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1. INTRODUCTION

In modern times, the traditional paper media is being replaced with its

digital versions for gaining the advantage of avoiding large storage and

preservation requirements, while making information easily available for a

larger number of users. Almost all commercial and private organizations are

now moving towards ‘paperless’ office, where common office devices such as

digital photocopiers, fax machines, scanners, digital cameras and camcorders

are increasingly used to create digital contents. The advantages of using digital

form can be easily created and stored.

In recent years, tremendous growth has been witnessed in the

development of modern technologies like Internet, P2P (Peer-to-Peer) and

MMS (Multimedia Messaging Services), which make an important evolution

towards digital distribution of data via networks. Moreover, several

transmission devices and techniques like General Packet Radio Service

(GPRS), Multimedia Messaging Services (MMS), Video Clip transmission,

High Definition Television (HDTV) and Video Conferencing are being used by

more and more people for faster and easy communication.

These digital transmission techniques have introduced flexible, cost-

effective communication models that are advantageous for electronic

commerce transactions. As a result, digital content appear widely in the

Internet and the World Wide Web (WWW) and in storage media such as CD-

ROM and DVD. On the other hand, easy access facilitates has introduced

information piracy, through unauthorized replication and manipulation of

digital data, beyond the terms and conditions agreed upon, with the help of

inexpensive tools (Surekha et al., 2010; Lin et al., 2000; Lin, 2001; Karen,

2003). This makes confidentiality, integrity, and authenticity as mandatory

requirements against unauthorized data duplication, and illegal distribution of

digital content (Bert and Cave, 2000).

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This has forced academicians, industrialists and researchers to focus on

the development of techniques for the protection of intellectual digital property.

Intellectual property is the intangible product and it can be in various forms as

(i) Multimedia contents - image, animated images, video, audio or

documents (text)

(ii) Software products – both customized and commercial software

(iii) New innovative hardware designs.

Some of the typical scenarios in which digital watermarking would be

useful are as follows.

Companies want to make their text reports available to the public while

maintaining their ownership over the document.

Organizations develop applications and want to establish their ownership

over whole or part of the software.

GPS companies implementing state-of-the-art real-time software systems

for generating maps want to protect the software from being duplicated by

rivals.

This research is focused on the intellectual content protection using

digital images. A digital image is the stored description of a graphic picture

using a set of brightness and color values of pixels and/or a set of instructions

for reproducing the picture (http://www.answers.com/topic/image).

The issue of intellectual content protection is addressed by Digital

Rights Management (DRM) to address the problem of piracy

(http://www.securitydocs.com/library/3461). It has been the focus of many

researches in both academia and industry. According to the MIT Technology

Review, (Singh, 2001) DRM was one of the top ten emerging technologies that

would change the world in content protection. DRM technologies are engaged

to control the use of digital media by preventing end user’s access, copying,

distribution, manipulation or conversion to other formats.

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DRM refers to a broad range of technologies and standards that use

information about rights and rights holders to manage copyright material and

the terms and conditions on which it is made available to users. Digital

watermarking and biometric signals are two components of DRM, serving the

above purpose as well as controlling the rights and privacy of the owners. This

research study is centered with watermarking techniques for digital images.

1.1. DIGITAL IMAGE WATERMARKING

Digital watermarking is a process to embed secret information into an

image. A watermark is a pattern of bits inserted into image data that helps to

identify the file’s security information (author, rights, secret message, etc.).

The name “watermark” is derived from the faintly visible marks imprinted on

organizational stationery. Unlike printed watermarks, which are intended to be

somewhat visible, most of the digital watermarks are designed to be completely

invisible. In addition, the bits representing the watermark must be scattered

throughout the file in such a way that they cannot be identified and

manipulated. The embedding technique must keep the original information

perceptually unchanged and the watermark data should be detected by an

extraction algorithm.

The main aim of watermarking is to provide robust watermarking

(copyright protection) and fragile watermarking (content authentication). A

third type of watermarking technique that is becoming popular is the

“Biometric Watermarking”. Biometric Watermarking is a technique that

creates a link between a human subject and the digital media by embedding

biometric information into the digital object. Information hiding is another area

where, as the name suggests, short messages or text files are hidden inside an

image before transmission. The main objective with all these techniques is to

use a method that hides secret data in a way that makes it difficult for the

hackers to decipher the original message.

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Irrespective of the operation being used for the general framework of

digital image watermarking consists of three main components, Watermark

Data Generator, Watermark Embedder and Watermark Extractor. The

watermark data generator is a procedure that creates the secret data that may

use a secret key, a signature or a combination of several original keys and the

original data. After watermark is generated, the embedding procedure begins,

where a cover image is selected into which the generated watermark is inserted

using a secret a key. The cover image is defined as an input image where

watermark is to be embedded. The output of this stage is called the

‘Watermarked Image’ or ‘Reference Image’ which is used by the Watermark

Extractor. The watermark extractor is a reverse procedure of the insertion

process where the embedded data is detected and recovered using the secret

key. The process is presented in a pictorial form in (Figure 1.1).

