A study of image fingerprinting by using visual cryptography

Computer Engineering and Intelligent Systems www.iiste.org
ISSN 2222-1719 (Paper) ISSN 2222-2863 (Online)
Vol 3, No.7, 2012

A Study of Image Fingerprinting by Using Visual Cryptography
Santosh Choudhary, Shrikrishan Yadav*, Neeta Sen, Gosiya Nasreen
Pacific College of Engineering, Pacific University, Udaipur-313001, Rajasthan, India
* E-mail of the corresponding author
Abstract
As digital media has made our life more colourful because of its advantages like easier to access, copy and distribute.
But as what we have seen, series of malice activities like copyright infringement, counterfeiting, piracy and
information distortion make damages to both the producers and the users of digital products. So we really need some
technology to protect the copyright, authenticity, integrity of the digital products and the intellectual property of the
users. There are many techniques such as Digital watermarking and Visual Cryptography both have been widely used
for protection of data either in text, video, sound or digital images form in modern network time. Digital
watermarking is an evolving field that requires continuous effort to find for the best possible method in protecting
multimedia content. But Visual Cryptography is a special encryption technique to hide information in images in such
a way that it can be decrypted by the human vision if the correct key image is used. Both techniques addresses the
growing concerns of theft and tampering of digital media through the use of advanced signal processing strategies to
embed copyright and authentication information within media content in their respective manner. In this paper we
have discussed image fingerprinting with the help of visual cryptography technique.
Keywords: Authentication, Digital Media, Digital Watermarking, Fingerprinting, Image, Piracy, Visual
Cryptography

1. Introduction
With the growth of the Internet, more and more information is being transmitted in digital format (image, audio,
video, etc.) now than ever before. However, the greatest drawback in transmission of digital information is its easy
weakness to have innumerable copies of the same nature and quality as that of the original. So, there is always the
chance of lack of authentication, ownership proof and copyright protection. Therefore, various cryptography
algorithms and embedding techniques have been established to solve this type’s problems that stress on copyright
marking. Some message is secretly inserted within the original digital message and that secret message is used to
emphasize copyright over the host digital message. But all such algorithms must satisfy a number of requirements to
maintain the quality and integrity of the resultant information. The watermarking and visual cryptography both
techniques are used to secure or protect or hide data from unauthorized users.
2. Digital Watermarking
Watermarking is the process of embedding information into a digital signal which may be used to verify its
authenticity or the identity of its owners, in the same manner as paper bearing a watermark for visible
identification. In digital watermarking, the signal may be audio, pictures, or video, in which a pattern of bits
inserted into a digital image, audio or video file that identifies the file's copyright information (author, rights,
etc.) If the signal is copied, then the information also is carried in the copy. A signal may carry several
different watermarks at the same time. Digital watermarking is also called data embedding and information
hiding. Digital

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Figure 1. Block Diagram of Watermarking Technique

Watermarking technology is an emerging field in computer science, cryptography, signal processing and
communications. According to Human Perception there are two types of watermarking, visible and invisible.
In visible digital watermarking, the information is visible in the picture or video. Typically, the information is a text
or a logo, which identifies the owner of the media. The image on the right has a visible watermark. For example
when a television broadcaster adds its logo to the corner of transmitted video, this also is a visible watermark. The
logo of the broadcaster such as Zee, Sony, ESPN and National Geographic etc is mainly on the right top corner of the
television, which is visible to every user.

In invisible digital watermarking, information is added as digital data to audio, picture, or video, but it cannot be
professed as such. The watermark may be projected for widespread use and thus, is made easy to retrieve or, it may
be a form of steganography, where a party communicates a secret message embedded in the digital signal. In either
case, as in visible watermarking, the objective is to attach ownership or other descriptive information to the signal in
a way that is difficult to remove. It also is possible to use hidden embedded information as a means of covert
communication between individuals.
One application of watermarking is in copyright protection systems, which are intended to prevent or frighten
unauthorized copying of digital media. In this use, a copy device retrieves the watermark from the signal before
making a copy; the device makes a decision whether to copy or not, depending on the contents of the watermark.
Another application is in source tracing. A watermark is embedded into a digital signal at each point of distribution.
If a copy of the work is found later, then the watermark may be retrieved from the copy and the source of the
distribution is known. This technique reportedly has been used to detect the source of illegally copied movies.
Annotation of digital photographs with descriptive information is another application of invisible watermarking.
While some file formats for digital media may contain additional information called metadata, digital watermarking
is distinctive in that the data is carried right in the signal.

