Digital Watermarking for ImagesAgendaMotivationDigital WatermarkingConceptsDiscrete Cosine Transform(DCT)DCT coding systemSemi Fragile Watermark (LPD)Semi Fragile Watermark (ctd.)Watermark DetectionBlock classificationExampleAttacksPrecautionsReferencesDigital Watermarking for ImagesAarathi RaghuCS 265Spring 2005Agenda1. Motivation2. What is digital watermarking?3. DCT4. A Semi-fragile watermarking algorithm5. Attacks and countermeasures6. ConclusionMotivationAnalog Digital Photographs JPEG imagesDistribution net requiredFree to distribute using internetHard to modify Easily modifiableSome level of copyright protectionNo copyright protectionDigital WatermarkingProcess of embedding information Information embedded is :•Imperceptible•Secure•RobustSemi-fragile watermarking–Uses:•Tamper detection•Image authenticationScenarioConceptsCompression is inevitable to accommodatedisk space, bandwidth and transmission time.Based on:–Redundancy reduction–Irrelevancy reductionDiscrete Cosine Transform(DCT)Divides image into parts based on the visual quality of the imageInput image is N*Mf(i,j) = intensity of pixel in row i and column jF(u,v) is DCT coefficient in DCT matrixLarger amplitudes closer to F(0,0) Compression possible because higher order coefficients are generally negligibleDCT coding systemimageDCT Transformation8*8 DCTQuantizationEntropy encodingLossy compressed dataSemi Fragile Watermark (LPD)Designed by Lin, Podilchuk, DelpWatermark:Pseudo-random zero-mean, unit variance Gaussian distributed numbersConstructed in DCT domainWatermark embedded in each DCT block selectivelySemi Fragile Watermark (ctd.)High frequency coefficients and DC coefficient – unmarkedInverse DCT produces spatial domain watermark WY = X + ßW, where ß is the strengthWatermark DetectionDone 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 otherwiseTb*= [(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 classificationCorrelation statistic, C, is compared to a threshold TC > = T : Block is authenticC < T : Block is alteredExampleDetection Original image Altered imageAttacksRemoval attacksGeometric attacksCryptographic attacksProtocol attacksPrecautionsWatermark should be present over more number of pixelsUsed keys should be secureUse of collusion-secure watermarksWatermarks should be non-invertiblePossible attacks need to be foreseenReferences1. ftp://skynet.ecn.purdue.edu/pub/dist/delp/ei00-water/paper.pdf2. http://www-nt.e-technik.uni-erlangen.de/~su/seminar/ws99/slides/amon.pdf3. http://www.lnt.de/~eggers/texte/IEEEcom2.pdf4.
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