Figure 1.1 : General Framework of Digital Image Watermarking System

To understand the concepts of watermarking, a clear understanding of

the difference between watermarking and other techniques like stegnography,

cryptography and digital signature is required and is presented in the following

sections.

Protected Image

Att

acke

dIm

age

Communication Channel

Watermark Generator

Cover ImageWatermark Signal

Secret Key

Watermark Data

Watermark Embedder

ATTACKSWatermark

Detector

Secret KeySecret Key

Watermark

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1.1.1. Watermarking and Steganography

Watermarking is a field that has emerged from 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.

1.1.2. Watermarking and Cryptography

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 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. Watermarking and Digital Signature

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

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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 sender cannot easily repudiate the original message received in a later

stage. A digital signature can be used with any kind of message, whether it is

encrypted or not, so that the receiver can confirm the sender's identity that the

message arrived intact. A digital signature protects a 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.

1.2. FEATURES OF DIGITAL WATERMARKING

An ideal digital watermark should have the following important

features. However, the relative importance of these properties is application

dependent.

Transparency (invisibility): Transparency or imperceptibility is the

characteristic of hiding a watermark in a way that does not degrade the

visual quality of an image. A closely related term is fidelity, which refers to

perceptual similarity between the watermarked image and the original

image. The watermark should be imperceptible both statistically and

perceptually. So that no visual effect is perceived by the end user. Further,

the embedding process should not degrade the image quality. However, in

some applications a little degradation is accepted to have higher robustness

or lower cost.

Robustness: A watermark algorithm is robust if it can survive against

changes made by signal processing operations. In contrary, a watermark

algorithm is termed as fragile if the watermark is destroyed by

modifications. If digital watermarking is used for ownership identification,

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then the watermark has to be robust against any modifications. The

watermarks should not get degraded or destroyed as a result of

unintentional or malicious signal and geometric distortions like analog-to

digital conversion, digital-to-analog conversion, cropping, resampling,

rotation, dithering, quantization, scaling and compression of the content. On

the other hand, if digital watermarking is used for content authentication,

the watermarks should be fragile i.e., the watermarks should get destroyed,

whenever the content is modified so that any modification to content can be

detected.

Capacity: A watermarking system must allow for a useful amount of

information to be embedded into an image. The amount of information that

can be embedded in a watermarked image is called data payload. The data

payload in image watermarking means the number of bits encoded with the

image. The payload of the embedded watermark information must be

sufficient to enable the envisioned application.

Inseparability: After the digital content is embedded with watermark,

separating the content from the watermark to retrieve the original content

should not be possible.

Security: The digital watermarking techniques should prevent unauthorized

users (hackers) from detecting and modifying the watermark (attacks)

embedded in the cover signal. It is the ability of the watermark to resist

malicious attacks. Secret keys can be used to ensure that only authorized

users are able to detect/modify the watermark.

Computation Cost: Computation cost is the measure of computing

resources required to perform watermark embedding or detection processes.

It can be measured using the processing time for a given computer

configuration.

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Complexity: Depending on the application, the insertion is done only once

and can be performed off-line. Consequently, the complexity of encoding

plays less important role than the complexity of the decoding. But in real-

time applications, both these procedures should be in a simple form.

1.3. APPLICATION AREAS

Both single and multiple watermarking algorithms have wide ranging

applications (Koz, 2002; Akhaee et al., 2011; Irany et al., 2011; Feng et al.,

2011) and this section discusses some common applications of digital

watermarking.

1. Copyright Protection: One of the most common applications of digital

watermarking is copyright protection. It can be used to identify and protect

copyright ownership. Copyright specifies rules for using and copying the

data and is inserted into digital object without loss of quality. This kind of

applications needs high robustness. It enables the identification of the

copyright holder and thus protects his or her rights in content distribution.

The successful detection of the watermark can positively identify the owner.

2. Authentication/Content Verification: In authentication the goal is to

detect any change or modification of the data, so that the information

required to authenticate the content should be watermarked. It refers to the

integrity of the image. An image is said to be authentic, if it has not been

modified. Authentication of digital images can be useful in insurance claims

by ensuring trustworthy photographs for court evidence. Other reported

applications related to image authentication are the validation of cultural

heritage paintings, medical records and digital artworks. This can be

possible through the fragile watermark, which has low robustness to any

modification. A subfield of authentication is the ‘content authentication’.

