Digitalwatermarking

1 DIGITAL WATERMARKING

VISVESVARAYA TECHNOLOGICAL UNIVERSITYJnana Sangama, Machche, Belgaum – 590 014

A seminar report on

DIGITAL WATERMARKING

Submitted by

MOHAMMED JASHIM1NH08EC034

VIII SEMESTER, Section ”A”

Department of Electronics & Communication Engineering

NEW HORIZONCOLLEGE OF ENGINEERING

(Accredited by NBA, Permanently affiliated to VTU)Outer Ring Road, Bellandur Post, Near Marathalli,

Bengaluru – 560 103

2011 – 12

Department of Electronics & Communication, NHCE 2011-12


NEW HORIZON COLLEGE OF ENGINEERINGOuter Ring Road, Bellandur Post, Near Marathalli,

Bengaluru – 560 103

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

Certificate

This is to certify that the seminar entitled “DIGITAL WATERMARKING” is a bonafide

work carried out by Mr.MOHAMMED JASHIM bearing the USN 1NH08EC034, in partial

fulfillment for the award of the degree of Bachelor of Engineering in Electronics &

Communication of the Visvesvaraya Technological University, Belgaum during the

academic year 2011 - 12. This seminar report has been approved as it satisfies the academic

requirements in respect of the Seminar Work (bearing the subject code 06EC86, for the VIII

semester during February-June 2012) prescribed for the Bachelor of Engineering Degree.

…………………………….. ……………………………..

Signature of the Coordinator Signature of the HOD

Mr. Aravinda K. Dr. T.N. BasavarajAssistant Professor, Professor, Dept. of E&C, NHCE, Dept. of E&C, NHCE,Bengaluru – 560 103 Bengaluru – 560 103



ABSTRACT

As more and more analogue systems are replaced by digital ones, the question of how to deal with piracy of copyrighted digital content is turning into a serious problem. For example, the speed with which the “Content Scrambling System” – the encryption standard used for DVD video – was broken, clearly shows, that encryption on its own is insufficient for many systems in protecting copyrighted material from misuse. Digital Watermarking, on the other hand, relies on the shortcomings of the “human visual system” (HVS) and is about embedding copyright information in a way that it is imperceivable by humans, while still being resistant to modifications of the cover media. Thus, digital watermarking can be considered to be a supplementary technique to well established encryption schemes.

(i)

ACKNOWLEDGMENT



I express my sincere thanks to Prof. Dr. T.N. Basavaraj (HOD E&C, NHCE).

Mr. Aravinda K(Assistant Professor,E&C,NHCE) for their kind co-operation for presenting

the seminar.

I also extend my sincere thanks to all other members of the faculty of Electronics

and Communication Department and my friends for their co-operation and

encouragement.

(ii)



TABLE OF CONTENTS

SI no DESCRIPTION PAGE NO :

1 INTRODUCTION 6

2 DIGITAL WATERMARKING TECHNOLOGY

OVERVIEW

10

3 CLASSIFICATION 11

4 DIGITAL WATERMARKING TECHNICAL DETAILS

14

5 WATERMARKING AS COMMUNICATION 17

6 DIGITAL WATERMARKING METHODS 19

7 DISTORTION AND ATTACKS 23

8 APPLICATION OF DIGITAL WATERMARKING

25

9 CONCLUSION 29

10 REFERENCES 30

CHAPTER 1



INTRODUCTION

Along with the explosive growth of the Internet not only desirable

new possibilities - like publicly available access to information databases around the

world, distributed project work across different countries, or fast and reliable means of

electronic communication - emerged, but the ease with which digital media can be

duplicated and modified, or the fact that legislation is seemingly unable to cope with its

rapid rate of change makes it also very attractive to people with dishonourable motives.

The advent of the Internet has resulted in many new opportunities for the creation and

delivery of content in digital form. Applications include electronic advertising, realtime

video and audio delivery, digital repositories and libraries, and Web publishing. An

important issue that arises in these applications is the protection of the rights of all

participants. It has been recognized for quite some time that current copyright laws are

inadequate for dealing with digital data. This has led to an interest towards developing

new copy deterrence and protection mechanisms. One such effort that has been

attracting increasing interest is based on digital watermarking techniques. Digital

watermarking is the process of embedding information into digital multimedia content

such that the information (which we call the watermark) can later be extracted or

detected for a variety of purposes including copy prevention and control. Digital

watermarking has become an active and important area of research, and development

and commercialization of watermarking techniques is being deemed essential to help

address some of the challenges faced by the rapid proliferation of digital content.

