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 AnalogDigital PhotographsJPEG images Distribution net required Free to distribute using internet Hard to modifyEasily 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 + 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 imageAltered 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/e i00-water/paper.pdf 2. erlangen.de/~su/seminar/ws99/slides/amo n.pdf erlangen.de/~su/seminar/ws99/slides/amo n.pdf pdf 2.pdf /sahaimgcoding.html 3/sahaimgcoding.html