State-of-the-Art Visual Cryptography Schemes

Abstract

Visual Cryptography (VC) is recent technology used to the strengthen security of many applications in various fields. It allows visual information like printed text, handwritten notes, and images to be encrypted by dividing it into shares. The most important characteristic of VCS is that one can visually decrypted the secret image by stacking shares without computation. The current paper aims at introducing a descriptive review for VC, which covering the "state-of-the-art" concept, and classification of schemes. In this paper, we have classified the VC schemes and provide some interpretation on the base of some various measures such as pixel expansion, share generated, format of secret image and number of secret image, which actually deemed as valuable contribution in the field of VC studies.

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Copyright © 2014 IJECCE, All right reserved
412
International Journal of Electronics Communication and Computer Engineering
Volume 5, Issue 2, ISSN (Online): 2249–071X, ISSN (Print): 2278–4209
State-of-the-Art Visual Cryptography Schemes
Mahmoud E. Hodeish
School of Technology, SRTM University,
Sub-Center, Latur, Maharashtra, India
Email: mah_hodeish@yahoo.com
Dr. V. T. Humbe
School of Technology, SRTM University,
Sub-Center, Latur, Maharashtra, India
Email: VikasHumbe@gmail.com
Abstract –Visual Cryptography (VC) is recent technology
used to the strengthen security of many applications in
various fields. It allows visual information like printed text,
handwritten notes, and images to be encrypted by dividing it
into shares. The most important characteristic of VCS is that
one can visually decrypted the secret image by stacking
shares without computation. The current paper aims at
introducing a descriptive review for VC, which covering the
"state-of-the-art" concept, and classification of schemes. In
this paper, we have classified the VC schemes and provide
some interpretation on the base of some various measures
such as pixel expansion, share generated, format of secret
image and number of secret image, which actually deemed as
valuable contribution in the field of VC studies.
Keywords –Visual Cryptography, Secret Sharing Scheme,
Pixel Expansion, Halftone VC, Authentication.
I. INTRODUCTION
There are a several techniques related to aspects of the
Information Security such as confidentiality, data security,
entity authentication and data origin authentication, One of
these techniques Visual Cryptography, which is a new
technique which provides information security which uses
simple algorithm unlike the complex, computationally
intensive algorithms used in other techniques in traditional
cryptography. This technique allows Visual information
(pictures, text, etc.) to be encrypted in such a way that
their decryption can be performed by the human visual
system, without any complex cryptographic algorithms.
Visual cryptography was originally invented and
introduced in 1994 by Noar and Shamir [1].
Visualcryptography is a cryptographic technique which
allows visual information (e.g. printed text,handwritten
notes and pictures) to be encrypted in such a way that the
decryption can beperformed by the human visual system,
without the aid of computers. Visual cryptographyScheme
eliminates complex computation problem in decryption
process, and the secretimages can be restored by stacking
operation. This property makes visual
cryptographyespecially useful for the low computation
load requirement. In a t-out-of-nscheme of VC, a secret
binary image (SI) is cryptographically encoded into n
shares of random binary patterns. The nshares are
Xeroxed onto ntransparencies, respectively, and
distributed amongst nparticipants, one for each
participant. No participant knows the share given to
another participant. Any tor more participants can visually
reveal the secret image by superimposing any t
transparencies together. The secret cannot be decoded by
any t-1 or fewer participants, even if infinite
computational power is available to them. Being a type of
secret sharing scheme, visual cryptography can be used in
a number of applications including access control.
II. VISUAL CRYPTOGRAPHY
As a result of information provided by the internet and
the increase in digital media, the need of protecting such
information becomes necessary and significant. The
amount of information that is downloaded and uploaded
increaseson a daily basis, with data ranging from simple
text documents to photos of individuals to hyper-spectral
image cubes of the world. Internet provides an easy access
that demands knowledgeof the best way to protect the
visual information available on Internet from attacks,
replication, or unauthorized use. This provides a
perfectlysecured system where secret image is contained
in "shares". Individually, these shares resemblerandom
noise, but when they are stacked and aligned perfectly,
their message is decrypted usingonly the human visual
system. The figure (1) shows the general visual
cryptography, which demonstrates theimage, is encoded
into multiple shares and later decoded without any
computation. This decoding is as simple as superimposing
transparences, which allows the secret to be recovered.
While this method gives security for text and binary
images, thegrowth of digital media requires the expansion
of this technique to provide security for gray andcolor
images. Several methods have been developed for
securing gray and color images, includinghalftoning [2],
dithering [3], color subpixel groupings [4], and meaningful
image shares [5][6].
Fig.1. General Visual Cryptography
A. History and Growing of Visual Cryptography:
The field of Visual Cryptography has evolved over the
past two decades. They are focused on a process for
perfectly encrypting digital media that could be decoded
using solely the human visual system. This idea would
allow written material to be digitally transmitted without
concern that the message could be intercepted and
accidentally revealed to unauthorized parties.The primary
description associated with Visual Cryptography is the
message being encoded into two shares. When looked at
individually, these shares reveal no information about the
message contained in them and resemble random noise.