With these drawbacks of the “digital age” in mind, creators of multimedia content may wish for a digital analogy to the watermarks that have been used in bookmaking since the 13th Century . This need for methods and tools to protect ones intellectual property rights initiated the relatively new research field of “digital watermarks”. Someone familiar with encryption techniques might be tempted to ask why there is such an amount of interest in the research community to develop robust watermarking techniques, if numerous secure encryption algorithms are readily available. There are several reasons for this:

1. Encryption alone often is insufficient to protect digital content, since unconsidered and erroneous usage by human operators often renders it useless.

2. If somebody breaks the encryption (e.g. breaking the “content scrambling system” used on DVDs with tools like “DeCSS”, “VobDec” or “SmartRipper”), copyright infringements can still be proven using the embedded watermark.

3. The decryption process usually depends on the data being unmodified.



4. Since rightful owners are to be allowed to access the data they paid for, the encryption needs to be undone at some point. As the unencrypted data is normally being held in the main memory of computers, it’s not too difficult to devise tools for storing it onto a local hard disk (e.g. many DVD players for the Windows OS use DirectShow for video output. This proves to be useful for multiangle DVDs, where applications like “DeCSS” fail. By using tools that implement appropriate DirectShow filters to write the decoded images to a user-specified file instead of displaying them on the screen (e.g. “DVDRip”), the raw video data is still accessible.

Because of these shortcomings, digital watermarking is sometimes referred to as being “the last line of defence”. Consequently, an effective watermark should normally have several properties, whose importance will vary depending upon the application.

Robustnesso Fragile Watermarks are highly sensitive to any modifications, their

sole purpose being to prove the authenticity of a document.

o Robust Watermarks should be embedded in a way, that they cannot be removed from the data without introducing noticeable defects.\

Perceptibility o Visible Watermarks are added as a perceivable additional layer to the original data

o Invisible Watermarks should not be perceivable by human senses.·

Security Unauthorized parties should not be able to read or alter the watermark, even if they have detailed knowledge about the used algorithms (Kerckhoffs’s maxim).

Multiple Watermarks Does the watermarking algorithm allow for multiple watermarks to be detected independently of each other?

Speed In video distribution systems it may be necessary to use asymmetric algorithms, which

offer very fast embedding methods, whereas watermark detection can take arbitrarily longer.

Digital watermarking is still a very young research area, with its first academic conference held in 1996 [Ande99]. Numerous algorithms have been proposed and dismissed since then. Therefore, this paper aims at describing one popular watermarking technique in detail (“Spread Spectrum Watermarking”), rather than providing a short and unsatisfactory explanation of a larger number of different methods.



INFORMATION HIDING: Steganography & Digital Watermarking

In addition to digital watermarking, the general idea of hiding some information in digital content has a wider class of applications that go beyond mere copyright protection and authentication.

The techniques involved in such applications are collectively referred to as information hiding. It contains a large range of problem beyond that of embedding message in content,keeping the existence of information secret and imperceptible.

For example, an image printed on a document could be annotated by information that could lead an user to its high resolution version

STEGANOGRAPHY:

Another topic that is related to watermarking is steganography. Steganography applications conceal information in other, seemingly innocent media. Steganographic results may masquerade as other file for data types, be concealed within various media, or even hidden in network traffic or disk space. We are only limited by our imagination in the many ways information and data can be exploited to conceal additional information. Although steganography has been studied as part of cryptography for many decades, the focus of steganography is secret communication. In fact, the modern formulation of the problem goes by the name of the prisoner’s problem. Here Alice and Bob are trying to hatch an escape plan while in prison. The problem is that all communication between them is examined by a warden, Wendy, who will place both of them in solitary confinement at the first hint of any suspicious communication. Hence, Alice and Bob must trade seemingly inconspicuous messages that actually contain hidden messages involving the escape plan. There are two versions of the problem that are usually discussed – one where the warden is passive, and only observes messages and the other where the warden is active and modifies messages in a limited manner to guard against hidden messages. Clearly the most important issue here is that the very presence of a hidden message must be concealed. Whereas in digital watermarking it is not clear that a good watermarking technique should also be steganographic.

Digital Watermarking

Digital information embedded within any digital media that can later be detected and extracted. Simply watermarking, a pattern of bits inserted into a digital image, audio or video file that identifies the file's copyright information (author, rights, etc.). The name comes from the faintly visible watermarks imprinted on stationery that identify the manufacturer of the stationery. The purpose of digital watermarks is to provide copyright protection for intellectual property that's in digital format.Unlike printed watermarks, which are intended to be somewhat visible, digital watermarks are designed to be completely invisible, or in the case of audio clips, inaudible. Moreover, the actual bits representing the watermark must be scattered throughout the file in such a way that they cannot be identified and manipulated. And finally, the digital watermark must be robust enough so that it can withstand normal changes to the file, such as reductions from lossy compression algorithms.