Traditional authentication algorithms aim at determining the authenticity by

determining the percentage of modifications seen. This percentage would be

higher when the image is compressed with low quality factor to reduce

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amount of storage space. In such cases, content authentication, which can

tolerate legitimate changes and highlight significant manipulations, would

be more promising.

3. Owner Identification/Proof of Ownership: The identification information

of the content owner can be embedded as a watermark data into the original

data to prove the ownership. This application requires high level of security.

4. Copy Control: One of the techniques of copy prevention is to have a copy

and consumer control mechanism to prevent illegal copying or recording of

the content by inserting a never-copy watermark or limiting the number of

times of copying.

5. Tamper Detection and Localization: Tamper detection is used to disclose

alterations made onto an image. It is closely related to authentication in the

sense that if tampering is detected in an image, then the image is considered

unauthentic. Tamper localiziation enables further investigation of an act of

tampering by identifying the tampered regions within the image. This

information can assist in media forensics. For example, the severity of the

tampering and the motives behind it can be established. For example,

consider pictures in Figure 1.2, the picture on the left show 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 watermark detection and localization

program on the tampered photo. The tampered areas are indicated in white

and it can be seen clearly that the detected areas correspond to the

modifications applied to the original photo.

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Figure 1.2 : Tamper Detection and Localization – An Example

6. Broadcast Monitoring: Advertisers use this kind of application to ensure

that the commercials are aired by the broadcasters at the time and location

desired. Watermarks can be embedded in any type of data to broadcast on

the network by automated systems, which are able to monitor distribution

channels to track the content in the time and the place they appear.

7. Tracking: Digital watermarks can be used to track the usage of digital

content. Each copy of digital content can be uniquely watermarked with

metadata specifying the authorized users of the content. Such watermarks

can be used to detect illegal replication of content by identifying the users

who replicated the content illegally. The watermarking technique used for

tracking is called as fingerprinting. The main challenge in fingerprinting is

to trace the source of illegal copies so that the owner can embed of a

different watermark key into each copy that distributed to a different

customer. Figure 1.3 shows how watermarking can be used for Tracking.

8. Covert Communication: Covert communication is another possible

application of digital watermarking. The watermark, secret message, can be

embedded imperceptibly to the digital image or video to communicate

information from the sender to the intended receiver while maintaining low

probability of intercept by other unintended receivers.

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Figure 1.3 : Watermarks for Tracking – An Example

9. Other Applications : There are many more applications where digital

watermarking algorithms have been used.

Device control applications watermarks embedded into radio and

television signals are used to control some features of a receiver.

Communication enhancement applications watermarks extracted are

used to repair error bits in transmission. Hence it saves time, cost and

bandwidth for retransmission.

Media forensics is an application which investigates digital data to

uncover scientifically valid information for court evidence. The

application of digital watermarks in media forensics include digital

camera, traitor tracing, transaction tracking and content recovery.

Annotation and privacy control applications use multi-bit

watermarking algorithms. For example, patient records and imaging

details related to a medical image can be carefully inserted into the

image. This would not only reduce storage space but also provides a

tight link between the image and its details.

Server

Receiver 1

Receiver 2

Receiver 3

Receiver 4

Copy 1 containing watermark 1

Copy 1 containing watermark 2

Copy 1 containing watermark 3

Copy 1 containing watermark 4

Illegal Copy

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Identity Card / Passport Security: Information in a passport or ID

card can also be included in the person’s photo that appears on the ID

card. Extracting the embedded information and comparing it to the

written text can verify the ID card. The inclusion of the watermark

provides an additional level of security in this application (Figure 1.4).

For example, if ID card is stolen and the person replaces the picture, the

failure in extracting the watermark will invalidate the ID card.

Figure 1.4 : Example of a protected identity card

Furthermore, digital watermarking could be used to save context or meta-

information in source documents. In using special watermarking agents,

generic search machines are able to retrieve such information and can offer

time-based media documents as a result.

1.4. CLASSIFICATION OF WATERMARKING TECHNIQUES

This section provides a brief discussion on the categorization of digital

watermarking algorithms. The digital watermarking algorithms can be

categorized in different manner (Lee and Jung, 2001; Yusof and Khalifa,

2007).

1.4.1. Types of Data Embedded

First, watermark techniques can be divided into four groups according to

the type of data to be watermarked.

Text watermarking - Text watermarking aims at embedding additional

information in the text itself with the goals of concealed communication

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and hidden information transport, of content and authorship authentication,

and finally of enriching the text with metadata.

Image watermarking – Image watermarking aim at embedding secret

information into a digital image with the goals of robustness for copyright

and authentication.

