International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
EXTENDING THE VISUAL CRYPTOGRAPHY ALGORITHM TO
PROVIDE SECURITY USING PIXEL SIEVE METHOD
Karuna Putta 1 , Dilip kumar Kotthapalli 2
1
M.Tech student, Department of Computer Science and Engineering,
Anna University, Chennai – 600 113 , T. N., INDIA
2
M.Tech student, Department of Electronics and Communication Engineering,
Sree Vidyanikethan Engineering College, TIRUPATI – 517 102, A. P., INDIA
Abstract: Visual cryptography is a simple and powerful
method which can provide high security for confidential
information. This technique generates noise-like random
pixels on share images to hide secret information. In this
paper a modified version of pixel sieve method is
proposed to achieve more security than existing method.
Based on key shifting scheme, the proposed method
generates quite noisy and highly secure encrypted
images. The simulation shows that the quality of the
encrypted images observably better than existing method.
Keywords - Extended visual cryptography (EVC), pixel
expansion, pixel sieve method.
I. INTRODUCTION
We need very efficient security systems for preventing
confidential information from being accessed by
unauthorized persons. As computing power becoming
more and more faster our older cryptographic systems
becoming less secure because an attacker can attempt
large number of random attack attempts in shorter time.
Visual cryptography [1][2] is a simple and
powerful method which can provide high security for
confidential information. Concept of visual cryptography
is introduced by Moni Naor and Adi Shamir in 1994. The
idea is to split a message into n different pieces such that
the original message is visible if any k (or more) of them
are used together, but totally invisible if fewer than k
pieces are used for getting the message.
In this method each message is considered as an
image of black and white pixels. This image is divided
into n slides called transparency. Each pixel of the
message appears in each transparency in a different
modified version. For getting the original information
from transparencies, all of them are stacked together with
proper alignment. The simplest example of visual
cryptography is a scheme in which we split the image
into two different shares.
ISSN: 2231-5381
The decryption of the image will be done by
overlapping the shares. When we place both the shares
one over another with proper alignment, we can interpret
the original image. Here occurs some management
problems which not only affects the practicability of
storage/transmission requirements for shares but also
decreases the contrast of the recovered secret images. To
the best of our knowledge, the existing Extended Visual
Cryptography Schemes (EVCS) algorithms for GASs
cannot avoid the pixel expansion problem. Therefore, we
are motivated to find a solution to this problem.
The encryption process can be divided into two
phases. The first phase of the algorithm, which uses
optimization techniques for a given access structure,
constructs a set of noise-like shares that are pixelexpansion free. The second phase of the algorithm
directly adds a cover image on each share via a stamping
algorithm. This solves the pixel expansion problem VC
implies a way to properly register the SI’s which is
always arduous when performed by hand, once the two
SI’s have been printed on transparencies, or one on a
transparency and the other on paper or some opaque
substrate. This problem, called the alignment problem in
literature, has so far hampered the deployment of the VC
technique.
Recently, various studies about visual
cryptography are proposed. A.Incze has proposed a
method for splitting the image into two different shares.
He proposed pixel sieve method which uses a key to split
the image. It is used to split a black and white image. The
image is rebuilt from the shares not by overlapping, but
by applying a cryptographic process using a key. The key
used in this method is a binary image which contains
holes like a sieve[3].
Here we are using pixel sieve method, where
key is used to split original image thus it forms shares.
Key provides relatively better security than existing
methods. Later we add cover images to overcome pixel
expansion problems and to reduce shares synchronization
duration.
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
III. MODULES
1.
2.
3.
4.
GRAYSCALE CONVERSION
IMAGE ENCRYPTION
ADDING COVERIMAGES
IMAGE DECRYPTION
1. Grayscale Conversion
Fig 1: An example of pixel sieve method
The method works as follows: we take the main image
and then sieve pixel by pixel. If the value of the pixel in
sieve is black then pixel from the main image goes to the
share1 otherwise pixel goes to share2. Pixel sieve
method is a powerful visual cryptographic algorithm. It
provides better security than older cryptographic
methods, but it has some limitations which can be solved
by proposed method.
II. SYSTEM ASSUMPTIONS
Conventional VSS schemes generate noise-like random
pixels on shares to hide secret images. In this manner, the
secret can be perfectly concealed on the share images.
However, these schemes suffer from a management
problem dealers cannot identify each share visually.
Hence, researchers have developed the extended visual
cryptography scheme, which adds a cover image to share
images.
Key sieve shifting:
In this method we iterate the sieve and cross merge
method several times with different shifted keys on the
original image. We shift the key in each round of
encryption process. In the decryption process the keys are
used in reverse order of encryption process. Shifting of the
encryption key is an important part of various
cryptographic algorithms. In this method the key sieve used
for pixel sieving is shifted in each round. We propose a key
shifting method with two steps.


In 1st step we circularly left shift each row of
the key sieve independently.
In 2nd step we circularly up shift each column
of the key independently.
ISSN: 2231-5381
In photography and computing, a grayscale digital image
is an image in which the value of each pixel is a single
sample, that is, it carries only intensity information.
Images of this sort, also known as black-and-white, are
composed exclusively of shades of gray, varying from
black at the weakest intensity to white at the strongest.