Satisfying all these requirements is no easy feat, but there are a number of companies offering competing technologies. All of them work by making the watermark appear as noise - that is,

random data that exists in most digital files anyway. To view a watermark, you need a special program that knows how to extract the watermark data.Watermarking is also called data embedding and information hiding.

Digital Watermarking describes methods and technologies that hide information, for example a number or text, in digital media, such as images, video or audio. The embedding takes place by manipulating the content of the digital data, which means the information is not embedded in the frame around the data. The hiding process has to be such that the modifications of the media are imperceptible. For images this means that the modifications of the pixel values have to be invisible. Furthermore, the watermark must be either robust or fragile, depending on the application. By "robust" we mean the capability of the watermark to resist manipulations of the media, such as lossy compression (where compressing data and then decompressing it retrieves data that may well be different from the original, but is close enough to be useful in some way), scaling, and cropping, just to enumerate some. In some cases the watermark may need to be fragile. "Fragile" means that the watermark should not resist tampering, or would resist only up to a certain, predetermined extent.

CHAPTER 2



DIGITAL WATERMARKING TECHNOLOGY

OVERVIEW

A watermark is a pattern of bits inserted into a digital image, audio or video file that identifies the file's copyright information (author, rights, etc.). The name “watermark” is derived from the faintly visible marks imprinted on organisational stationery. There are many types of digital information and data. The types concentrated on in this report are: • Digital Images

• Digital Audio, and • Digital Videos

Unlike printed watermarks, which are intended to be somewhat visible (like the very light compass stamp watermarking this report), digital watermarks are designed to be completely invisible, or in the case of audio clips, inaudible. In addition, the bits representing the watermark must be scattered throughout the file in such a way that they cannot be identified and manipulated. And finally, a digital watermark must be robust enough to survive changes to the file its embedded in, such as being saved using a lossy compression algorithm eg: JPEG.

Satisfying all these requirements is no easy feat, but there are a number of companies offering competing technologies. All of them work by making the watermark appear as noise- that is, random data that exists in most digital files anyway. Digital Watermarking works by concealing information within digital data, such that it cannot be detected without special software with the purpose of making sure the concealed data is present in all copies of the data that are made whether legally or otherwise, regardless of attempts to damage/remove it. The purpose of digital watermarks is to provide copyright protection for intellectual property that is in digital format.

CHAPTER 3



CLASSIFICATION

Classification according to visibility:

Visible digital watermarking

Invisible digital watermarking

VISIBLE DIGITAL WATERMARKING

In visible digital watermarking, the information is visible in

the picture or video. Typically, the information is text or a logo, which identifies the owner of

the media. The image on the right has a visible watermark. When a television broadcaster

adds its logo to the corner of transmitted video, this also is a visible watermark.

INVISIBLE DIGITAL WATERMARKING

In invisible digital watermarking, information is added as

digital data to audio, picture, or video, but it cannot be perceived as such (although it may be

possible to detect that some amount of information is hidden in the signal). The watermark

may be intended for widespread use and thus, is made easy to retrieve or, it may be a form

of steganography, where a party communicates a secret message embedded in the digital

signal. In either case, as in visible watermarking, the objective is to attach ownership or other

descriptive information to the signal in a way that is difficult to remove. It also is possible to

use hidden embedded information as a means of covert communication between individuals.

PURPOSE VISIBLE INVISIBLEValidation of intended recipient - PrimaryNon-repudiable transmission - PrimaryDeterrence against theft Primary SecondaryDiminish commercial value without utility Primary PrimaryDiscourage unauthorized duplication Primary SecondaryDigital notarization and authentication Secondary Primary

Identify source Primary Secondary



Classification Based On Digital Watermarking techniques used :

Robustness Perceptibility Capacity Embedded method

ROBUSTNESS

A digital watermark is called fragile if it fails to be detectable after the

slightest modification. Fragile watermarks are commonly used for tamper detection (integrity

proof). Modifications to an original work that clearly are noticeable, commonly are not

referred to as watermarks, but as generalized barcodes.

A digital watermark is called semi-fragile if it resists benign transformations, but fails

detection after malignant transformations. Semi-fragile watermarks commonly are used to

detect malignant transformations.

A digital watermark is called robust if it resists a designated class of transformations. Robust

watermarks may be used in copy protection applications to carry copy and no access control

information to form correct order and get the digital water marking

PERCEPTIBILITY

A digital watermark is called imperceptible if the original cover signal

and the marked signal are perceptually indistinguishable.

A digital watermark is called perceptible if its presence in the marked signal is noticeable.