Video watermarking – Video watermarking involves embedding information

into digital video frames. Ideally, a user viewing the video cannot perceive a

difference between the original, unmarked video and the marked video, but a

watermark extraction application can read the watermark and obtain the embedded

information.

Audio watermarking – Audio watermarks are special signals embedded

into digital audio. Audio watermarking schemes rely on the imperfection of

the human auditory system. However, human ear is much more sensitive

than other sensory motors. Thus, good audio watermarking schemes are

difficult to design

1.4.2. Human Perception

Based on human perception, watermark algorithms are divided into two

categories, namely, visible watermarking and invisible watermarking. Visible

watermark is associated with perception of the human eye, so that if the

watermark is embedded in the data in the way it can be seen without extraction.

Examples of visible watermarks are logos that are used in papers and video. On

the other hand, an invisible watermarking cannot be seen by human eye. So it is

embedded in the data without affecting the content and can be extracted only

by the owner or the person who has right for that. For example images

distribute over the internet where the watermarked is invisible for copy

protection. An example is given in Figure 1.5. Dual watermark is a

combination of a visible and an invisible watermark (Boland et al., 1995).

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Figure 1.5 : Visible and Invisible Watermarking – An Example

1.4.3. Extraction Method

Watermark algorithms, based on the method used for detection is

classified as blind, non-blind and semi-blind techniques.

Blind or public watermarking: In public watermarking, there is no need

for original signal during the detection processing to detect the watermark.

Only the secret key is required and the users of the content are authorized to

detect the watermark.

Non-blind or private watermarking: In non-blind or private watermark,

original signal is required for detecting the watermark and the users are not

authorized to detect the watermark.

Semi-blind watermarking: In semi-blind watermarking, sometimes some

extra information is needed to correctly detect the watermark. Some

algorithms need to access the original image just after adding the

watermarking, which is called published watermarked signal.

1.4.4. Processing Domain

Finally, based on processing-domain, watermark techniques can be

divided into spatial and transform domain.

In spatial domain based watermarking techniques, the pixel values are

modified to embed the watermark data into the cover image. These methods

exploit the statistical properties of the image pixels during embedding and

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extraction processes. Techniques in spatial domain class generally share the

following characteristics:

The watermark is applied in the pixel domain.

No transforms are applied to the host signal during watermark

embedding.

Combination with the host signal is based on simple operations, in the

pixel domain.

The watermark can be detected by correlating the expected pattern with

the received signal.

The main strengths of pixel domain methods are that they are conceptually

simple and have very low computational complexities. Some example includes

gray scale watermarking techniques like tagging, Least Significant Bit (LSB),

predictive coding techniques and texture block coding (Hartung and Kutter,

1999).

In transform domain methods, transform coefficients are modified for

embedding the watermark. Transform domain is also called frequency domain

because values of frequency can be altered from their original. The main

strength offered by transform domain techniques is that they can take

advantage of properties of alternate domains to address the limitations of pixel-

based methods or to support additional features. Some examples of transform

domain are Discrete Fourier Transform (DFT), Discrete Cosine Transform

(DCT) and Discrete Wavelet Transform (DWT).

1.4.5. Robustness Feature

Additionally, classification can be based on the robustness feature.

Different techniques of this category are as follows.

Robust watermark: One of the properties of the digital watermarking

is robustness. A watermark algorithm is robust if it can survive after

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common signal processing operations such as filtering and lossy

compression.

Fragile watermark: A fragile watermark should be detected after any

change in signal and also possible to identify the signal before

modification. This kind of watermark is used more for the verification or

authenticity of original content.

Semi-fragile watermark: Semi-fragile watermark is sensitive to some

degree of the change to a watermarked image.

1.4.6. Applications

Furthermore, from application point of view, watermark techniques can

be grouped as source-based or destination-based. In source-based, all copies of

a particular data have a unique watermark, which identifies the owner of that

data, while in the destination-based, each distributed copy is embedded using a

unique watermark data, which identifies a particular destination.

Figure 1.6 depicts different classification for digital watermarking

algorithms.

1.5. ATTACKS ON WATERMARKS

Watermark attacks are defined as intentional or unintentional

manipulations of the watermarked image. The attacks can change the

watermarked image in two ways (Lin, 2000). The first one change the visual

meaning of the image and in the second one, change is made to obtain

information about the watermarking algorithm. The former is to manipulate the

image by cut/copy paste, cloning the image pixels or attempt to mix the pixels

with adjacent areas. In the later one, the attacker may know the secrets of the

watermarking algorithm and is able to create different strategies to hack the

digital content.