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2.1Elements of a Watermarking System
A watermarking system can be viewed as a communication system consisting of three main components: an
embedder, a communication channel and a detector. Watermark information is embedded into the signal itself,
instead of being placed in the header of a file or using encryption like in other security techniques, in such a
way that it is extractable by the detector. To be more specific, the watermark information is embedded within
the host signal before the watermarked signal is transmitted over the communication channel, so that the
watermark can be detected at the receiving end, that is, at the detector. A general digital watermark life-cycle has
different phases which are embedding, attacking, detection and retrieval functions. The information to be embedded
in a signal is called a digital watermark, although in some contexts the phrase digital watermark means the difference
between the watermarked signal and the cover signal. The signal where the watermark is to be embedded is called
the host signal. A watermarking system shown in fig. 2 is usually divided into three distinct steps, embedding, attack,
and detection. In embedding, an algorithm accepts the host and the data to be embedded, and produces a
watermarked signal.
Then the watermarked digital signal is transmitted or stored, usually transmitted to another person. If this person
makes a modification, this is called an attack. While the modification may not be malicious, the term attack arises
from copyright protection application, where pirates attempt to remove the digital watermark through modification.
There are many possible modifications, for example, Lossy compression of the data, cropping an image or video or
intentionally adding noise. Detection often called extraction is an algorithm which is applied to the attacked signal to
attempt to extract the watermark from it. If the signal was unmodified during transmission, then the watermark still is
present and it may be extracted. In robust digital watermarking applications, the extraction algorithm should be able
to produce the watermark correctly, even if the modifications were strong. In fragile digital watermarking, the
extraction algorithm should fail if any change is made to the signal. Thus the robust digital watermarking is better as
compare to fragile because the watermark can be extract even if the information is modified strongly by unauthorized
users.
Watermarking is always regarded as a subset of steganography. However, the message hidden in watermarking is not
the major information that we want to convey. On the converse, the information hidden in steganography is always
the most important. Because of invisibility, what is hidden in both watermarking and steganography is the existence
of the signal. However, for the cryptography, what is hidden is the content of information.

Figure 2. General Watermarking Systems

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3. Visual Cryptography
Visual Cryptography is a secret sharing scheme that uses the human visual system to perform computations. It is a
new type of cryptography technique in which no cryptographic computations is needed as the decryption end. The
secret sharing was invented independently by Adi Shamir.
The cryptography was generalised to visual cryptography by Mony Naor and Adi Shamir in 1994. When the random
image contains truly random pixels it can be seen as a one-time pad system and will offer unbreakable encryption. In
the overlay animation you can observe the two layers sliding over each other until they are correctly aligned and the
hidden information appears. To try this, you can copy the example layers 1 and 2 given in fig. 3, and print them onto
a transparent sheet or thin paper.
Always use a program that displays the black and white pixels correctly and set the printer so that all pixels are
printed accurate. You can also copy and paste them on each other in a drawing program like paint and see the result
immediately, but make sure to select transparent drawing and align both layers exactly over each other. Therefore,
the protection of rightful ownership of digital data has become an important issue in recent years. Nowadays, many
researchers are working and have proposed many techniques to protect the intellectual property rights for digital
images or information. So both methods are used to hide a meaningful data, in a host image for the purpose of
copyright protection, integrity checking, and captioning.
It is a special encryption technique to hide information in images in such a way that it can be decrypted by the
human vision if the correct key image is used. Visual Cryptography uses the basic technique to hide
information in which it uses two transparent images. One image contains random pixels and the other image
contains the secret information. It is impossible to retrieve the secret information from one of the images. Both
transparent images and layers are required to reveal the information.

3.1 Implementation of Visual Cryptography when Information is in Image Form
It is easy to implement visual cryptography to secure data when it is in image form. In the visual cryptography, if the
information is in images form than it works like this. Each pixel of the images is divided into smaller blocks. There
are always the same number white (transparent) and black blocks. If a pixel is divided into two parts, there are one
white and one black block. If the pixel is divided into four equal parts, there are two white and two black blocks. The
example images from above uses pixels that are divided into four parts.
In the fig. 3 we can see that a pixel, divided into four parts, can have six different states. If a pixel on layer 1 has a
given state, the pixel on layer 2 may have one of two states: identical or inverted to the pixel of layer 1. If the pixel of
layer 2 is identical to layer 1, the overlaid pixel will be half black and half white. Such overlaid pixel is called grey or
empty. If the pixels of layer 1 and 2 are inverted or opposite, the overlaid version will be completely black. This is an
information pixel.