CAPACITY



The length of the embedded message determines two different main classes of digital

watermarking schemes:

The message is conceptually zero-bit long and the system is designed in order to detect

the presence or the absence of the watermark in the marked object. This kind of

watermarking scheme is usually referred to as zero-bit or presence watermarking

schemes. Sometimes, this type of watermarking scheme is called 1-bit watermark,

because a 1 denotes the presence (and a 0 the absence) of a watermark.

The message is a n-bit-long stream ( , with )

or and is modulated in the watermark. These kinds of schemes usually

are referred to as multiple-bit watermarking or non-zero-bit watermarking schemes.

EMBEDDING METHOD

A digital watermarking method is referred to

as spread-spectrum if the marked signal is obtained by an additive modification. Spread-

spectrum watermarks are known to be modestly robust, but also to have a low information

capacity due to host interference.

A digital watermarking method is said to be of quantization type if the marked signal is

obtained by quantization. Quantization watermarks suffer from low robustness, but have a

high information capacity due to rejection of host interference.

A digital watermarking method is referred to as amplitude modulation if the marked signal is

embedded by additive modification which is similar to spread spectrum method, but is

particularly embedded in the spatial domain.

CHAPTER 4



DIGITAL WATERMARKING TECHNICAL DETAILS

The above block diagram show the digital watermarking of a signal. Digital watermarking technology makes use of the fact that the human eye has only a limited ability to observe differences. Minor modifications in the colour values of an image are subconsciously corrected by the eye, so that the observer does not notice any difference. While vendors of digital watermarking schemes do not publicly release the exact methods used to create their watermarks, they do admit to using the following basic procedure

A secret key (string or integer) produces a random number which determines the particular pixels, which will be protected by the watermarking. The watermark is embedded redundantly over the whole image, so that every part of the image is protected. One way of doing this is by “Patchwork”. This technique uses a random number generator to select n pairs of pixels and slightly increases or decrease their luminosity (brightness level). Thus the contrast of this set is increased without any change in the average luminosity of the image. With suitable parameters, Patchwork even survives compression using JPEG.

Although the amount of secret information has no direct impact on the visual fidelity of the image or the robustness of the watermark, it plays an important role in the security of the


http://en.wikipedia.org/wiki/File:Watermark_life_cycle.svg


system. The key space, that is the range of all possible values of the secret information, x must be large enough to make exhaustive search attacks impossible.

In the process of extracting the watermark, the secret key is used to identify the manipulated pixels and finally to decode the watermark. The quality of digital watermarks can be judged in two ways; firstly it must be able to resist intentional and unintentional attacks and secondly the embedded watermark must not detract from the quality of the image. The higher the resistance of a watermark against attacks, the higher the risk of the quality of the image being reduced, and the greater the chance of obvious visual artefacts being created.

Every watermarking system consists at least of two different parts: watermark embedding unit and watermark detection and extraction unit.The above figure shows an example of embedding unit for still images. The unmarked image is passed through a perceptual analysis block that determines how much a certain pixel can be altered so that the resulting watermarked image is indistinguishable from the original. This takes into account the humaneye sensitivity to changes in flat areas and its relatively high tolerance to small changes inedges. After this so-called perceptual-mask has been computed, the information to be hidden is shaped by this mask and spread all over the original image. This spreading technique is similar to the interleaving used in other applications involving coding, such as compact disc storage, to prevent damage of the information caused by scratches or dust. In our case, the main reason for this spreading is to ensure that the hidden information survivescropping of the image. Moreover, the way this spreading is performed depends on the secretkey, so it is difficult to recover the hidden information if one is not in possession of this key. In fact,a similar technique is used in spread spectrum systems (more precisely, in Code-Division MultipleAccess) to extract the desired information from noise or other users. Additional key-dependentuncertainty can be introduced in pixel amplitudes (recall that the perceptual mask imposes only anupper limit). Finally, watermark is added to the original image.



