CS 161 Watermarking 13 November 2006 2006 Doug Tygar CS 161 13 November 2006 How can we mark data We want to protect data Video sound music Digimarc Intertrust etc Programs Collberg Thomborson Statistical data Examples of traditional protection methods False entries in biographical dictionaries Copyright notices Licensing agreements Secure coprocessors 2006 Doug Tygar CS 161 13 November 2006 1 Watermarking Watermarking include low level bit data that marks information Either on a per copy basis or a per provider basis Example temperature database slightly adjust temps to mark uniquely Store copies of info released If reused prove using similarities But what if adversary changes low level info 2006 Doug Tygar CS 161 13 November 2006 Can watermarking work for data It is not clear how applicable watermarking will be for data A perfect technique immune against strong tampering is probably impossible But some watermarking techniques may be usable DMCA removing watermarks is illegal In this talk I survey watermarking techniques for photographic data 2006 Doug Tygar CS 161 13 November 2006 2 Motivation Intellectual property is important for the Internet IP images are valuable Costly to create high quality images Users are attracted by good design Binary data is trivial to copy The web is a headache for copyright protection Many methods for free data exchange Watermarking is seen as the white knight of copyright protection 2006 Doug Tygar CS 161 13 November 2006 Part 1 Making Image Watermarks 2006 Doug Tygar CS 161 13 November 2006 3 Secrets of a image watermarking salesman This slide can transform you into an experienced watermarking salesman Show two identical images Claim that one is watermarked Assert that it s robust against attacks Get signature on big contract 2006 Doug Tygar CS 161 13 November 2006 Companies to apply to Digimarc Bluespike MediaSec Signafy Signum signumtech com ARIS musicode com Intertrust But also some of the 2 3 letter companies IBM HP NEC 2006 Doug Tygar CS 161 13 November 2006 4 Applications Copyright protection Content owner embeds a secret watermark Proof of ownership by disclosing the secret key Fingerprinting Embed a serial number describing the recipient Later we can detect which user copied the image Authentication Integrity verification A fragile watermark assures integrity Content labeling Rights management Content protection 2006 Doug Tygar CS 161 13 November 2006 Watermarking concepts Watermarks visible Verifiability invisible private Watermarking technique fragile robust public Key symmetric asymmetric Original necessary yes no 2006 Doug Tygar CS 161 13 November 2006 5 Visible watermarks Visible watermarks are used in special domains Vatican library Swiss paper museum Issues with visible watermarks Content producer does not like to degrade the image Customers don t appreciate them either Visible watermarks are easier to remove Easy to detect for people But more difficult to detect automatically 2006 Doug Tygar CS 161 13 November 2006 The watermarking process private wm Embed a watermark W Watermark insertion Watermark extraction K Secret key extraction 2006 Doug Tygar W CS 161 13 November 2006 6 Watermarking process II Detecting a watermark Watermark W insertion K Yes No Secret key 2006 Doug Tygar CS 161 13 November 2006 Requirements of invisible watermarks Robust against tampering un intentional Various image transformations RST Image compression Color requantization Non linear transformations print and scan Non perceptible hard to detect Easy to use exportable etc How can watermarking be possible The visual system has very strong error correction An images contains a lot of redundancies Small changes are undetected People are used to low image quality TV newspaper images 2006 Doug Tygar CS 161 13 November 2006 7 Example The NEC watermark There is no perceptible difference between the original and watermarked image But the difference image looks interesting The watermark is present everywhere 2006 Doug Tygar CS 161 13 November 2006 Early aproaches Spatial Domain Embedding Original idea LSB is insignificant JK PGS Jordan Kutter pretty good signature The watermark was embedded directly in the LSB of the pixels of the blue plane in the spatial domain For robustness every possibility of rotation translation scale was searched Flaws Blue plane is insignificant Least significant bits are unimportant Possible search space is huge Not secure against say compression Tirkel van Schnydel and Osborne scheme Embed m sequences in the LSB of the spatial domain But also not robust against tampering 2006 Doug Tygar CS 161 13 November 2006 8 Spatial Domain Embedding II Bender 95 Nikolaidis and Pitas 96 Randomly divide image into disjunct pixel set A and B For most images statistically A pixels pixel B pixels pixel 0 Insertion choose k small A pixels add k B pixels subtract k Merge A and B to get watermarked image Detection divide image again into A and B set x A pixels pixel B pixels pixel if x close to 0 then no watermark is present if x close to N k then a watermark is present 2006 Doug Tygar CS 161 13 November 2006 Transformation Domain Encoding An early goal was robustness against JPG compression Hence design watermarking into JPG compression New ideas Use strong error correction Spread spectrum encoding Embed the mark in the perceptually important regions Tradeoff robustness vs degradation artifacts Robustness against RST is essential O Ruanaidh uses Fourier Mellin transform to achieve RST invariance Reed Solomon error correction Spread spectrum encoding Strong error correction also gives JPG robustness Does not need the original image for watermark extraction 2006 Doug Tygar CS 161 13 November 2006 9 Signal Processing Primer The Fourier transform analyzes image frequencies 1 N 1 2 f 2 f F w cos t j sin t x t N N N t 0 Properties of the magnitude spectrum Translation invariance Rotation of the image translates to a rotation in the Fourier domain Scaling results in zoom in The inverse Fourier transform returns the original image 2006 Doug Tygar CS 161 13 November 2006 Examples of the Fourier transform Fourier transform of a photograph 2006 Doug Tygar CS 161 13 November 2006 10 Example Robustness to cropping Let s use the Fourier transform to construct a scheme which is robust against cropping Tile the image with small blocks of watermarks For each block we compute the Fourier transform The watermark is embedded in the Fourier domain each block Then we compute
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