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CHAPTER 1

INTRODUCTION

The growth of high speed computer networks and World Wide Web (WWW) have explored

means of new business, scientific, entertainment and social opportunities in the form of

electronic publishing and advertising, massaging, real-time information delivery, data

sharing, collaboration among computers, product ordering, transaction processing, digital

repositories and libraries, web newspapers and magazines, network video and audio, personal

communication and lots more. The cost effectiveness of selling softwares in the form of

digital images and video sequences by transmission over WWW is greatly enhanced due to

the improvement in technology.

We know that one of the biggest technological events of the last two decades was the

invasion of digital media in an entire range of everyday life aspects. Digital data can be

stored efficiently and with a very high quality, and it can be manipulated very easily using

computers. Furthermore, digital data can be transmitted in a fast and inexpensive way

through data communication networks without losing quality. Digital media offer several

distinct advantages over analog media. The quality of digital audio, images and video

signals are higher than that of their analog counterparts. Editing is easy because one can

access the exact discrete locations that need to be changed. Copying is simple with no loss

of fidelity. A copy of a digital media is identical to the original. With digital multimedia

distribution over World Wide Web, authentications are more threatened than ever due to the

possibility of unlimited copying. The easy transmission and manipulation of digital data

constitutes a real threat for information creators, and copyright owners want to be

compensated every time their work is used. Furthermore, they want to be sure that their

work is not used in an improper way (e. g. modified without their permission). For digital

data, copyright enforcement and content verification are very difficult tasks. One solution

would be to restrict access to the data using some encryption techniques. However,

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encryption does not provide overall protection. Once the encrypted data are decrypted, they

can be freely distributed or manipulated.

Unauthorized use of data creates several problems. For example, if we visit http:\\www.

wallpaper.com, we observe that all the wallpaper images are created by the owners, which

are their Intellectual Property Right (IPR). Any user can download the wallpapers. Now,

consider that a user downloads the images and posts those images (either after modifying or

original) on his/her website. Three issues may arise in this situation:

1) How will the owner of wallpaper.com know that there is one more web server on

WWW posting their wallpapers?

2) If the owner knows about this fact, where shall he go to make a complaint?

3) The last but very important issue is that even if first two problems are resolved, how

the owner will prove the ownership on the wallpaper images posted on another

server?

The first issue is related to network technologies and involves issues like ‘web crawler’ and

‘pattern matching’ etc. Second issue is related to the international copyright laws and is

another very tricky issue. This thesis does not deal with these 2 issues. This thesis covers

the third issue, the authentication i.e. how to prove the ownership?

The above problem can be solved by hiding some ownership data into the multimedia data,

which can be extracted later to prove the ownership. This idea is implemented in bank

currency notes embedded with the watermark which is used to check the originality of the

note. The same “watermarking” concept may be used in multimedia digital contents for

checking the authenticity of the original content.

To begin with a quick background of watermarking, first we present the history of data

hiding and related terminologies. Then, we will move on to a discussion on the

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watermarking, requirements that watermarking system must meet, types of the watermarking,

applications and then various attacks on a watermarking system.

1.1 DATA HIDING BACKGROUND The solution of the problem discussed above seems to lie in a technique that dates back to

ancient Egypt and Greece: data hiding or steganography. Steganography deals with the

methods of embedding data within a medium (host or cover medium) in an imperceptible

way. All forms of digital data (still images, audio, video, text documents and multimedia

documents) can be used as a cover medium for information hiding.

The history of steganography goes all the way back to the 5th Century. The earliest known

writings about steganography were by the Greek historian Herodotus. The historian relates

how a slave had a message tattooed on his head by Histiaeus who was trying to get a

message to his son-in-law Aristagoras. Once the slaves’ hair was long enough to cover the

message he was sent to Aristagoras in the city of Miletus [92].

Stegnography has been used in many different ways. The simplest was the use of invisible

inks that a person could use to send a message to another person without anyone else

knowing. Different forms of invisible ink were used to conceal messages. Some of the more

common forms of invisible ink have been lemon juice, milk, and urine to name a few. If

someone wanted to conceal a message, he would simply write a message, using one of these

inks, on a sheet of paper that already had something written on it. The person receiving the

message would then hold the paper over a flame and the transparent message would appear.

