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SJSU CS 265 - Digital Watermarking Technique

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IntroductionWatermarking in GeneralDesirable Characteristics of Invisible WatermarksSpread Spectrum WatermarkingWatermark InsertionWatermark ExtractionAdvantage and Disadvantage of Spread Spectrum WatermarkingSample WatermarkConclusionA STUDY ON A DIGITAL WATERMARKING TECHNIQUEBy: Elo Leung and Wing WongCS265 Section 2Spring 20040IntroductionIn recent years, digital watermarking has been studied intensively to provide copyrightprotection to multimedia data including image, audio and video. Watermarkingtechniques can be classified, according to the domain of watermark insertion, into spatial-domain and frequency-domain watermarking. Alternatively, watermarks can be classifiedas visible, invisible and dual watermark according to human perception. In this project, an invisible watermark proposed by I. J. Cox et al. [1] is studied. Theirmethod applies the watermark in the frequency domain and the watermark is inserted intothe spectral components of the data using technique analogous to spread spectrumcommunication. We examine how their watermarking works and the pros and cons of themethod. Watermarking in GeneralWatermarking is the process of embedding data called a watermark or tag or label into amultimedia object such that it can be detected or extracted later to make an assertionabout the object. In [2], Mohanty identified the three components in any watermarkingalgorithm:  the watermark; the encoder (insertion algorithm);  the decoder and comparator (verification and extraction algorithm).Each owner has a unique watermark. An owner can also put different watermarks indifferent objects to identify the purchasers of the different copies of the protectedmaterial. The encoder takes an image I, a signature S and generates a watermarked image I’ by anencoder function E as shown in Fig. 1.EncoderOriginal WatermarkedImage (I ) Image (I' )Signature (S )EFigure 1: Encoder [2]A decoder function D takes an image J, whose ownership needs to be determined, andextracts a signature S’. Some algorithms require the use of the original un-watermarkedimage I in this process. A comparator function C then compares S’ with the originalsignature S and determines whether the two signatures match. Since it is highly unlikelythat S’ be identical to the original signature, the comparator considers it a match if thecorrelation of the two signatures passes a certain threshold. Fig. 2 shows the process.1Decoder ComparatorTest Extracted match/Image (J ) Signature (S' )mismatch Origianal Image (I ) Origianal Signature (S )DCFigure 2: Decoder and Comparator [2]Attacks on WatermarksA watermarked document is likely to be subject to common processing operations or intentional manipulations. These attacks are summarized in Fig. 3. TransmissionLossy OtherCompression International TamperingsProcessingII' Watermarked Object Corrupted Object TransmissionTypical Distortions and International TamperingGeometricalDistortionCommonSignalFigure 3: Attacks on watermarks [1] Lossy compression schemes like JPEG and MPEG can degrade the data’s qualitythrough irretrievable loss of data [1]. Geometric distortions include “rotation, translation, scaling and cropping”[1] and “arespecific to images and videos” [1]. Common signal processing operations include “digital-to-analog and analog-to-digitalconversion, resampling, requantization, … dithering, recompression and commonsignal enhancements to image contrast and/or color and audio frequencyequalization” [1]. Other intentional attacks include rewatermarking, “collusion and forgery attacks” [1]. Desirable Characteristics of Invisible WatermarksAn invisible watermark must be perceptually unnoticeable. Adding the watermark shouldnot corrupt the original audio, video, or image. An invisible watermark should also be2robust to common signal distortions and the removal of the watermark should result indegradation of the quality of the original digitized medium. Moreover, the watermarkshould serve as an original signature of the owner, so that retrieving the watermark froma digitized medium would readily identify the original owner. Spread Spectrum WatermarkingCox et al. [1] argued that “a watermark must be placed in perceptually significantcomponents of a signal if it is to be robust to common signal distortions and maliciousattack”. To insert a watermark into the most perceptually significant regions of aspectrum without introducing visible or audible distortions, they adopted the idea ofspread spectrum communication, in which a narrowband signal is transmitted over amuch larger bandwidth such that the signal energy in any single frequency isimperceptible. Cox’s watermark is spread throughout an image spectrum so that themodification to each component is very small and that the location of the watermark isnot obvious to attackers. Detection of the watermark is done by concentrating the weaksignals into a single signal with high signal-to-noise ratio. Since the magnitude of thewatermark at each location is unknown to an attacker, much noise energy would need tobe added to each spectral coefficient in order to remove the watermark and this processwould destroy the image. The insertion and the extraction of the watermark are discussedin the following sub-sections.Watermark InsertionThe watermark is inserted in the perceptually significant components to maximize thedegradation of the original data when attackers attempt to remove the watermark. Prior toinserting any watermark, the original image is transformed using Discrete CosineTransform (DCT) or Fast Fourier Transform (FFT) to obtain a spectrum. Once thespectrum of the original data is obtained, the perceptually significant regions aredetermined and the watermark is inserted into these regions of the spectrum. “Thewatermark X = x1, x2, …, xn is computed where each xi is chosen according to N(0,1),where N(0, 1) denotes a normal distribution with mean 0 and variance 1”[2]. Cox et al.used DCT for frequency transformation and they chose the 1000 largest coefficients toadd the watermark. The watermark is inserted in the DCT domain of the image by settingthe frequency


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SJSU CS 265 - Digital Watermarking Technique

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