2. Outline
Introduction, Applications
Characteristics and classification
Popular techniques for watermarking
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3. Definition: A digital watermark is a
digital signal or pattern inserted into a
digital document such as text, graphics
or multimedia, and carries information
unique to the copyright owner, the
creator of the document or the
authorized consumer.
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5. Watermarking Vs Encryption
Encryption involves document transformation so that
the contents of the document are not visible without
a decryption key
Watermarking leaves the original file/image intact
and recognizable
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7. Digital Watermarking Applications
Ownership Assertion
‘A’ uses a private key to generate a watermark and embeds it in the
document
‘A’ makes the watermarked image publicly available
‘B’ claims that he owns the image derived from the public image
‘A’ produces the unmarked original and establishes the presence of ‘A’s
watermark
Fingerprinting
Used to avoid unauthorized duplication and distribution.
A distinct watermark (a fingerprint) is embedded in each copy of the data.
If unauthorized copies are found, the origin of the copy can be determined
by retrieving the fingerprint.
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8. Digital Watermarking Applications (2)
Authentication & integrity verification
Watermarks should be able to detect even the slightest
change in the document.
A unique key associated with the source is used the
create the watermark and then embed in the document.
This key is then used to extract the watermark and the
integrity of the document verified on the basis of the
integrity of the watermark.
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9. Digital Watermarking Applications (3)
Content labeling
Bits embedded in the data, comprise an annotation,
giving some more information about the data.
Digital cameras annotate images with the time and
date, when the photograph was taken.
Medical imaging machines annotate images (X-Rays)
with patients name, ID.
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10. Digital Watermarking Applications (4)
Usage control & Copy protection
Digital watermark inserted to indicate the number of
copies permitted.
Every time a copy is made the hardware modifies the
watermark and at the same time it would not create any
more copies of the data.
Commonly used in DVD technology.
Content Protection
Content owner might want to publicly and freely
provide a preview of multimedia content being sold.
To make the preview commercially useless, content is
stamped with visible watermarks.
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11. Characteristics of Digital Watermarks
Readily Detectable: the data owner or an independent control authority
should easily detect it.
Unambiguous: retrieval of it should unambiguously identify the data
owner.
Robust: difficult to remove for an attacker, who would like to
destroy it in order to counterfeit the copyright of the data.
Moreover, removal of it should cause a considerable degradation
in the quality of the data.
Visible watermarks should be visible enough to discourage theft.
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12. Digital Watermark
Classification
Based on visibility of watermarks
- Visible Watermarks
- Invisible Watermarks
Based on the content to be watermarked
- Text Watermarking
- Image, Audio, Video Watermarking
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13. Techniques for Texts
Text Line Coding: Change the spacing
between lines.
Word-shift Coding: Change the spacing
between words.
Character Encoding: Alter the shapes of
characters.
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16. Easily defeated…
Retyping the text destroys the watermark
Word processors change the spacing between
words and lines
Character encoding can be defeated by changing
the font
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17. Techniques for Images
Spatial Watermarking: Just change some of the values of the
pixels in the lower bit plane; e.g., Change some of the bits from 1
to 0 or 0 to 1.
Frequency Domain Watermarking: First convert the image to the
frequency domain and then apply the watermark in the low
frequency regions.
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18. Checksum Technique for images
Watermark is formed from the 7 most significant bits of each pixel.
Eight 7-bit segments (from eight different pixels) are concatenated
and the final checksum is thus 56-bit.
Locations of the pixels that are to contain one bit each of the
checksum are randomly chosen.
These pixel locations along with the checksum form the watermark,
W.
Last bit of each pixel is then changed to the corresponding checksum
bit.
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21. Advantages/Disadvantages
Embedding the checksum only changes (on average) half the number
of pixel. So less visual distortion.
Can hold multiple watermarks as long as they don’t overlap.
Extremely simple and fast.
Extremely fragile. Any change to the checksum causes the failure of
the verification procedure.
Forger could replace a section with another one of equal size and
checksum.
Entire watermark can be removed by removing the LSB plane. Can’t
survive lossy compression.
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22. Conclusion
First generation of copyright marking schemes is not
strong enough
Existing schemes provide only limited measures of
marking
Can only meet few requirements at a time
Tradeoff - Bandwidth vs. robustness
No single problem but a constellation!
Real problem: watermark restoration
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