Image stegnography was done during the early twentieth century. During the Boer War in

South Africa, the British were using Lord Robert Baden-Powell as a scout. He was scouting

the Boer artillery bases mapping their positions. He took his maps and converted them into

pictures of butterflies with certain markings on the wings that were actually the enemies’

positions [92].

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During World War II, Nazis introduced a new concept in espionage, which was called the

microdot. This simple device could conceal a full typewritten page within the size of a

common period. A microdot could hold valuable information such as charts, diagrams and

drawings.

Figure 1.1: Watermark on the bank currency note

Thus, stegnography is an area which is, more or less, a Hide-&-Seek game. Some important

data or information is hidden in another medium. The cover medium has no relationship

with the data or information hidden. Data or information which is hidden is not encrypted

also. The key issue in a stegnography system becomes that no one should suspect that a

particular medium is carrying any hidden data or information.

We can extend the stegnography concept for the authentication of digital multimedia data.

Digital multimedia data which has to be protected is now the cover medium and then we can

hide the copyright data into it. In this case, there will be two major requirements as follows:

1) Imperceptibility: After hiding the copyright data, cover medium should not be

affected, and

2) Robustness: No body should be able to remove the data without affecting the cover

medium.

Watermark symbol is added here to prove the originality

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The copyright data may be termed as digital watermark data. This area of application of

stegnography is known as Digital Watermarking. Therefore, digital watermark is a

message/data/information which is embedded into digital content (audio, video, images or

text) that can be detected or extracted later. Such message/data/information mostly carries

the copyright or ownership information of the content. The process of embedding digital

watermark information into digital content is known digital watermarking.

Before moving further in this discussion, we must first understand the difference of the

digital watermarking with other related terms like stegnography, cryptography and digital

signature.

1.1.1 STEGANOGRAPHY VS WATERMARKING

Watermarking is the subset of Stegnography. In Stegnography, data which is hidden has no

relationship with the cover medium and the requirement from such a system is that no

suspicion should arise that a medium is carrying any hidden data. In watermarking, unlike

stegnography, the data which is hidden has relationship with the cover medium data. Data

hidden is the ownership data of the cover medium and there is no issue like suspecting that a

particular medium is carrying some copyright data.

As the purpose of stegnography is to have a covert communication between two parties i.e.

existence of the communication is unknown to a possible attacker, and a successful attack

shall detect the existence of this communication. On the contrary, watermarking, as opposed

to stegnography, requires a system to be robust against possible attacks. Other requirements

of watermarking are entirely different from stegnography and these are discussed in detail in

Section 1.3.

1.1.2 CRYPTOGRAPHY VS. WATERMARKING

Cryptography can be defined as the processing of information into an unintelligible form

known as encryption, for the purpose of secure transmission. Through the use of a “key”,

the receiver can decode the encrypted message (the process known as decryption) to retrieve

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the original message. So, cryptography is about protecting the contents of the message. But

as soon as the data is decrypted, all the in-built security and data is ready to use.

Cryptography "scrambles" a message so that it can not be understood by unauthorized user.

This does not happen in watermarking. Neither the cover medium nor the copyright data

changes its meaning. Rather, copyright data is hidden to give the ownership information of

the medium in which it is hidden.

1.1.3 DIGITAL SIGNATURE VS. WATERMARKING

Digital signatures, like written signatures, are used to provide authentication of the associated

input, usually called a "message”. Digital signature is an electronic signature that can be used

to authenticate the identity of the sender of a message or the signer of a document, and

possibly to ensure that the original content of the message or document that has been sent is

unchanged. Digital signatures are easily transportable, cannot be imitated by someone else,

and can be automatically time-stamped. The ability to ensure that the original signed

message arrived means that the sender cannot easily repudiate it later. A digital signature can

be used with any kind of message, whether it is encrypted or not, simply so that the receiver

can be sure of the sender's identity and that the message arrived intact. A digital signature is

apart from the protected message, whereas a digital watermark is inside a multimedia

message. Both, digital signature and watermarking protect integrity and authenticity of a

document. Digital signature system is vulnerable to distortion but a watermark system has to

tolerate a limited distortion level.

