Image Processing Reference
In-Depth Information
them. DW, as a rule, contain some authentic code, the information on the proprietor or the
operating information for reproduction and copying tools. Unlike usual watermarks, DW
can be not only visible, but also (as a rule) invisible because by the nature DW are
distortions of a signal, which is intended for perception by the person in the first place, and,
hence, for preservation of consumer qualities of protected audiovisual production should be
as less as possible. Invisible DW are analyzed by the special decoder which renders the
decision on their presence, and if necessary, extracts the hidden message. The most suitable
objects of protection by means of DW are static images, files of audio- and the video data[1-
3]. DW applications are not limited to information security applications. The basis areas of
DW technology can be united in four groups:
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Copy protection;
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Hidden labeling of documents;
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Proof of authenticity of the information;
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Hidden communication channels.
Definition of the received information authenticity, plays a special role in a modern
information exchange. Usually the digital signature is used for authentication. However it is
not quite appropriate for authentication of multimedia information. The message with
attached digital signature should be stored and transferred absolutely precisely, «bit-to-bit»,
while multimedia information can slightly be changed both at storage (at the expense of
compression and due to insufficient correcting ability of a code), and at transfer (influence of
single or package errors in a communication channel). Thus its quality remains admissible
for the user, but the digital signature will not work, so the addressee cannot distinguish
true, though and a little changed message from the completely false one. Besides, the
multimedia data can be transformed from one format to another, thus traditional means of
definition of integrity also will not work.
It is possible to tell that DW are capable to protect the content of digital audio/video,
instead of its digital representation in the form of sequence of bits. An essential lack of the
digital signature is also that it is easy to completely remove it from the message and attach
the new signature. Signature removal will allow the infringer to refuse authorship or to
mislead the lawful addressee concerning authorship of the message. Modern systems of DW
are projected so that to minimise possibility of similar infringements without simultaneous
essential deterioration of record. DW should be robust or fragile (depending on application)
to deliberate and casual influences. If DW is used for authenticity acknowledgement,
inadmissible change of the container should lead to DW destruction (fragile DW). If DW
contains an identification code, a firm logo, etc. it should remain at the maximum distortions
of the container, of course, not leading to essential distortions of an initial signal. Thus, at
use DW the basic problem are the attacks, which aim is infringement of their integrity. It is
possible to distinguish the following attacks: the attacks directed on DW removal, the
geometrical attacks directed on distortion of the container, cryptographic attacks, attacks
against the used embedding method and DW checking procedure [4-6]. Researching new
methods of embedding DW, robust against malicious attacks is base problem in researching
new methods of protection of the multimedia information presented in a digital form.
Along with clear advantages of a digital watermarks embedding, its application demands
inclusion of the additional block of embedding in structure of each recording device. For
already existing modern mobile digital recording devices it leads to at least updating of the
microprogram and it can be impossible if computing resources of the device are limited.