The above figure shows the typical configuration of a watermark detection and extraction unit. Watermark detection involves deciding whether a certain image has been watermarked with a given key. Note then that a watermark detector produces a binary output. Important considerations here are the probability of correct detection PD (i.e., the probability of correctly deciding that a watermark is present) and the probability of false alarm PF (i.e., the probability of incorrectly deciding that an image has been watermarked with a certain key).These two measures allow us to compare different watermarking schemes: One method will be superior if achieves a higher PD for a fixed PF . Note also that for a watermarking algorithm to be useful it must work with extremely low probabilities of false alarm. Watermark detection is usually done by correlating the watermarked image with a locally generated version of the watermark at the receiver side. This correlation yields a high value when the watermark has been obtained with the proper key. As we have shown, it is possible to improve the performance of the detector by eliminating original image-induced noisewith signal processing. It is worthy of remark that some authors propose using the original image in the detection process. Although this simplifies further treatment of the watermark in the reeiver end, it is quite unrealistic for most applications, particularly those related to E-commerce. Once the presence of the watermark has been correctly detected, it is possible to extract the hidden information. The procedure is also generally done by means of a cross-correlation but in this case, an independent decision has to be taken forevery information bit with a sign slicer. In fact, we have also shown that this correlation structure has not been well-founded and significant improvements are achievable when image statistics are available. For instance, the widely-used DCT coefficients used in the JPEG and MPEG-2 standards are well approximated by generalized gaussian probability density functions that yield a considerably different extraction scheme. Obviously, when extractingthe information the most adequate parameter for comparison purposes is the probability ofbit error Pb, identical to that used in digital communications. This is not surprising because watermarking creates a hidden (sometimes called steganographic)channel on which information is conveyed.



CHAPTER 5

WATERMARKING AS COMMUNICATION

It is quite common and popular to adapt techniques from standard communication theory to study and improve watermarking algorithms using models similar to the ones shown in Figure,below.And it shows how the information bits are first encoded (to suit the modulation type, error control etc.) followed by modulating a carrier signal that carries this information across a noisy channel. At the decoder side, this carrier is demodulated and then the information (possibly corrupted due to channel noise) is decoded.

In a digital watermarking system the modulator is replaced by the watermark embedder thatplaces the watermark in the media content. Distortions to the watermarked media is induced by known or unknown attacks or signal processing operations such as compression, decompression, cropping, scaling etc. The embedded watermark is finally retrieved by the watermarked decoder or detector. One major difference between the two models can be seen in the encoder side. While, in communication systems, the encoding is done in order to protect the information bits from channel distortion, in watermarking, emphasis is usually placed on techniques that minimize perceptual distortions to the watermarked content. Some analogies between the traditional communication system and the watermarking system are summarized.COMMUNICATION SYSTEM WATERMARKING SYSTEMInformation Watermark

Communication channel Host signal(such as image,video etc.)

Power constraint on transmitted signal due to physical limitations

Power constraint on watermark due to audio/visual quality limitations

Interference Host signal and watermark attacks



Side information at transmitter and/or receiver

Knowledge of host signal,watermarking parameters such as key etc.at the encoder and/or decoder.

Channel capacity Watermarking capacity



CHAPTER 6

DIGITAL WATERMARKING METHODS

IMAGE These are some of the methods that can be used to test whether a watermark can survive different changes to the image it is embedded in.

Compare this Original Image with the attacked images below, and see if you can spot any changes in quality.

HORIZONTAL FLIPPING Many images can be flipped horizontally without losing quality. Few watermarks survive flipping, although resilience to flipping is easy to implement.

ROTATION AND CROPPING

A small rotation with cropping doesn’t reduce image quality, but can make watermarks undetectable as rotation realigns horizontal features of an image used to check for the presence of a watermark. The example at left has been rotated 3 degrees to the right, and then had its edges cropped to make the sides straight again.

JPEG COMPRESSION/RE-COMPRESSION JPEG is a widely used compression algorithms for images and any watermarking system should be resilient to some degree to compression or change of compression level e.g. from 71% to 70% in quality like the example at down.



SCALING Uniform scaling increases/decreases an image by the same % rate in the horizontal and vertical directions. Non-uniform scaling like the example at left increases/decreases the image horizontally and vertically at different % rates. Digital watermarking methods are often resilient only to uniform scaling.

DITHERING

Dithering approximates colours not in the current palette by alternating two available similar colours from pixel to pixel. If done correctly this method can completely obliterate a watermark, however it can make an image appear to be “patchy” when the image is over-dithered (as in the elbow area of the image at left).

MOSAIC

A mosaic attack doesn’t damage the watermarked image or make it lose quality in any way, but still enables the image to be viewed in eg: a web browser by chopping the image into subsections of equal size and putting it back together again. To the viewer a “mosaic” image appears to look the same as the original but a web crawler like DigiMarc’s MarcSpider sees many separate images and doesn’t detect that these separate images are parts of a watermarked image. This means that the watermark cannot be detected, as a problem common to all image watermarking schemes is that they have trouble embedding watermarks into small images, (less than 256 pixels in height or width).



STIRMARK StirMark is the industry standard software used by researchers to automatically attempt to remove watermarks created by Digimarc, SysCoP, JK_PGS,Signum Technologies and EIKONAmark. Stirmark attacks a given watermarked image using all the techniques mentioned in this report as well as more esoteric techniques such as low pass filtering, gamma correction, sharpening/unsharpening etc.