So, to conclude, Watermarking is adding“ownership” information in multimedia contents to

prove the authenticity. This technology embeds a data, an unperceivable digital code,

namely the watermark, carrying information about the copyright status of the work to be

protected. Continuous efforts are being made to device efficient watermarking schema but

techniques proposed so far do not seem to be robust to all possible attacks and multimedia

data processing operations. The sudden increase in watermarking interest is most likely due

to the increase in concern over IPR. Today, digital data security covers such topics as access

control, authentication, and copyright protection for still images, audio, video, and

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multimedia products. A pirate tries either to remove a watermark to violate a copyright or to

cast the same watermark, after altering the data, to forge the proof of authenticity. Generally,

the watermarking of still images, video, and audio demonstrate certain common fundamental

concepts.

1. 2 APPLICATION AREAS OF DIGITAL WATERMARKING Watermarking techniques may be relevant in the following application areas [26]:

1.2.1 COPYRIGHT PROTECTION

The primary use of watermarking is where an organization wishes to assert its ownership of

copyright for digital objects. This application is of great interest to ‘big media’

organizations, and of some interest to other vendors of digital information, such as news and

photo agencies. These applications require a minimal amount of information to be

embedded, coupled with a high degree of resistance to signal modification (since they may

be subjected to deliberate attack). For example, now a days, a news channel “AAJ-TAK” is

showing the animal’s clips (which are already shown on “Discovery” Channel) by hiding the

Discovery channel’s logo on the video clips. As per the law, The AAJ-TAK should show the

curtsey-sign and should pay the copyright fee to the Discovery channel. In such cases,

There is a strong need of watermarking as once the digital data is broadcasted, any body else

can start selling it without paying the IPR value to its owner.

1.2.2 COPY PROTECTION

Watermarking can be used as a strong tool to prevent illegal copying. For example, if an

audio CD has a watermark embedded into it, then any of the system (Hardware like DVD, or

software) can not make a copy of it, and even if it copies, the watermark data will not get

copied to new duplicate audio CD. Now the duplicate CD can be easily found because it

does not have watermark data. Some schemes have attempted to satisfy more complex copy

protection requirements. An early example is the Serial Copy Management System (SCMS),

introduced in the 1980s, which enabled a user to make a single digital audio tape of a

recording they had purchased but prevented the recording of further copies (i.e. second

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generation) from that first copy. The scheme failed ultimately because not all manufacturers

of consumer equipment were prepared to implement the scheme in their products.

1.2.3 TEMPER DETECTION

In this application area, it is necessary to assure that the origin of a data object is

demonstrated and its integrity is proved. One example of temper detection is photographic

forensic information which may be presented as evidence in the court. Given the ease with

which digital images can be manipulated, there is a need to provide proof that an image has

not been altered. Such a mechanism could be built into a digital camera [29]. For example,

if a cop’s camera catches an over speeding vehicle then when proving the driver guilty in

front of the judge, the accused may claim that the video presented in the court is tempered

and the car shown in the video does not belong to him. A watermarking system which is

embedded in digital cameras may help to resolve the issue. If somebody tries to temper the

data, the watermark will get destroyed indicating that the data is tempered. In our country, a

well-known example is the “Tahalka-Scam”.

1.2.4 BROADCAST MONITORING

There are several types of organizations and individuals interested in monitoring the

broadcast of their interest. For example, advertisers want to ensure that they receive the exact

airtime that they have purchased from broadcasting firms. Musicians and actors want to

ensure that they receive accurate royalty payments for broadcasts of their performances and

copyright owners want to ensure that their property is not illegally rebroadcast by pirate

stations. In 1997, a scandal broke out in Japan regarding television advertising. At least two

stations had been routinely overbooking air time. Advertisers were paying for thousands of

commercials that were never aired [16]. The practice had remained largely undetected for

over twenty years because there were no systems in place to monitor the actual broadcast of

advertisements.

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This broadcast monitoring can be implemented by putting a unique watermark in each video

or sound clip prior to broadcast. Automated monitoring stations can then receive broadcasts

and look for these watermarks identifying when and where each clip appears.