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Landscape figure - (Intro landscape figure .doc)

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Attacks can be divided into two parts based on their strength: incidental

and malicious. Incidental manipulations are friendly and sometimes required

and are usually cannot be considered as an attack. For example, JPEG

compression is very necessary in internet application to save time and to reduce

load on the communication channels. Robust watermarking approaches are

proposed to allow both the incidental and malicious manipulations. Fragile

watermarking does not allow any kind of manipulation and semi-fragile

watermarking techniques are designed that are robust against friendly

manipulations but fragile against malicious manipulations. On the other hand,

an intentional manipulation is considered as an attack and large numbers of

watermarking methods are described to deal with different kind of attacks. A

robust watermark should survive a wide variety of attacks both incidental and

malicious attacks (Nikolaidis et al., 2001; Hartung and Kutter, 1999). It is very

difficult to resist the malicious attacks but the watermarking techniques must

have the potential to deal with the requirements of robustness (Khan, 2006;

Barni and Bartolini, 2004).

Attacks can be classified in four categories, namely, simple attacks,

filtering attacks, detection-disabled attacks, ambiguity attacks and removal

attacks.

Simple attacks otherwise called as waveform attacks or noise attacks are

conceptually simple attacks that attempt to impair the embedded watermark by

manipulations of the whole watermarked data (host data plus watermark)

without an attempt to identify and isolate the watermark. Examples include

filtering, compression (JPEG, MPEG), addition of noise, addition of an offset,

cropping, geometrical operations, Digital to analog and analog to digital

conversion.

Image is generally stored in lossy compressed format. These

compressions separate important and unimportant parts of data and discard the

unimportant parts. This distortion may cause damage to watermark data too.

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Therefore, a simple attack is compressing multimedia data in a lossy way and

destroying watermark. In case of image multimedia, a rotation or scaling can

change pixel values and damage watermark, while preserving the visual

content of the image. Signal processing operations such as quantization,

decompression, re-sampling, color reduction, swapping some pixels and so on

can damage the watermark data. Adding noise can also affect the inserted

watermark data.

Many techniques are developed to deal with filtering,

collage/counterfeiting, removal, copy/paste (Cox et al., 2008). An attacker can

use filtering technique to remove watermark i.e. if the watermark is embedded

in the high frequencies of the image and low pass filter is applied, then the

watermark will be filtered (destroyed), e.g. Weiner filtering is an optimal linear

filtering (Su and Girod, 1999). Holliman and Memon (2000) develop an attack

called collage/counterfeiting attack which is undetectable by the traditional

watermarking algorithms. The techniques proposed by Chamlawi et al. (2010)

and Liu and Steinebach (2006), make it possible to detect collage attack by

applying the watermark correlation with the original cover work.

Detection-disabling attacks: (other possible names include

“synchronization attacks”) are attacks that attempt to break the correlation and

to make the recovery of the watermark impossible or infeasible for a watermark

detector, mostly by geometric distortion like zooming, shift in (for video)

direction, rotation, cropping, pixel permutations, subsampling, removal or

insertion of pixels or pixel clusters, or any other geometric transformation of

the data.

Ambiguity attacks, otherwise called as deadlock attacks, inversion

attacks, fake watermark attacks and fake-original attacks, are attacks that

attempt to confuse by producing fake original data or fake watermarked data.

An example is an inversion attack that attempts to discredit the authority of the

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watermark by embedding one or several additional watermarks such that it is

unclear which was the first, authoritative watermark.

Removal attacks are attacks that attempt to analyze the watermarked

data, estimate the watermark or the host data, separate the watermarked data

into host data and watermark, and discard only the watermark. Examples are

collusion attacks, denoising, certain filter operations, or compression attacks

using synthetic modeling of the image (e.g., using texture models or 3-D

models). Also included in this group are attacks that are tailored to a specific

watermarking scheme. Apart from this cryptographic and protocol attacks also

exist. A cryptographic attack is a method for circumventing the security of a

watermarking system by finding a weakness in a code, cipher, protocol or key

management schemes. Protocol attack exploit a specific feature or

implementation bug of some protocol installed in order to consume excess

amounts of its resources. Sometimes the transitions between the grouped

mentioned above are sometimes fuzzy and some attacks may not clearly belong

to one group.

1.6. MULTIPLE WATERMARKING SCHEME

More recently, different watermarking techniques and strategies have

been proposed in order to solve a number of problems, ranging from the

detection of content manipulations, to information hiding, to document usage

tracing. In particular, the insertion of multiple watermarks provides the

possibility of directly detecting from the image, who is the creator and who has

access to the data. (Noore et al., 2007; Jain, 2000; Zebbiche et al. 2006; Hui et

al., 2008). Multiple watermarking is defined as a process that embeds more

than one watermark into the cover image using different sets of secret keys,

where each set of key is corresponding for embedding one watermark only.