AUDIO

The most common method of watermarking audio is to mark

every xth

bit in an audio file depending on the random generator seed calculated from the watermarking key applied to the audio.

These are some of the ways watermarks can be removed from audio files.

MPEG1 LAYER III (MP3) AUDIO COMPRESSION

A digital audio compression algorithm that achieves a compression factor of about twelve while preserving sound quality. What this lossy compression does is remove the frequencies not heard by the human ear from the audio. If a raw audio file is converted to MP3 at a bit-rate of 128kbps than roughly 90% of the frequencies are removed. This means that a search for the watermark needs to find an unaltered length of samples that contains at least 2 watermarked bits to prove the watermarks existence.

AUDIO RESTORATION PROGRAMS

Audio restoration programs are designed to remove hisses, crackles and pops from audio recordings. They do this by searching through the wavelength, removing samples that don’t “fit in” amongst neighbouring samples, and replacing them with an average of the two neighbour samples. Although the removal of digital watermarks is obviously not a purpose of these programs, they work remarkably well at doing so as the sample bits inserted to watermark the audio don’t fit in with their surrounding pixels, and are therefore removed.

ECHO HIDING REMOVAL



Echo hiding relies on the fact that we cannot perceive short echoes, eg: 1 millisecond(ms) and embeds data into a cover audio signal by introducing an echo characterised by its delay and its relative amplitude compared to surrounding samples. The echo delays are chosen between 0.5 ms and 2 ms and the best relative amplitude of the echo is around 0.8 ms. However specialised software which looks for echoes with a length between 0.5 ms and 2 ms (as seen below), can be used to detect and remove these echoes without effecting sound quality.

JITTER

The simplest and most effective attack on any audio watermarking scheme is to add jitter to the signal. In our first implementation, we split the signal into chunks of 500 samples, either duplicated or deleted a sample at random in each chunk (resulting in chunks of 499 or 501 samples long) and stuck the chunks back together. This turned out to be almost imperceptible after altering, even in classical music; but the jitter prevents the marked bits from being located”, and therefore the watermark is obliterated. In his paper titled “Audio watermarking: Features, Applications and Algorithms“, Michael Arnold agrees with the Cambridge team stating that “one of the greatest challenges [of watermarking] is the robustness against the so-called jitter attack”.

VIDEO

At present there is no known method to remove a digital watermark from a stream of video. This is probably because those who trade in pirated video, (especially in DivX format), store their pirated movies locally on their hard disk drives or on CD-R disks where they cannot be checked for watermarks by anyone.

CHAPTER 7

DISTORTION AND ATTACKS



In practice, a watermarked object may be altered either on purpose or accidentally, so the watermarking system should still be able to detect and extract the watermark. Obviously, the distorsions are limited to those that do not produce excessive degradations, since otherwise the transformed object would be unusable. These distorsions also introduce a degradation on the performance of the system as measured by the probabilities defined in the previous section (i.e., PD and Pb would decrease for a fixed PF). For intentional attacks, the goal of the attacker is to maximize the reduction in these probabilities while minimizing the impactthat his/her transformation produces on the object; this has to be done without knowing the value of the secret key used in the watermarking insertion process, which is where all the security of the algorithm lies.Next, we introduce some of the best known attacks. Some of them may be intentional or unintentional, depending on the application:

Additive Noise. This may stem in certain applications from the use of D/A and A/D converters or from transmission errors. However, an attacker may introduce perceptually shaped noise (thus, imperceptible) with the maximum unnoticeable power. This will typically force to increase the threshold at which the correlation detector works.

Filtering. Low-pass filtering, for instance, does not introduce considerable degradation in watermarked images or audio, but can dramatically affect the performance, since spread-spectrum-like watermarks have a non negligible high-frequency spectral contents.

Cropping. This is a very common attack since in many cases the attacker is interested in a smallportion of the watermarked object, such as parts of a certain picture or frames of a video sequence.With this in mind, in order to survive, the watermark needs to be spread over the dimensionswhere this attack takes place.

Compression. This is generally an unintentional attack which appears very often in multimediaapplications. Practically all the audio, video and images that are currently being distributed viaInternet have been compressed. If the watermark is required to resist different levels of compression,it is usually advisable to perform the watermark insertion task in the same domain wherethe compression takes place. For instance, DCT domain image watermarking is more robust to JPEGcompression than spatial-domain watermarking.