We can now create the two layers. One transparent image, layer 1, has pixels which all have a random state, one of
the six possible states. Layer 2 is identical to layer 1, except for the pixels that should be black (contain information)
when overlaid. These pixels have a state that is opposite to the same pixel in layer 1. If both images are overlaid, the
areas with identical states will look gray, and the areas with opposite states will be black. The system of pixel can
be applied in different ways. In our example, each pixel is divided into four blocks. However, you can also use pixels,
divided into two rectangle blocks, or even divided circles. Also, it doesn't matter if the pixel is divided horizontally
or vertically. There are many different pixel systems, some with better contrast, higher resolution or even with colour
pixels. The pixels size or colour and resolution can be used according the requirements of the condition or problem.
In fig.3 the outcomes 12 are given but the overlaid or combination of empty pixel and information pixel can be done
in many ways. If the pixel states of layer 1 are truly (crypto secure) random, both empty and information pixels of
layer 2 will also have completely random states. One cannot know if a pixel in layer 2 is used to create a grey or
black pixel, since we need the state of that pixel in layer 1 (which is random) to know the overlay result. If all

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requirements for true randomness are fulfilled, Visual Cryptography offers absolute secrecy according to the
Information Theory.

If Visual Cryptography is used for secure communications, the sender will distribute one or more random layers 1 in
advance to the receiver. If the sender has a message, he creates a layer 2 for a particular distributed layer 1 and sends
it to the receiver. The receiver aligns the two layers and the secret information is revealed, this without the need for
an encryption device, a computer or performing calculations by hand. The system is unbreakable, as long as both
layers don't fall in the wrong hands. When one of both layers is intercepted it's impossible to retrieve the encrypted
information.

Figure 3. Two Image Layers Overlapping to Secure Data
3.2 Implementation of Visual Cryptography by using Fingerprinting
Fingerprinting of an image is a method by which the information related to the image and its owner can be kept

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hidden in the image (cover image) itself. A unique image identification number, customer id and name of the image
will be used as the fingerprint of each image buy-sell transaction. This unique fingerprint will be embedded into that
particular image. The fingerprint will be generated using a text to image conversion algorithm by which secret data
will be prepared. The novelty of the algorithm lies in the fingerprinting method. It is visually encrypted to enhance
the security. The fingerprint is divided into some shares of equal size which alone does not contain any significant
information about the fingerprint unless they are visually overlapped one above the other. The complete information
in the fingerprint is divided equally among all the shares.

It is visually encrypted to enhance the security. The fingerprint is divided into some shares of equal size which alone
does not contain any significant information about the fingerprint unless they are visually overlapped one above the
other. The complete information in the fingerprint is divided equally among all the shares. To serve the fingerprinting
purpose the shares are embedded into the image in different blocks in the frequency domain. A single cover image
will contain all the shares of the fingerprint. So whole information about the fingerprint is inside the cover image but
embedded into different spatial locations. This involves a new strategy of embedding multiple watermarks in the
same image. Each share can be seen as an independent watermark and will be embedded into a different block of the
image.

The method proposed here is supposed to be very robust. The fingerprint is secure against the general image
processing attacks like noise addition and image compression. The main advantage of using a visually encrypted
fingerprint is that there is no effect of the normal correlation of the fingerprint to the image. Blind detection of the
fingerprint to illegally detect it is not possible since the correlated fingerprint will appear as noise only. Even some
shares of the fingerprint get detected but without the successful detection of every share it is impossible to regenerate
the fingerprint. So no illegal person can detect the fingerprint without knowing the all the shares altogether.

Generation of fingerprint: The logos of organizations or some standard images have been used by the
watermarking fraternity worldwide. But these are not image and customer dependent, which is the basic need for a
fingerprint. So what we propose here to generate a unique fingerprint for every image. This fingerprint will contain
in information about the customer and the image itself in the form of text. This text will be converted into image
which will finally be used as a fingerprint.

Visual cryptography: This is a very popular strategy to encrypt the images. The image is divided into some number
of equally sized shares. Individual shares do not contain any information. They all appear as noise. The only way to
extract the original image information is to overlap over each other. We are going to visually encrypt our fingerprint
into 4 shares. These shares will be embedded into the image not the original fingerprint as it is done in all the
watermarking strategies.

