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OutlineIntroduction, Applications
Characteristics and classification
Popular techniques for watermarking
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Example
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Watermarking Vs
EncryptionEncryption involves document transformation
so that the contents of the document are notvisible without a decryption key
Watermarking leaves the original file/imageintact and recognizable
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Digital Watermarking ApplicationsOwnership Assertion
A uses a private key to generate a watermark and embeds it inthe 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 As 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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Digital Watermarking Applications
(2)Authentication & integrity verification
Watermarks should be able to detect even theslightest change in the document.
A unique key associated with the source is usedthe create the watermark and then embed in thedocument.
This key is then used to extract the watermarkand the integrity of the document verified on thebasis of the integrity of the watermark.
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Digital Watermarking Applications
(3)Content labeling
Bits embedded in the data, comprise anannotation, giving some more information aboutthe data.
Digital cameras annotate images with the timeand date, when the photograph was taken.
Medical imaging machines annotate images (X-Rays) with patients name, ID.
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Digital Watermarking Applications
(4)Usage control & Copy protectionDigital watermark inserted to indicate the
number of copies permitted.
Every time a copy is made the hardwaremodifies the watermark and at the same time itwould not create any more copies of the data.Commonly used in DVD technology.
Content ProtectionContent owner might want to publicly and freely
provide a preview of multimedia content beingsold.To make the preview commercially useless,
content is stamped with visible watermarks.
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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 wouldlike to destroy it in order to counterfeit the copyright ofthe data. Moreover, removal of it should cause aconsiderable degradation in the quality of the data.
Visible watermarks should be visible enoughtodiscourage theft.
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Digital Watermark
ClassificationBased 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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Techniques for Texts
Text Line Coding:Change the spacing
between lines.
Word-shift Coding:Change the spacing
between words.
Character Encoding:Alter the shapes ofcharacters.
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Word-shift coding example
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Easily defeatedRetyping the text destroys the watermark
Word processors change the spacing
between words and lines
Character encoding can be defeated bychanging the font
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Techniques for ImagesSpatial Watermarking:Just change some of the values of
the pixels in the lower bit plane; e.g., Change some ofthe bits from 1 to 0 or 0 to 1.
Frequency Domain Watermarking:First convert theimage to the frequency domain and then apply thewatermark in the low frequency regions.
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Checksum Technique for
imagesWatermark 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 thewatermark, W.
Last bit of each pixel is then changed to the correspondingchecksum bit.
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Bavarian couple
Original Watermarked Version
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Advantages/Disadvantag
esEmbedding the checksum only changes (on average) half
the number of pixel. So less visual distortion.
Can hold multiple watermarks as long as they dont overlap.
Extremely simple and fast.
Extremely fragile. Any change to the checksum causes thefailure of the verification procedure.
Forger could replace a section with another one of equalsize and checksum.
Entire watermark can be removed by removing the LSBplane. Cant survive lossy compression.
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ConclusionFirst generation of copyright marking
schemes is not strong enough
Existing schemes provide only limitedmeasures 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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Q&A
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