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However, when these shares are printed on transparencies,
overlaid, and perfectly aligned, the message contained in
the shares is revealed. The message is revealed without
additional calculation or manipulation. This feature
assures that the secure process can be used by someone
who has no previous knowledge of Visual Cryptography,
programming background, or cryptographic analysis
experience. Since the development of this idea, several
different variations and modifications have been
developed to explore many diverse aspects of Visual
Cryptography. Some of these include an algorithm for
encrypting specific image regions [7], an algorithm using
share rotation for revelation [8], and an algorithm which
usesa probabilistic scheme for share generation [9]. As
these algorithms are explored and developed, they have
deferent approaches and techniques but the fundamental
underlying architecture draws directly from the original
Visual Cryptography technique.
III. VISUAL CRYPTOGRAPHY SCHEMES
1. Traditional secret sharing Scheme
The traditional secret sharing scheme which was
produced by Shamir [10] and Blakley [11] isactually,
separated. Concerning the nature of the secret sharing
scheme, it is supposed to be a bank vaultthatmustpass by a
secret key. The bank has three tellers, but the system of
bank does not trust any of them individually. Therefore,
they must be to design a system such that any two of the
three tellers can pass the vault together. This problem can
be known as a
)3,2(
secret sharing scheme.
-
),( nk
Secret sharing scheme is a method to share a
secretKamongstnparticipants onefor each participant such
that the following conditions should be hold:
1- No participant knows the share given to another
participant.
2- ktogether can reveal the secret image by
superimposing k (transparencies).
3- Any ttransparencies, t < k, the secret cannot be
decoded.
Here is an example to illustrate the
)2,2(
secret
sharing scheme. Suppose that the secret
K
is a binary
sequence of length m, i.e.
),,,( 21 m
kkkK 
. The two
shares,
1
s
and
2
s
can be constructed as follow.
- The first share is chosen to be a random binary sequence
of length m, say
),,,( 112111 m
ssss 
. Then, we can
compute the second share by doing “exclusive-or

” on
K
and
1
s
.
iii sks 12 
,
mi ,,1 
(1)
For example, assume that
2m
,
)1,0(k
. Then the
two shares can be constructed as following in table (1):
Table 1: Construction of two shares
1
s
Kss  12
(0,0)
(0,1)
(0,1)
(0,0)
(1,0)
(1,1)
(1,1)
(1,0)
However, looking only at one share, say
1
s
, any four
values of
K
are possible. In other words, it gains no
information about
K
if another share
2
s
is unknown.
2. Visual Secret Sharing Scheme (VSSS)
In the Visual Secret sharing scheme, there is a secret
picture to be shared among nparticipants. The picture is
divided into ntransparencies (shadows) such that if any m
transparencies are placed together, the picture becomes
visible. However, if fewer than m transparencies are
placed together, or analyzed by any other means; nothing
can be seen. Visual Secret Sharing scheme uses
mathematical secret sharing but implements in hardware,
printed on transparencies. It once created, it requires no
technology, and however resolution and contrast is
lost.Naor and Shamir [1] introduced a visual secret sharing
scheme to decrypt the secret image without performing
any cryptographic computation; this scheme uses human
visual system. Depending on how the decryptionof secret
image, the difference between a VSSS and a traditional
secret sharing scheme is usually, the traditional secret
sharing scheme requires computation over a finite field.
In a VSSS, however, the computation is simply performed
by the human visual system of the users.
To construction of a secure VSSS is difficult. Assume
that a particular pixel
P
on a share
i
s
is black.
Whenever a set of shares (including
i
s
) is stacked
together, the result must be black. It means that in the
secret image, the pixel
P
must be black. That is mean
that by examining one of the shares we can gotsome
information about the secret image, but the security
condition does not allow this. Naor and Shamir [1]
introduced a VSSS that solved this problem by expanding
each original pixel into msubpixels.
Suppose that the image secret is a collection of black
and white pixels, or a binary image, and each pixel is
encrypted individually. Each original pixel encodes into n
shares, and each share is a set of m black and white
subpixels, which are printed near to each other such that
human visual system averages their individual black/white
contribution. The VSSS can be described by an
mn
Boolean matrix
M
where
1],[ jiM
if the j-thsubpixel
in the i-th shares is black, and
0],[ jiM
if the j-
thsubpixel in the i-th shares is white.
Definition 1. Hamming weight: The number of non-
zero symbols in a symbol sequence. In a binary
representation, Hamming weight is the number of "1" bits
in the binary sequence.
Definition 2. OR-ed k-vector: Given a j x k matrix, it is
the k-vector where each tuple consists of the result of
performing boolean OR operation on its corresponding jx1
column vector.
Definition 3. An VCS scheme is a 6-tuple (n, m, S, V,
α, d). It assumes that each pixel appears in n versions
called shares, one for each transparency. Each share is a