1.2.5 FINGERPRINTING

If monitoring and owner identification applications place the same watermark in all copies of

the same content, it may create a problem. If out of n number of legal buyers of a content,

one starts selling the contents illegally, it may be very difficult to catch who is redistributing

the contents without permission. Allowing each copy distributed to be customized for each

legal recipient can solve this problem. This capability allows a unique watermark to be

embedded in each individual copy. Now, if the owner finds an illegal copy, he can find out

who is selling his contents by finding the watermark which belongs to only singly legal

buyer. This particular application area is known as fingerprinting. This is potentially

valuable both as a deterrent to illegal use and as a technological aid to investigation.

1.2.6 ANNOTATION APPLICATIONS

In this applications area, watermarks convey object-specific information (“feature tags” or

“captions”) to users of the object. For example, patient identification data can be embedded

into medical images. These applications require relatively large quantities of embedded data.

While there is no need to protect against deliberate tampering. Normal use of the data object

may involve such transformations as image cropping or scaling and will require the use of a

technique that is resistant to those types of modification.

For more details of various watermarking applications, one may refer [20].

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1. 3 CHARACTERISTICS OF WATERMARKING SCHEMES An effective watermarking scheme should have the following characteristics:

1) Imperceptibility: In terms of watermarking, imperceptibility means that after inserting

the watermark data, cover medium should not alter much. In other words, the

presence of the watermark data should not affect the cover medium being protected.

If a watermarking scheme does not ensure this requirement, it may happen that after

inserting a watermark data in a cover medium (say an image), image quality may alter

which the owner of the image will never like that a protecting mechanism modifies

his work.

2) Robustness: Robustness of the watermark data means that the watermark data should

not be destroyed if someone performs the common manipulations as well as

malicious attacks. It is more of a property and also a requirement of watermarking

and its applicability depends on the application area.

3) Fragility: Fragility means that the watermark data is altered or disturbed up to a

certain extent when someone performs the common manipulations & malicious

attacks. Some application areas like temper detection may require a fragile

watermark to know that some tempering is done with his work. Some application

may require semi-fragility too. The semi-fragile watermark comprises a fragile

watermark component and a robust watermark component i.e. semi-fragile

watermarks are robust to some attacks but fragile to others attacks.

4) Resilient to common signal processing: The watermark should be retrievable even if

common signal processing operations are applied to the watermarked cover medium

data. These operations include digital-to-analog and analog-to-digital conversion (i.e.

taking the printout of an image and then scan it to create another digital copy of the

image), re-sampling, re-quantization (including dithering and recompression), and

common signal enhancements such as image contrast, brightness and color

adjustment, or audio bass and treble adjustment, high pass and low pass filtering,

histogram equalization of an image and format conversion (BMP image to JPEG

image, MPEG movie to WMV movie, mp3 song to mp4 etc.)

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5) Resilient to common geometric distortions (image and video data): Watermarks in

image and video data should also be immune from geometric image operations such

as rotation, translation, cropping and scaling. This property is not required for audio

watermarking.

6) Robust to subterfuge attacks (collusion and forgery): In addition, the watermark

should be robust to collusion attack. Multiple individuals, who possess a watermarked

copy of the data, may collude their watermark copies to destroy the watermark

presence and can generate a duplicate of the original copy. Further, if a digital

watermark is to be used in litigation, it must be impossible for colluders to combine

their images to generate a different valid watermark.

7) Unambiguousness: Retrieval of the watermark should unambiguously identify the

owner. Furthermore, the accuracy of owner identification should not degrade much

in the case of an attack. The Unzign and Stirmark [97] have shown remarkable

success in removing data embedded by commercially available programs.

Watermarking of watermarked image (re-watermarking) is also a major threat [97].

1.4 TYPES OF DIGITAL WATERMARKS Prof. S. Mohanty presents a very good classification of watermarking areas in his paper [62].

We can classify the types of watermarking based on the cover medium, embedding domain,

perception and application domain. Figure 1.2 shows the various classifications of

watermarking.

Based on their embedding domain, watermarking schemes can be classified as follows:

1) Spatial Domain: The watermarking system directly alters the main data elements (like

pixels in an image) to hide the watermark data.

2) Transformed Domain: The watermarking system alters the frequency transforms of

data elements to hide the watermark data. This has proved to be more robust than the

spatial domain watermarking.

