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