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
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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.