Abstract
Digital watermarking is the practice of inserting a signal, known as the watermark, into an original signal in an imperceptible manner. The watermark encodes or represents information that can protect the watermarked signal, typically identifying the owner (source) or the intended recipient (destination) of the signal. The embedded watermark may be detected by using a watermark detector, which enables an application to react to the presence (or absence) of the watermark in a signal. However, the watermarked signal may be processed, or attacked, prior to watermark detection. Attacks may remove the embedded watermark or make the watermark more difficult to detect. One type of attack that has received considerable attention is synchronization attacks. A synchronization attack confuses the watermark detector by re-positioning the embedded watermark. Most watermark detectors will fail to detect the watermark embedded in the attacked signal unless the position of the watermark can be identified. This is a significant vulnerability in robust watermark detection. The process of identifying the position of the watermark is known as watermark detector synchronization. A new framework is developed for temporal synchronization in blind symmetric video watermarking. Embedding and detection models are proposed that encompass the behavior of many video watermarking techniques. These models demonstrate that synchronization is challenging when the watermark lacks redundancy, but also that efficient synchronization can be achieved by designing the watermark with temporal redundancy. The temporal synchronization models are adapted to spatial synchronization in still image watermarks. For spatial synchronization, redundancy is obtained by constructing a watermark which induces a pattern in the auto-correlation. Experimental results support the theoretical foundations for both temporal and spatial synchronization. In addition, earlier exploration in watermarking led to the development of a semifragile watermarking technique for image authentication. The semi-fragile technique is capable of detecting significant alterations to the watermarked image, but is tolerant to lossy JPEG compression and other, more subtle alterations. This earlier work is not related to watermark synchronization.
