Active and Passive Multimedia Forensics

Thanks to their huge expressive capability, coupled with the widespread use of the Internet and of affordable and high quality cameras and computers, digital multimedia represent nowadays one of the principal means of communication. Besides the many benefits, the wide proliferation of such contents has lead to problematic issues regarding their authen- ticity and security. To cope with such problems, the scientific community has focused its attention on digital forensic techniques. The objective of this doctoral study is to actively contribute to this field of research, developing efficient techniques to protect digital contents and verify their integrity. Digital Watermarking has been initially proposed as a valuable instrument to prove con- tent ownership, protect copyright and verify integrity, by imperceptibly embedding a mes- sage into a documents. Such message can later be detected and used to disclose possible copyrights violations or manipulations. For specific applications, such as copyright pro- tection, the watermark is required to be as robust as possible, surviving possible attack a malevolent user may be willing to apply. In light of this, we developed a novel watermark- ing benchmarking tool able to evaluate the robustness of watermarking techniques under the attack of multiple processing operators. On the other hand, for specific applications, such as forensic and medical, the robustness requirement is overtaken by integrity preser- vation. To cope with this aim, fragile watermarking has been developed, assuming that the watermark is modified whenever a tampering occurs, thus its absence can be taken as ev- idence of manipulation. Among this class of techniques, we developed a prediction-based reversible watermarking algorithm, which allows a perfect recovery of both the original content and the watermark. More recently, passive forensics approaches, which work in absence of any watermark or special hardware, have been proposed for authentication purposes. The basic idea is that the manipulation of a digital media, if performed properly, may not leave any visual trace of its occurrence, but it alters the statistics of the content. Without any prior knowledge about the content, such alterations can be revealed and taken as evidence of forgery. We focused our study on geometric-based forensic techniques both for images and videos au- thentication. Firstly we proposed a method for authenticating text on signs and billboards, based on the assumption that text on a planar surface is imaged under perspective projec- tion, but it is unlikely to satisfy such geometric mapping when manipulated. Finally, we proposed a novel geometric technique to detect physically implausible trajectories of ob- jects in video sequences. This technique explicitly models the three-dimensional trajectory of objects in free-flight and the corresponding two-dimensional projection into the image plane. Deviations from this model provide evidence of manipulation.