Transcript watermark.ppt

EMTM 553: E-commerce Systems
Lecture 7a: Digital Watermarking
Insup Lee
Department of Computer and Information Science
University of Pennsylvania
[email protected]
www.cis.upenn.edu/~lee
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Outline
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Introduction, Applications
Characteristics and classification
Popular techniques for watermarking
Attacks on watermarks
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History
Paper Watermarks: Formed during the
manufacturing process of the paper when the
paper is wet, so that fibers can be disturbed
according to the design of the watermark.
Used mainly for identification and
authentication
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Common Examples
• Currency Notes
• Stamps
• Passports
• Legal Documents
• Logos used by cable TV channels
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Digital Watermarking
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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Example
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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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Watermarking Process
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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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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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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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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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Characteristics of Digital Watermarks
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Unobtrusive: invisible enough not to degrade the data quality and to
prevent an attacker from finding and deleting it.
Readily Detectable: the data owner or an independent control
authority should easily detect it.
Unambiguous: retrieval of it should unambiguously identify the data
owner.
• Innumerable: it should be possible to generate a large
number of watermarks, all distinguishable.
• 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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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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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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Text line coding example
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Word-shift coding example
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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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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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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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Example
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Bavarian couple
Original
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Watermarked Version
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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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Attacks on Copyright
Marking Systems
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Watermarking Attack
• Iterative progress
– Attacks will lead to more robust systems
• Various attacks on marking systems
– Very simple (manual) - Jitter
– General but still simple (StirMark)
• Idea is to be able to produce images very similar
to the original except that it has the watermark
removed.
• Another way of saying - Hacking!!
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Early experiment—‘jitter’ attack
SysCoP
skytale:SysCoP$ imageread_demo syscop_watermarked.ppm
Key:
skytale:SysCoP$ imageread_demo syscop_jitter.ppm
Key:
No certificate file.
------------------------------------------------A valid watermark found - estimated correction
percentage is : 100
Retrieved Secret Label (string) : SysCoP(TM)
No certificate file.
------------------------------------------------Cannon find valid watermark - failed.
Image syscop_jitter.ppm has been tampered or has
not been watermarked.
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Jitter Attack
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Tweak the lower order bits and add jitter
Split signal into number of chunks
Duplicate or Delete at random
Imperceptible even in classical music!
Jitter prevents marked bits from being
located
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Jitter attack example
Watermarked
Jitter added
SysCoP
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StirMark
• Most watermarking schemes are robust to simple
manipulations:
• Rotation, cropping, rescaling,etc. …separately
• By using these transforms in tandem it’s possible
to distort the watermark
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StirMark
– Apply minor geometric distortion:
o Stretching, shearing, shifting and rotation
o Simulate printing/scanning process
o Defeats most commercial image marking systems
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Example
Original
JK_PGS(314,27)
StirMark
PictureMarc 1.51, SysCoP (Demo), JK_PGS, SureSign, EIKONAmark (Pitas), NEC (Cox et al.)
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Web crawlers for watermarks
• Interesting application
– Web crawler download images
– Check whether they contain a watermark
– Produce reports for copyright owners
e.g. MarcSpider by Digimarc, Portland, OR
• How do you prevent such crawlers from reporting
your pictures?
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Attacks on web-crawlers I
• ‘Mosaic’ attack
– Designed to defeat systems combining watermarks and
web-crawlers
– Split images in small pieces (e.g. 100100)
– Each image is small enough that partial watermark cannot
be detected
– Confuse the mark detector
• Java applets/ActiveX objects
– Image displayed with Java applet automatically or after
certain actions of the user
– Can even be de-scrambled ‘on the fly’
• Refuse connection, login, payment
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‘Mosaic’ attack
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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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Q&A
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