Digital Watermarking for Images

Digital Watermarking for Images

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 Transfor mation 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 + W, where 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 Original image Altered image Detection

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/IEEEcom 2.pdf 4. http://www.acm.org/crossroads/xrds6-3/sa haimgcoding.html

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