Multiple watermarking combine the advantages of single watermarking

algorithms to create a sophisticated multiple watermarking scheme

(Lähetkangas, 2005), which is efficient in terms of robustness and security.

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Moreover multiple watermarks prevent the crosstalk between different

watermarks and to decode or detect a particular watermark. Only the

corresponding key set is needed at the decoder. Further each embedded

watermark bit sequence can be decoded independently.

Thus, the technology of multiple watermarking extends single

watermarking techniques for embedding more than one watermark into the

same image and simultaneously meet the requirements of operations such as

copyright protection, authentication and information hiding. As this paradigm

is used for multiple operations, it is often referred to as multipurpose

watermarking (Lu et al., 2001; 2005).

A general framework of multiple watermark embedding and extraction

procedure is shown in Figure 1.7.

Notwithstanding the application potential of such methodologies,

multiple watermarking is still an open problem. The general problem of

multiple watermarking has been the object of several investigations since the

pioneering contribution (Cox et al., 1997), where the possibility of recovering

different watermarks in the same image was first shown. Mintzer and

Braudaway (1999) suggested that the insertion of multiple watermarks can be

exploited to convey multiple sets of information, while Stankovic et al. (2001)

and Hsu and Wu (1999) discuss specific extensions of single watermarking

algorithms to the case of multiple watermarks, by introducing orthogonal

watermarks. A multiple watermark-embedding procedure (Wong et al., 2003)

that allows simultaneous insertions without requiring the key sets to be

orthogonal to each other has also been probed. Specific applications such as

medical image management (Woo et al., 2005; Giakoumaki et al., 2003) may

even require the insertion of two different types of watermark, namely, a robust

one for authentication purposes, and a fragile one for data integrity control.

However, this technique is also becoming a widely sought after techniques by

commercial and e-publication sectors.

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Figure 1.7 : Multiple Watermarking System

The existing multiple watermarking methods can be improved

(i) by enhancing the underlying single watermarking techniques,

which help to improve the multiple watermarking scheme

(ii) in terms of capacity, validity and robustness.

Apart from the above requirements, it is important to find a balance among the

aspects such as robustness to various attacks, security and invisibility while

designing a multiple watermarking system.

1.6.1. Paradigms of Multiple Watermarking Algorithms

The first categorization is based on the usage of multiple watermarks.

(i) Multiple Purpose : This can be used for different applications, like

copyright protection, integrity verification and annotation watermark.

Copyright Watermark Generator

Authentication Watermark Generator

Secret Message Watermark Generator

Cover ImageWatermark Data

Watermark Data

Watermark Data

Watermark embedder

Communication channel

Watermark Extractor

Copyright data

Attacks

Copyright Secret Key

Authentication Secret Key

Secret Message Secret Key

Watermark Extractor

Watermark Extractor

Authentication Data

Secret Data

Watermark Extractor

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(ii) Single Purpose : The second scenario is to embed multiple watermarks

for a single purpose. For example, this can be one watermark for the

copyright holder and one for the consumer or in the case where there are

multiple owners.

(iii) Distribution of chain tracking : Here a watermark identifying the

receiver of the media is embedded and if the media gets sold/transmitted

multiple times, a watermark is embedded for every receiver and also for

the original owner of the media (Mintzer and Braudaway, 1999).

Secondly, the existing multiple watermarking algorithms can be divided

into three classes, namely,

(i) Rewatermarking

(ii) Segmented watermarking and

(iii) Composite watermarking.

Re-watermarking is a very straight forward approach. The watermarks

are simply embedded one after the other. The problem which arises is that the

watermarks interfere with each other. As a consequence earlier embedded

watermarks possibly get erased by later embedded ones. The advantage is that

no central party for embedding is necessary and the embedders do not need to

know each other and also the number of watermarks to embed do not need to

be known in advance.

Composite watermarking builds a single composite watermark from a

collection of watermarks and then embeds the composite watermark in to the

cover image in a usual way. A good signal merging methods is required for

increased performance. This approach has the need for a trusted party which

does the composition and embedding of the single watermarks and all

watermarks have to be present at once.

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Segmented watermarking divides the cover image into several partitions

and allocate each partition for a different watermark. Here, the number of

divisions limits the number of watermark signals to be embedded. Besides,

when the number of watermarks increases the size of each block decreases.

Furthermore the location of the embedding partitions has to be opened to the

embedding algorithm and each embedding algorithm has to know which

partitions are already occupied. As an alternative a trusted embedder who

records all occupied blocks can be used during the embedding process. This

type is one of the most commonly used algorithms.

