irrefutably prove it, and digital forensics certainly plays the main role in it.

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