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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.
3
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.
4
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
5
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
6
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,
7
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.
8
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
9
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.
10
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.
11
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
12
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
13
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).
14
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
15
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
16
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.
17
Landscape figure - (Intro landscape figure .doc)
18
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.
19
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
20
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.
21
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.
22
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.