1.7. VISUAL CRYPTOGRAPHY

Visual Cryptography (VC) is a Secret Sharing Scheme extended for

images. It has the ability to restore the secret data without the use of

computations. It is a paradigm introduced by Naor and Shamir (1994), was

initially used to encrypt material like written text, printed text, pictures, in a

secure manner. It is a Visual Secret Sharing Scheme (VSSS) which uses the

Human Visual System (HVS) to decrypt a secret message without expensive

and complicated decoding process (Tai and Chang, 2005). The basic VC

system starts with the encoding phase, where a secret image is divided into a

collection of ‘m’ black and ‘n’ white pixels. Each collection of m x n pixels is

referred to as a share, which will resemble a noisy image when separated.

During decoding phase, these shares or subset of shares are stacked together

which will allow the visual recovery of the secret message. A simple VC

example is given in Figure 1.8.

Visual Cryptograpy has been applied in many applications, including

information hiding, general access structures (Ateniese et al., 1996), visual

authentication and identification (Naor and Pinkas, 1997). The solutions

normally operate on binary or binarized inputs.

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Secret Image

Share 1

Stacking the share reveals the secret image

Share 2

Figure 1.8 : Visual Cryptography – An Example

Visual cryptographic solutions normally operate on binary or binarized

inputs. After its initial introduction, many researchers have found different

variations of VC (Yang, 2010). The improvement varies from binary image to

gray scale and colour images. In halftone VC, the natural (continuous-tone)

images are first converted into halftone images by using the density of the net

dots to simulate the original gray or color levels in the target binary

representation. Then, the halftone version of the input image is used instead of

the original secret image to produce the shares. A half tone image is the binary

version of the gray scale image. The halftoning technique is used in many

applications such as facsimile (FAX), electronic scanning and copying, and

laser printing etc. The decrypted image is obtained by stacking the shares

together.

Visual Cryptographic systems are being used in various applications

such as E-voting system, financial documents and secure image transmission.

In recent years, it is also used with another important technique

‘Watermarking’. VC used in conjunction with watermarking, allows multiple

watermarks to be embedded in the same image without modifying the host

image (Luo et al., 2008; 2009). In addition, it has the advantage that the

watermarks can be extracted without using the original image. Thus, they are

very suitable for many applications including medical images and financial

document images.

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1.8. MOTIVATION

The past few decades, owing to the increased popularity of multimedia

applications along with World Wide Web, have envisaged tremendous increase

in the usage of multimedia content, especially, digital images. Several

innovative techniques are used both by professionals and common population

to create digital images. With the ever changing advancements in imaging

software like Adobe PhotoShop, it is now easy to manipulate and edit images

without noticeable traces. A simple example is shown in Figure 1.9.

Figure 1.9 : Image Manipulation Example (Before and After)(Source : http://photo-retouching.jaincotech.com/gallery_manipulation.htm)

In the above figure, the lady (left picture) is removed entirely without a

trace and the after manipulated photo has the picture of only two friends (right

picture). Easy reproduction, retransmission and manipulation techniques allows

a pirate (a person or organization) to violate the copyright of real owner. Thus,

it becomes imperative to have some protection techniques to make an image

data trustworthy for use and also to protect the image content against illegal

tampering and manipulation.

Apart from this there is a growing population who are using digital

images to hide or embed details such as owner information, date, time, camera

settings, event/occasion of the image, image title, secret information for value

added functionalities and secret communication. Techniques like cryptography

and encryption have been used heavily for this purpose in the past. These

techniques disguise the data inside an image and transform the image by

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making it unreadable. The original image can be obtained only by the use of

correct secret key. The drawback of this data protection strategy is that once

such a data is decrypted by a pirate, there is no way to protect the data and

track the illegal distribution. Also it is impossible to prove the ownership

legally.

Watermarking technique, on the other hand, allows a person to view an

image while hiding the information stored inside and can also be used to prove

ownership of digital information, thus enhancing the act of security.

Application areas like medical, businesses and military demand high content

protection and demand advanced protection schemes that make their

intellectual property difficult to steal. In order to address the differing

requirements of these various applications, a variety of methods have been

proposed for watermark embedding.

There have been many watermarking schemes proposed (El-Hadedy et

al., 2011; Yu et al., 2012) each aiming to develop robust watermarks that

protects digital contents during transmission. However, the continuing

revolution in the communication medium is demanding and thus, it has become

imperative to improve watermarking techniques that can satisfy the property of

CAR (confidentiality, availability and reliability) along with maximum

transparency, capacity and robustness. Search for techniques to answer the

above questions is the focus of the present research work. The main motivation

is to find a technique that can simultaneously protect, preserve security without

destroying or modifying the content of the digital image. For this purpose, the

present research work, proposes the use of multiple watermarking and visual

cryptography.

