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Digital Watermarking
Multimedia Security
2
What is the Watermark ?
• Paper Watermark– the technique of impressing into
the paper a form, image, or text• to make forgery more difficult• to record the manufacturer’s
trademark
3
Digital Watermark
• A digital watermark– a digital signal or pattern imposed on a digital document ( text,
graphics, multimedia presentations )
• visible watermark– the more obvious means of discouraging unauthorized use by
reducing the commercial value of a document
• invisible watermark– the watermark is imperceptible to the human eye– when the ownership of data is in question, the watermark will then
be extracted to characterize the ownership
4
Visible Watermarking• G. W. Braudaway, K. A. Magerlein and F. Mintzer, “Protecting publicly-
available images with a visible watermark,” SPIE, vol. 2659, 1996, pp. 126-133.
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Invisible Watermark• Motivation
– The distribution of digital media is becoming faster, easier andrequiring less effort to make exact copies
• How to protect the intellectual property?
• Conventional approaches– In analog world
• signature, steel seal, embossed portrait, copyright label...– In digital world: cryptology
M
encryption key
E
decryption key
D M
6
Cryptology vs. Watermarking
• CryptologyOnce the data is decrypted, subsequent retransmission or dissemination is not encrypted
• Watermarking– Copyright information is hidden into digital data itself– Not restrict to access the data– Its objective is to permanently and unalterably reside in
the data
7
Watermarking Requirements
• Imperceptible
• Undeletable
• Statistically undetectable
• Robustness– resistant to lossy data compression– resistant to signal manipulation and
processing operation• Unambiguous
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Watermarked Image
Transmission
Corrupted Watermarked Image
Transmission
Lossy Compression
Geometric Distortions
Signal Processing
D/A - A/D Conversion
Typical Distortions or Intentional Tampering
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Watermark Embedding
• Making the watermark robustness is not trivial– with complete knowledge
• any watermark can theoretically be removed
– with partial knowledge• the removal may interfere with the viewing of the data• the effort of removal is greater than the value of the data
• Challenges from data compression– Whatever hole one may find to fill with watermark is likely to be
eliminated by data compression
10
Watermarking for Text
• Line-Shift Coding• Word-Shift Coding
• Feature Coding
J.T. Brassil,”Electronic Marking and Identification Techniques to Discourage Document Copying,” IEEE JSAC, vol 13, no.8, Oct, 1995.
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Watermarking for Images & Videos
• Watermarking in
• Watermarking in
spatial domaintransform domainraw datacompressed data
• Detection/extraction the original datawith
without
• Watermarking withrandom number visually recognizable pattern
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LSB Flipping Method
• Generate the random walk sequence for each watermark (e.g.. 00112)• Force the LSB to match the watermark bit
0 1 2 3 4 5 0 1 2 3 4 5
Bit 1 0 55 73 7171
123 123 205 0
Bit 0 1 120121
123 70 72 147 199 1
Watermark 1 Bit 1 Bit 2 2 130 123123
67 6868
73 123 2
Watermark 2 Bit 3 3 140 133 120 72 7070
117 3
Bit 0 Bit 2 Bit 3 4 158159
142 123122
123 69 7170 4
5 195 178 150 112 67 70 5
This works will not survive any modification
S. Walton, “Image Authentication for a Slippery New Age,” Dr. Dobb’s Journal, pp. 18-26, April 1995.
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Spread Spectrum Method
• Spread spectrum coding of a watermark – frequency domain of the image communication channel– watermark the signal transmitted through the channel
the watermark W = w1,...,wn
each wi is chosen according to zero-mean Gaussian Distributionthe image X is transform by full-frame DCT
n highest magnitude coefficients (except DC) are chosen: y1,...,yn
Embedding: y’i = yi + α wi
Extracting: wi = (yi* - yi) / α
similarity = correlation (W,W *)
I. J. Cox,”Secure Spread Spectrum Watermarking for Multimedia,” IEEE Trans. IP, vol 6, no.12, pp. 1673-1687, Dec. 1997.
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Spread Spectrum Method (cont.)
OriginalImage
FFT/DCT
Determine Perceptually Significant Regions
WatermarkedImage
Inverse FFT/DCT
Watermark
FFT/DCT
OriginalImage
ReceivedImage
OriginalWatermark
ExtractedWatermark
-
Similar
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Spread Spectrum Method (cont.)
• Watermark detector
Watermark detector response to 1000 randomly generated watermarks
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Perceptually Masking Method
DetectionH0 : X = F* - F = NH1 : X = F* - F = W * + N
the hypothesis decision is obtained bysimilarity = correlation (X,W)
Spatial Masking
F
W
F’ WatermarkedImage
OriginalImage
m-sequence
DCT
Perceptual Analysis
DCT
+ IDCT
x
M. D. Swanson,”Transparent Robust Image Watermarking,” Proc. ICIP, pp. 211-214, Sep. 1996.
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Perceptually Masking Method (cont.)
• Frame from “pingpong”
• The watermark
Similarity value
18
Digimarc Watermarking• A commercial watermarking software
– http://www.digimarc.com
Creator ID100024
19
Watermarking for Audio
• Phase Coding– Inserting the watermark by modifying the phase of each
frequency component
• Spread Spectrum Method– The watermark code is spread over the available frequency band,
and then attenuated and added as additive random noise
• Perceptual Method– The watermark is generated by filtering a PN-sequence with a
filter that approximates the frequency masking characteristics of HAS
– Weighting the watermark in the time domain to account for temporal masking
20
Watermarking for Audio (cont.)
ExtractEnvelop
Quantization
M(w) +× ×
FrequencyMasking
InformationHanningWindow FFT
AudioSignal
PNsequence
ScaleFactor
w
w +originalsignal
• Watermark generator
Filtered PN-sequence
L. Boney, “Digital Watermarks for Audio Signals, “ IEEE Proc. Multimedia’96, pp. 473-480, 1996.
21
Watermarking for Audio (cont.)
• Watermark detection
22
Watermarking for Polygonal Models
• 3D models watermarking– vertex coordinates– vertex topology (connectivity)
Embedded pattern Simplified polygonal
R. Ohbuchi, “Watermarking 3-D Polygonal Models,” ACM Multimedia 97, pp. 261-272, 1997.
23
Limitations of Watermarking
• Basic watermarking steps
Image I
Watermark WE I’
Image I
Test image J D W’ C
Watermark W
y/n ?
S. Craver, “Can Invisible Watermarks Resolve Rightful Ownerships?,” IBM Research Report, July, 1996.
24
Limitations of Watermarking (cont.)
• Counterfeit
I
I’ D W C
Watermark W
y/n ?
Watermarked image I’ Dinv Counterfeit image I*
W*
I*
I’ D W* C
Watermark W*
y/n ?
25
Visible watermark Invisible watermark
Random sequence watermark
Visually recognizable watermark
Digital watermark
Watermark :
Verification :
ID number (random number)
Quantitative measurement of the detection
Visually recognizable pattern
Extracted pattern & Quantitative measurement