Rotation and Scaling. This has been the true battle horse of digital watermarking, especially becauseof its success with still images. Correlation based detection and extraction fail when rotation or scaling are performed on the watermarked image because the embedded watermark and the locallygenerated version do not share the same spatial pattern anymore. Obviously, it would be possible to do exhaustive search on different rotation angles and scaling factors until a correlation peakis found, but this is prohibitively complex. Note that estimating the two parameters becomes simplewhen the original image is present, but we have argumented against this possibility in previoussections. The authors have shown that although the problem resembles synchronization fordigital communications, the techniques applied there fail loudly. Some authors have recently proposedthe use of rotation and scaling-invariant transforms (such as the Fourier-Mellin ) but this dramaticallyreduces the capacity for message hiding. In any case, publicly available programs like Strirmarkbreak the uniform axes transformation by creating an imperceptible non-linear resampling of the image that renders invariant transforms unusable. In audio watermarking it is also quite simple to perform a non-linear transformation of the time axis that considerably difficults watermark detection.

Statistical Averaging. An attacker may try to estimate the watermark and then ‘unwatermark’the object by substracting the estimate. This is dangerous if the watermark does not depend substantially on the data. Note that with different watermarked objects it would be possible to improve the estimate by simple averaging. This is a good reason for using perceptual masks to createthe watermark.



Multiple Watermarking. An attacker may watermark an already watermarked object and latermake claims of ownership. The easiest solution is to timestamp the hidden information by a certification authority.

Attacks at Other Levels. There are a number of attacks that are directed to the way the watermarkis manipulated. For instance, it is possible to circumvent copy control mechanisms discussedbelow by super scrambling data so that the watermark is lost or to deceive web crawlers searchingfor certain watermarks by creating a presentation layer that alters they way data are ordered.The latter is sometimes called ‘mosaic attack’ .

CHAPTER 8

APPLICATION OF DIGITAL WATERMARKING



VideoWatermarking. In this case, most considerations made in previous sections hold. However, now the temporal axis can be exploited to increase the redundancy of the watermark. As in the still images case, watermarks can be created either in the spatial or in the DCT domains. In the latter, the results can be directly extrapolated to MPEG-2 sequences, although different actions must be taken for I, P and B frames. Note that perhaps the set of attacks that can be performed intentionally is not smaller but definitely more expensive than for still images.

Audio Watermarking. Again, previous considerations are valid. In this case, time and frequency masking properties of the human ear are used to conceal the watermark and make it inaudible. The greatest difficulty lies in synchronizing the watermark and the watermarked audio file, but techniques that overcome this problem have been proposed.

Hardware/Software Watermarking. This is a good paradigm that allows us to understand how almost every kind of data can be copyright protected. If one is able to find two different ways of expressing the same information, then one bit of information can be concealed, something that can be easily generalized to any number of bits. This is why it is generally said that a perfect compression scheme does not leave room for watermarking. In the hardware context, Boolean equivalences can be exploited to yield instances that use different types of gates and that can be addressed by the hidden information bits. Software can be also protected not only by finding equivalences between instructions, variable names, or memory addresses, but also by altering the order of non-critical instructions. All this can be accomplished at compiler level.

Text Watermarking. This problem, which in fact was one of the first that was studied within the information hiding area can be solved at two levels. At the printout level, information can be encoded in the way the textlines or words are separated (this facilitates the survival of the watermark even to photocopying). At the semantic level (necessary when raw text files are provided), equivalences between words or expressions can be used, although special care has to be taken not to destruct the possible intention of the author.

Executable Watermarks. Once the hidden channel has been created it is possible to include even executable contents, provided that the corresponding applet is running on the end user side.

Labeling. The hidden message could also contain labels that allow for example to annotate images or audio. Of course, the annotation may also been included in a separate file, but with watermarking it results more difficult to destroy or loosethis label, since it becomes closely tied to the object that annotates. This is especially useful inmedical applications since it prevents dangerous errors.

Fingerprinting. This is similar to the previous application and allows acquisition devices (such as video cameras, audio recorders, etc) to insert information about the specific device (e.g., an ID number) and date of creation. This can also be done with conventional digital signature techniques but with watermarking it becomes considerably more difficult to excise or alter the signature. Some digital cameras already include this feature.

Authentication. This is a variant of the previous application, in an area where cryptographic techniques have already made their way. However, there are two significant benefits that arise from using watermarking: first, as in the previous case, the signature becomes embedded in



the message, second, it is possible to create ‘soft authentication’ algorithms that offer a multivalued ‘perceptual closeness’ measure that accounts for different unintentional transformations that the data may have suffered (an example is image compression with different levels), instead of the classical yes/no answer given by cryptography-based authentication.Unfortunately, the major drawback of watermarking-based authentication is the lack of public key algorithms that force either to put secret keys in risk or to resort to trusted parties.