However practical applications require the use of multiple watermarks

for different purposes embedded with different techniques into the same cover

image. Existing multiple watermarking schemes has disadvantages like

(i) introduction of perceptual distortions (ii) high time complexity and (iii) low

28

immunity to attacks. Thus, advanced schemes that can solve the above

problems are to be designed to achieve more comprehensive and sustained

privacy control and tamper detection. Further, presently the speed of the

embedding and extraction procedures is high and depends on each watermark.

An ideal multiple watermarking scheme should be able to embed multiple

watermarks quickly and in an efficient manner.

All the above challenges motivated this research work to focus on

providing solutions that can enhance the process of multiple watermarking for

copyright, authentication and information hiding. In essence the present study

is focused on answering questions concerned about ‘who is this? (Copyright)’,

‘how to know? (Authentication)’ and ‘what is it ? (Secret message hiding)

(Figure 1.10).

1.9. PROBLEM STATEMENT AND OBJECTIVES

The main goal of the present research work is to develop sophisticated

multiple watermarking schemes with visual cryptography for information

hiding, copyright protection and authentication with important characteristics

like robustness, high immunity against attacks and high reliability. To achieve

this goal, the work is divided into two stages.

Stage 1 : Propose and develop single watermarking algorithms for information

hiding, copyright protection and authentication and select a winning

algorithm for each area.

Stage 2 : Use the selected algorithm and develop multiple watermarking

schemes.

29

Figure 1.10 : Motivations Behind the Research Work

The objectives formulated for both the stages are given below.

To propose and identify three enhanced watermarking schemes using pixel-

based, feature-based and transformation-based techniques for the following

operations

o Information hiding

o Copyright protection

o Authentication

SOLUTION : IMAGE WATERMARKING

Hacker

Receiver

Intellectual Property

Owner

‘Who is this? ‘ –Copyright ‘How to know? – Authentication ‘What is it ? – Secret message hiding

Researcher

RESEARCH PROBLEM

30

To develop a visual cryptographic scheme that is compatible with the single

watermarking schemes.

To propose hybrid multiple watermarking techniques for

o Information hiding and Copyright protection

o Copyright protection and Authentication

o Authentication and Information Hiding

To conduct performance evaluation of the proposed models in two stages.

o Stage 1 : to select an efficient single watermarking algorithm that

best suits information hiding, copyright protection and

authentication.

o Stage 2 : to identify the advantages and disadvantages of the three

proposed multiple watermarking schemes.

To achieve the above set objectives, the watermarking models are

built based on the following problem statement.

“Let I be the original (cover) image. The research problem is to

develop embedding function that inserts multiple watermarks

{W1, W2 and W3) representing the copyright, authentication and

information into I to obtain the watermarked image I . That is, I =

(I, W1, W2, W3). Further, the developed scheme should be robust

and reduce distortion and should be able to survive intended and

unintended attacks.”

1.10. ORGANIZATION OF THE CHAPTERS

The underlying objective of this research work is to develop multiple

watermarking algorithms digital images. This chapter introduced the concepts

behind the research topic and formulated the research problem and goals. The

rest of the dissertation is organized as follows.

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The literature review is a critical look at the existing research that is

significant to the work that is carried out. In case of watermarking, several

researchers have addressed the problem of digit recognition. A critical look at

the various available literatures related to the present research work is given in

Chapter 2, Review of Literature.

The main component of the selected algorithm is Visual Cryptography.

The underlying concept behind Visual cryptography, the proposed single and

multiple watermarking schemes are described along with the research design in

Chapter 3, Methodology.

To analyze the performance of the proposed single watermarking

systems and proposed multiple watermarking systems, several experiments

were conducted using different cover and watermark images. The experimental

results obtained are tabulated and discussed in Chapter 4, Results and

Discussion.

The research study is concluded along with future research directions in

Chapter 5, Summary and Conclusion.

The work of several researchers are quoted and used as evidence to

support the concepts explained in this dissertation. All such evidences used are

listed in the reference section of the dissertation.

1.11. CHAPTER SUMMARY

Digital watermarking techniques are already effectively used in

associated copy control applications and broadcast monitoring systems. In

combination with digital rights management frameworks, the techniques can

solve the limitation of the intellectual property dilemma in image-related

business areas. However, watermarking techniques behave differently in

different attack operations or applications. A desirable watermarking algorithm

should not rely on a certain method. It should be designed as a single model

algorithm that inserts watermark in different ways for different applications.

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The watermarks should survive attacks and present a unified method. It is still

a wide and attractive field for further research in which innovative methods and

techniques needs to be established. This research work studies the applicability

of multiple watermarking and visual cryptography techniques to improve the

existing single watermarking schemes. To develop such a model, a detailed

review study of the previous research works was conducted and the scrutinized

works are summarized in the next chapter, Review of Literature.