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3) Feature Domain: The watermarking system takes into account the region, boundary

and object characteristics. It presents better detection and recovery from attacks.

Figure 1.2: Various classifications of watermarking

Watermarking techniques can also be divided into four categories, according to the type of

document to be watermarked, as follows.

1) Image Watermarking: Figure 1.3 and 1.4 represent the general scheme of an image

watermarking, embedding and decoding (specifically key based, invisible and fragile)

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system respectively. ‘E’ represents the watermarking embedding algorithm and ‘D’

represents the watermarking decoding algorithm.

2) Other types of watermarking, according to the type of document to be watermarked

are Video Watermarking, Audio Watermarking and Text Watermarking.

Figure 1.3: Image watermark embedding scheme

Figure 1.4: Image watermark detection scheme

According to the human perception, the digital watermarks can be divided into 4 different

types: Visible watermark, Invisible-Robust watermark, Invisible-Fragile watermark, and dual

watermark. Visible watermark is a secondary translucent overlaid into the primary image.

The watermark appears visible to a casual viewer on a careful inspection. The invisible-

robust watermark is embedded in such a way that alternations made to the pixel value are

perceptually not noticed and it can be recovered only with appropriate decoding mechanism.

The fragile watermark is embedded in such a way that any manipulation or modification of

the image would alter or destroy the watermark. Dual watermark is a combination of a

visible and an invisible watermark [8].

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According to application domain, Source-based watermarks are desirable for ownership

identification or authentication where a unique watermark identifies the owner. A source-

based watermark could be used for authentication and to determine whether a received image

or other electronic data has been tampered. The watermark could also be destination based

where each distributed copy gets a unique watermark identifying the particular buyer. The

destination based watermark could be used to trace the buyer in the case of illegal reselling.

This is used in fingerprinting. A watermark is said private if only authorized readers can

detect it. In other words, in private watermarking, a mechanism is envisaged that makes it

impossible for unauthorized people to extract the watermark. A watermarking algorithm is

said blind if it does not resort to the comparison between the original non-marked and the

marked document to recover the watermark. Conversely, a watermarking algorithm is said

non-blind if it needs the original data to extract the information contained in the watermark.

The definition of invertible and quasi-invertible is more technical and can be given as

follows [2]:

If E is the Embedding algorithm, D is detection algorithm, Cδ is Comparator function, I is

original cover image, Î is watermarked image, J is recovered attacked image, S is watermark

signal and S’ is extracted watermark data, then:

1) E (I, S) = Î

2) D (J, I) = S’ or D (J) = S’

3) Comparator Cδ:

A watermarking scheme (E, D, Cδ) is invertible if:

1) Inverse mapping E-1 does exist such that E-1 (Î) = (Î’, S’) &E (Î’, S’) = Î;

2) E-1 is computational feasible;

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3) S’ is an allowed watermark;

4) Î and Î’ are perceptually similar; and

5) Comparator output Cδ (D (Î, Î’), S’) = 1

Otherwise the watermarking scheme is non-invertible.

A watermarking scheme (E, D, Cδ) is quasi-invertible if:

1) Properties for invertible watermarking schemes apply;

2) Only difference E (Î’, S’) = Î’’ ≠ Î; and

3) Î’’ and Î perceptually similar.

Otherwise the watermarking scheme is non-quasi-invertible. A Non-invertible scheme can

be quasi-invertible and Non-quasi-invertibility implies non-invertibility.

1.5 STRUCTURE OF THE THESIS This thesis comprises of the following chapters:

Chapter 2 describes the image watermarking literature survey and problem statement.

Chapter 3 describes the preliminaries (like background of JPEG compression, 2D–DCT and

DWT, image quality parameter, some standard watermarking techniques which are used to

compare the performances of the proposed techniques etc and test images data). The

watermarking techniques for gray images have been proposed in Chapter 4. Chapter 5

describes the proposed watermarking techniques and issues related to colored BMP images.

In Chapter 6, the proposed watermarking techniques for JPEG images have been given.

Finally the summary of results, conclusions and future work is given in Chapter 7 followed

by references, author’s publications and synopsis at the end.