The broad availability of tools for the acquisition and processing of multimedia signals has recently led to the concern that images and videos cannot be considered a trustworthy evidence, since they can be altered rather easily. This possibility raises the need to verify whether a multimedia content, which can be downloaded from the internet, acquired by a Video forensics surveillance system, or received by a digital TV broadcaster, is original or not. To cope with these issues, signal processing experts have been investigating effective video forensic strategies aimed at reconstructing the processing history of the video data under investigation and validating their origins. The key assumption of these techniques is that most alterations are not reversible and leave in the reconstructed signal some “footprints”, which can be analyzed in order to identify the previous processing steps. This paper presents an overview of the video forensic techniques that have been proposed in the literature, focusing on the acquisition, compression, and editing operations, trying to highlight strengths and weaknesses of each solution. It also provides a review of simple processing chains that combine different operations. Anti-forensic techniques are also considered to outline the current limitations and highlight the open research issues.
In the recent years the availability of inexpensive, portable, and highly usable digital multimedia devices (such as cameras, mobile-phones, digital recorders, etc.) has increased the possibility of generating digital audiovisual data without any time, location, and network-related constraints. In addition, the versatility of the digital support allows copying, editing, and distributing the multimedia data with little effort. As a consequence, the authentication and validation of a given content have become more and more difficult, due to the possible diverse origins and the potential alterations that could have been operated. This difficulty has severe implications when the digital content is used to support legal evidences. Digital videos and photographs can be no longer considered “proof of evidence/occurrence” since their origin and integrity cannot be trusted .Moreover, the detection of copyright infringements and the validation of the legal property of multimedia data may be difficult since there is no way to identify the original owner.
From these premises, a significant research effort has been recently devoted to the forensic analysis of multimedia data. A large part of the research activities in this field are devoted to the analysis of still images, since digital photographs are largely used to provide objective evidence in legal, medical, and surveillance applications. In particular, several approaches target the possibility of validating, detecting alterations, and recovering the chain of processing steps operated on digital images. As a result, nowadays digital image forensic techniques enable to determine: whether an image is original or artificially created via cut and paste operations from different photos; which source generated an image (camera model, vendors); whether the whole image or parts of it have been artificially modified and how; what was the processing history of an image. These solutions rely on the consideration that many processing steps are not reversible and leave some traces in the resulting signal (hereby called “footprints”). Detecting and analyzing these footprints allow the reconstruction of the chain of processing steps. In other words, the detection of these footprints allows a sort of reverse engineering of digital content, in order to identify the type and order of the processing steps that a digital content has undergone, from its first generation to its actual form.