Averaged Block based DCT Fingerprinting: This technique is far advanced and beneficial than the classical mid
band coefficients exchange scheme. Since the coefficients in which the watermarking information is entered are
unique in every image, so it is very difficult for the users to collude the fingerprint and its embedding location in the
image.
Multi-fingerprint Algorithm: The general watermarking techniques embed only one watermark into the image.
Some have watermarked the image using more than one watermark but in a sequential manner, i.e. next watermark is
embedded into the image in the coefficients returned by first watermarking steps. This is not effective way to do so,
since if one watermarking algorithm fails then the whole chain of watermarks is lost. Also the noise added to the
image is very high. But we propose the multi-fingerprint Algorithm in which different shares generated through
visual cryptography will be embedded into the image at different spatial location altogether.

3.2.1 Proposed Steps to Secure Information in Cover Image in Visual Cryptography with Fingerprint
Step1: Generate Watermarked Image by Using Visual Cryptographic Encryption
1. Start
2. Take a Cover Image

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3. Subdivision of Image into four parts LL, LR, UR, and UL (As shown in Fig. 3)
4. Apply DCT method using mid-band coefficient

Step 2: Generation of Fingerprint
1. Start
2. We take input data in form of text data for example consumer id, unique image no., image name etc
3. This secure information is embedded into cover image
4. Then we get watermarked secure image

Figure 4. Proposed Flow Chart for Visual Cryptography with Fingerprint

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4. Conclusion and Future Scope
The digital watermarking and visual cryptography methods are used to hide or protect information from
unauthorized users to maintain the privacy and originality. The field of digital watermarking is young and there
are some anticipated limitations, it has potential and some unique features which other alternatives lack. Also, it
will be fruitful if the connection between data hiding and cryptography is further investigated in future.
The information hiding in visual cryptography can be applied to many applications in real and cyber world. The
advantage is that the final decryption process is done by human visual system instead of complex computations, so it
is less complex to implement as compare digital watermarking. The techniques will have a significant effect on
defence, business, copyright protection and other fields where information needs to be protected at all costs from
attackers. Intelligent Watermarking Techniques will be of great value to undergraduate and postgraduate students in
many disciplines, including engineering and computer science. It is also targeted at researchers, scientists and
engineers.

References
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Mrs Santosh Choudhary: received her B.E. degree in Computer Science and Engineering from Modi Institute of
Technology, India and M. Tech in Computer Science & Engineering. She is pursuing Ph.D degree in “Image
processing” from Pacific University, Udaipur. She has currently associate professor & HOD in Pacific College of
Engineering, Udaipur, India and having more than 5 year teaching experiences. She guided many M. Tech scholars
and presented more than 5 papers in International and National conferences. Her current research interests include
Image Processing, Fuzzy Logic, Wireless Sensor Network and Data Structure.

Shrikrishan Yadav: working as an Assistant Professor in Computer Science and Engineering Department in PAHER
University, Udaipur, India. He has completed B. E. in Computer Science and Engineering from Mohanlal Shukhadia
University, Udaipur and pursued M.Tech. in Information Communication from Gyan Vihar University, Jaipur. He has
more than three years of teaching experience. He is also published and presented 16 papers in International and
National journals and conferences. He is an associate member of Computer Society of India (CSI), a member of
International Association of Engineers (IAENG), International Association of Engineers and Scientists (IAEST),
International Association of Computer Science and Information Technology (IACSIT) and Universal Association of
Computer and Electronics Engineers (UACEE). His current research interest includes Cognitive Radio, Wireless
Sensor Networks, Artificial Intelligence, Information Communication, and Wireless Communication System.

Neeta Sen: received her B.Sc degree in Computer Science from Mohanlal Shukhadia University (MLSU), Udaipur
India and master degree in computer application form MLSU, Udaipur. She is pursuing Ph.D degree in “Digital
Watermarking” from Pacific University, Udaipur. She is currently working as Senior Lecturer in Pacific College of
Engineering, Udaipur, India and having more than 3 year teaching experiences. She presented and published more
than 3 papers in International and National conferences. Her current research interests include Digital Watermarking,
Networking, Compiler, Computer Architecture, and Software Engineering.

Gosiya Nasreen: : received her B. Tech. degree in Computer Science and Engineering from Rajasthan Technical
University (RTU), Kota, India and pursuing master degree in computer science and engineering form Pacific
University, Udaipur. She is currently working as Lecturer in Pacific College of Engineering, Udaipur, India and
having more than 1 year teaching experiences. She presented and published more than 3 papers in International
conferences. Her current research interests include Web Services, Operating System, Embedded System, and
Software Engineering.

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