Copy and Playback Control. The message carried by the watermark may also contain information regarding copy and display permissions.Then, a secure module can be added in copy or playback equipment to automatically extract this permission information and block further processing if required. In order to be effective, this protection approach requires agreements between content providers and consumer electronics manufacturers to introduce compliant watermark detectors in their video players and recorders. This approach is being taken in Digital Video Disc (DVD).

Signalling. The imperceptibility constraint is helpful when transmitting signalling information in the hidden channel. The advantage of using this channel is that no bandwidth increase is required.An interesting application in broadcasting

copyright protection of digital media. In the past duplicating art work was quite complicated and required a high level of expertise for the counterfeit to look like the original. However, in the digital world this is not true. Now it is possible for almost anyone to duplicate or manipulate digital data and not lose data quality. Similar to the process when artists creatively signed their paintings with a brush to claim copyrights, artists of today can watermark their work by hiding their name within the image. Hence, the embedded watermark permits identification of the owner of the work. It is clear that this concept is also applicable to other media such as digital video and audio. Currently the unauthorized distribution of digital audio over the Internet in the MP3 format is a big problem. In this scenario digital watermarking may be useful to set up controlled audio distribution and to provide efficient means for copyright protection, usually in collaboration with international registration bodies.

Are there any other applications where digital watermarking may be used?

There are a number of possible applications for digital watermarking technologies and this number is increasing rapidly.

In the field of data security, watermarks may be used for certification, authentication, and conditional access. Certification is an important issue for official documents, such as identity cards or passports.

Example on the left of a protected identity card. The identity number "123456789" is written in clear text on the card and hidden as a digital watermark in the identity photo. Therefore switching or manipulating the identity photo will be detected



Digital watermarking permits linking information on documents.

That means that key information is written twice on the document. For instance, the name of a passport owner is normally printed in clear text. But it would also be hidden as an invisible watermark in the passport photo. If anyone tries to tamper with the passport by replacing the photo it would be possible to detect the change by scanning the passport and verifying the name hidden in the photo.

The picture shows a printing machine from Intercard for various types of plastic cards (Courtesy of Intercard, Switzerland).

TAMPERING WITH IMAGES:Another application is the authentication of image content. The goal of this application is to detect any alterations and modifications in an image.

The three pictures below illustrate this application. The picture on the left shows an original photo of a car that has been protected with a watermarking technology. In the center, the same picture is shown but with a small modification: the numbers on the license plate have been changed. The picture on the right shows the photo after running the digital watermark detection program on the tampered photo. The tampered areas are indicated in white. We can clearly see that the detected area corresponds to the modifications applied to the original photo.

Using digital watermarks for integrity verification: the protected image is the image (a) above; a modified image is obtained by swapping the numbers 9 and 4 of the number plate (b); digital watermarking technology allows detecting and highlights the modified areas, as shown on (c).



Invisible marking on blank paper: Digital watermarks can also be adapted to mark white paper with the goal of authenticating the originator, verify the authenticity of the document content, or to date the document. Such applications are especially of interest for official documents, such as contracts. For example, the digital watermark can be used to embed the name of the lawyer or important information such as key monetary amounts. In the event of a dispute, the digital watermark is then read allowing authentication of key information in the contract. AlpVision developed genuine process to invisibly mark white blank paper with normal and visible ink. This patented technology is now known as Cryptoglyph.The image on the down shows blank paper marked by the invisible digital watermark using standard visible ink, with the Cryptoglyph technology.

Digital Media Management: Beside applications in the fields of copyright protection, authentication and security, digital watermarks can also serve as invisible labels and content links. For example, photo development laboratories may insert a watermark into the picture to link the print to its negative. This way is very simple to find the negative for a given print. All one has to do is scan the print and extracted the information about the negative. In a completely different scenario digital watermarks may be used as a geometrical reference which may be useful for programs such as optical character recognition (OCR) software. The embedded calibration watermark may improve the detection reliability of the OCR software since it allows the determination of translation, rotation, and scaling.



CONCLUSION

Digital watermarking is a rapidly evolving area of research and development. An exhaustive list of digital watermarking applications is of course impossible. However, it is interesting to note the increasing interest in fragile watermarking technologies. Especially promising are applications related to copy protection of printed media. Examples here include the protection of bills with digital watermarks. Various companies have projects in this direction and it is very likely that fully functioning solutions will soon be available.One key problem that we still face today is the development of truly robust, transparent and securewatermarking technique for different digital media including images, video and audio. Another key problem is the development of semi-fragile authentication techniques. The solution to these problem will require application of known results and development of new results in the fields of information and coding theory, adaptive signal processing, game theory, statistical decision theory, and cryptography.



REFERENCES

http://www.10gea.org

http://standards.ieee.org/resources/glance.html

IEEE

Digital watermarking,R chandramauli,Nasir Memon

Wikipedia
