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Digital Watermarking for Images
Aarathi Raghu
CS 265
Spring 2005
Agenda
1. Motivation
2. What is digital watermarking?
3. DCT
4. A Semi-fragile watermarking algorithm
5. Attacks and countermeasures
6. Conclusion
Motivation
Analog Digital
Photographs JPEG images
Distribution net required
Free to distribute using internet
Hard to modify Easily modifiable
Some level of copyright protection
No copyright protection
Digital Watermarking
Process of embedding information Information embedded is :
• Imperceptible
• Secure
• Robust
Semi-fragile watermarking– Uses:
• Tamper detection
• Image authentication
Scenario
Concepts
Compression is inevitable to accommodate
disk space, bandwidth and transmission time.
Based on:– Redundancy reduction– Irrelevancy reduction
Discrete Cosine Transform(DCT) Divides image into parts based on the visual quality
of the image
Input image is N*M f(i,j) = intensity of pixel in row i and column j F(u,v) is DCT coefficient in DCT matrix Larger amplitudes closer to F(0,0) Compression possible because higher order
coefficients are generally negligible
DCT coding system
image
DCT Transformation
8*8 DCT
Quantization
Entropy encoding
Lossy compressed data
Semi Fragile Watermark (LPD)
Designed by Lin, Podilchuk, Delp Watermark:Pseudo-random zero-mean, unit
variance Gaussian distributed numbers Constructed in DCT domain Watermark embedded in each DCT block
selectively
Semi Fragile Watermark (ctd.)
High frequency coefficients and DC coefficient – unmarked
Inverse DCT produces spatial domain watermark W
Y = X + ßW, where ß is the strength
Watermark Detection
Done block-by-block (col)(B(x,y))=B(x,y)-B(x+1,y) if x E {1,2, ….,
blocksize –1}, 0 otherwise (row)(B(x,y))=B(x,y)- B(x,y+1) if y E {1,2,
……,blocksize –1}, 0 otherwise Tb*= [(col)(Tb(x,y)) | (row)(Tb(x,y))] Wb*= [(col)(Wb(x,y)) | (row)(Wb(x,y))] C = (Tb*.Wb*)
sqrt ((Tb*.Tb*) (Wb*.Wb*))
Block classification
Correlation statistic, C, is compared to a threshold T
C > = T : Block is authentic C < T : Block is altered
Example
Detection
Original image Altered image
Attacks
Removal attacks Geometric attacks Cryptographic attacks Protocol attacks
Precautions
Watermark should be present over more number of pixels
Used keys should be secure Use of collusion-secure watermarks Watermarks should be non-invertible Possible attacks need to be foreseen
References
1. ftp://skynet.ecn.purdue.edu/pub/dist/delp/ei00-water/paper.pdf
2. http://www-nt.e-technik.uni-erlangen.de/~su/seminar/ws99/slides/amon.pdf
3. http://www.lnt.de/~eggers/texte/IEEEcom2.pdf
4. http://www.acm.org/crossroads/xrds6-3/sahaimgcoding.html