Conversion of a color image to grayscale is not unique; a
common strategy is to match the luminance of the
grayscale image to the luminance of the color image. In
fact a gray color is one in which the red, green and blue
components all have equal intensity in RGB space. The
grayscale intensity is stored as an 8-bit integer giving 256
possible different shades of gray. which converts the
given original image to a 256 bits gray-level bitmap
image.
2. Image Encryption
In cryptography, encryption is the process of encoding
messages in such a way that hackers cannot read it, but
that authorized parties can view that message. In an
encryption scheme, the message or information is
encrypted using an encryption algorithm, turning it to an
unreadable cipher-text. This image is divided into n
slides called transparency. Each pixel of the message
appears in each transparency in a different modified
version. For getting the original information all of them
are stacked together with proper alignment.
Shifting of the encryption key is an important
part of various cryptographic algorithms. In this method
the key sieve used for pixel sieving is shifted in each
round. In this method we iterate the sieve and cross
merge method several times with different shifted keys
on the original image. We shift the key in each round of
encryption process.
3. Adding cover images
Conventional VSS schemes generate noise-like random
pixels on shares to hide secret images. However, these
schemes suffer from a management problem dealers
cannot identify each share visually. Hence, researchers
have developed the extended visual cryptography
scheme, which adds a cover image to share images. By
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
adding cover images it reduces the pixel expansion
problems and also reduces shares synchronization time.
4. Image Decryption
The decryption of the image will be done by overlapping
the shares. When we place both the shares one over
another with proper alignment, we can interpret the
original image. In the decryption process the keys are
used in reverse order of encryption process. Then we can
overlap the two shares.
IV. IMPLEMENTATION ISSUES
Visual cryptography [1][2] is a simple and powerful
method which can provide high security for confidential
information. Concept of visual cryptography is
introduced by Moni Naor and Adi Shamir in 1994. The
idea is to split a message into n different pieces such that
the original message is visible if any k (or more) of them
are used together, but totally invisible if fewer than k
pieces are used for getting the message. In this method
each message is considered as an image of black and
white pixels.
The simplest example of visual cryptography is
a scheme in which we split the image into two different
shares. The decryption of the image will be done by
overlapping the shares. When we place both the shares
one over another with proper alignment, we can interpret
the original image successfully.
Recently, various studies about visual
cryptography are proposed. A.Incze has proposed a
method for splitting the image into two different shares.
He proposed pixel sieve method which uses a key to split
the image. It is used to split a black and white image. The
image is rebuilt from the shares not by overlapping, but
by applying a cryptographic process using a key. We use
pixel sieve method to provide better security when
compared with previous methods.
Fig 2: Sieve and cross merge process
In this method we iterate the sieve and cross merge
method several times with different shifted keys on the
original image. We shift the key in each round of
encryption process. In the decryption process the keys are
used in reverse order of encryption process, then we
overlap the two shares to get original image.
V. EXPERIMENTAL RESULTS
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. Visual Cryptography 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.
When the random image contains truely random
pixels it can be seen as a one-time pad system and will
offer unbreakable encryption. The two layers sliding over
each other until they are correctly aligned and the hidden
information appears. Before overlaying the shares we are
using pixel sieve method where we iterate the sieve and
cross merge method several times with different shifted
keys on the original image at encryption. Later in the
decryption process the keys are used in reverse order of
encryption process. Then we can overlap the two shares
to get original image.
VI. CONCLUSION
The major contributions of our work is first solution that
addresses the pixel expansion problem of the EVCS for
GAS. Key sieve shifting method enhances the security of
the pixel sieve method. This method provides security
against nearly equal keys used for decryption. Another
advantage of this method is that it also increases the
randomness in the decrypted image.
ISSN: 2231-5381
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REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
M. Naor and A. Shamir, “Visual cryptography,”
in
Proc.
Advances
in
Cryptology
(Eurprocrypt’94), 1994, pp. 1–12.
E. R. Verheul and H. C. A. v. Tilborg,
“Constructions and properties of k-out-of-n
visual secret sharing schemes,” Designs Codes
Crypto., vol. 11, pp. 179–196, 1997.
A.Incze, “Pixel Sieve method for secret sharing
& visual cryptography”. 9th RoEduNet IEEE
International Conference,2010
H. Koga, “A general formula of the (t,n)threshold visual secret sharing scheme,” in Proc.
Advances in Cryptology (Asiacrypt), 2002, pp.
328–345.
C. Blundo, S. Cimato, and A. D. Santis, “Visual
cryptography schemes with optimal pixel
expansion,” Theor. Comput. Sci., vol. 369,
pp.169–182, 2006.
D. Wang, L. Zhang, N. Ma, and X. Li, “Two
secret sharing schemes based on boolean
operations,” Pattern Recognit., vol. 40, pp.
2776–2785, 2007.
C. S. Hsu, S. F. Tu, and Y. C. Hou, “An
optimization model for visual cryptography
schemes with unexpanded shares,” Found.
Intelligent Syst., LNAI, vol. 4203, pp. 58–67,
2006.
Gamil
R.S.
Qaid
and
Sanjay
N.
Talbar,’’Encryption and Decryption of Digital
Image Using Color signal” IJCSI International
Journal of Computer Science Issues, Vol. 9,
Issue 2, No 2, March 2012
Vaibhav Choudhary,” Modified Pixel Sieve
Method for Visual Cryptography”, Indian
Journal of Computer Science and Engineering
Vol. 1 No. 4 321-326
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