23-07-2012, 03:43 PM
Multimedia Watermarking Techniques
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INTRODUCTION
Multimedia production and distribution, as we see it
today, is all digital, from the authoring tools of content
providers to the receivers. The advantages of digital
processing and distribution, like noise-free transmission,
software instead of hardware processing, and improved
reconfigurability of systems, are all well known and obvious.
Not so obvious are the disadvantages of digital
media distribution. For example, from the viewpoint of
media producers and content providers, the possibility for
unlimited copying of digital data without loss of fidelity
is undesirable because it may cause considerable financial
loss. Digital copy protection or copy prevention mechanisms
are only of limited value because access to cleartext
versions of protected data must at least be granted to
paying recipients which can then produce and distribute
illegal copies. Technical attempts to prevent copying have
in reality always been circumvented.
STEGANOGRAPHY AND WATERMARKING—HISTORY
AND TERMINOLOGY
The idea to communicate secretly is as old as communication
itself. First stories, which can be interpreted as early
records of covert communication, appear in the old Greek
literature, for example, in Homer’s Iliad, or in tales by
Herodotus. The word “steganography,” which is still in use
today, derives from the Greek language and means covert
communication. Kobayashi [67] and Petitcolas et al. [99]
have investigated the history of covert communication in
great detail, including the broad use of techniques for secret
and covert communication before and during the two World
Wars, and steganographic methods for analog signals. Although
the historical background is very interesting, we do
not cover it here in detail. Please refer to [67] and [99] for
an in-depth investigation of historic aspects.
DIGITAL WATERMARKING
The basic requirements in watermarking apply to all
media and are very intuitive.
1) A watermark shall convey as much information as
possible, which means the watermark data rate should
be high.
2) A watermark should in general be secret and should
only be accessible by authorized parties. This requirement
is referred to as security of the watermark and
is usually achieved by the use of cryptographic keys.
TEXT DOCUMENT WATERMARKING
Methods for embedding information into text documents
have been used for a long time by secret services.
For text watermarking, we have to distinguish between
methods that hide information in the semantics, which
means in the meaning and ordering of the words, and
methods that hide information in the format, which means
in the layout and the appearance.
The first class designs a text around the message to be
hidden. In that sense, the information is not really embedded
in existing information, but rather covered by misleading
information. This class of techniques is outside the scope of
this paper and will not be considered here. In the following,
we concentrate on the latter type of information-embedding
methods which use an existing text document into which
data are embedded.