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International Journal of Electronics Communication and Computer Engineering
Volume 5, Issue 2, ISSN (Online): 2249–071X, ISSN (Print): 2278–4209
collection of m black and white subpixels. The resulting
structure can be described by an n x m Boolean Matrix
S=[Sij ] where Sij = 1 if the j-th sub-pixel in the i-th share
is black. Therefore, the grey level of the combined share,
obtained by stacking the transparencies, is proportional to
the Hamming weight H(V) of the OR-ed m-vector V. This
grey level is usually interpreted by the visual system as
black if H(V)¸d and as white if H(V ) < d - αm for some
fixed threshold 1 ≤ d ≤ m and relative deference α > 0. αm,
the difference between the minimum H(V) value of a
black pixel and the maximum allowed H(V) value for a
white pixel is called the contrast of a VCS scheme.
To decrypt the secret image, we simply xerox
t
shares
onto transparencies, and then stacking them together with
perfect alignment. We can see a stacked version share
V
whose black subpixels are represented by the Boolean “or”
of row
t
sss ,,, 21 
in
M
.
t
sssV  
21
(2)
The gray level of this stacked share
V
is proportional to
the Hamming weight
)(VH
of
V
. This gray level is
interpreted by the visual system of the users as black if
dVH )(
and as white if
mdVH

)(
for some
fixed threshold
md 1
and relative difference
0

.
Visual Secret Sharing is based on the access structure
schemes specified as follows :
2.12 out of 2 Scheme (2 subpixels):
In this scheme, a binary image pixel is divided into two
sub-pixels out of which black or white is randomly chosen
depending on the current pixel as following in figure (2):
Fig.2. A random column-permutation of white pixel and black
pixel is done from S0 and S1
If the image pixel is white, then chose one of the two
rows for white, else if it is black, then choose between one
of the two rows for black. A random column-permutation
of white pixel and black pixel is done from S0 and S1 to
generate a given matrix as C0 and C1 represents white and
black pixel respectively which produces two vectors V0
and V1 corresponding to occurrence of the pixel i.e. either
white or black in a secret image as shown in figure (2). If
the pixel is white then V0 will be outputted which is of
either 1 0 or 0 1 with gray-level ½ and if black pixel then
V1 will be outputted with either 1 1 or 1 1.
2.2 2 out of 2 Scheme (4 subpixels): In this scheme, each
black/white pixel is encoded as a 2x2 cell into two shares
where each share has 2 black, 2 white subpixels. When
stacked, shares combine to give solid black or half black
(seen as grey) as shown in figure (3).
Fig.3. C0and C1are the two column matrices of white and
black pixel respectively which is selected on random basis.
2.3 3out of 3 Scheme:This scheme encodes the secret
image into three shares based on pixel expansion such that
when all the three shares are combined, the secret image
will be revealed.
3. Visual cryptography for general access structures
The drawback of (k,n) Basic model that is any "k"
shares will decode the secret image which reduces security
level. To overcome this issue G. Ateniese, C. Blundo, A.
De Santis, and D. R. Stinson [12] extended thebasic model
to general access structures by, where an access structure
is a specification of all qualified and forbidden subsets of
‘n’ shares. Any subset of ‘k’ or more qualified shares can
decrypt the secret image but no information can be
obtained by stacking lesser number of qualified shares or
by stacking disqualified shares. Construction of k out of n
threshold visual cryptography scheme for general access
structure is better with respect to pixel expansion than [1].
To illustrate the idea behind the extension of basic
model to general access structure, suppose that a bank has
a vault. In this time, the bank employs three senior tellers
and a manager. They would like to design a system such
that one of the three senior tellers together with the
manager can open the vault. However, two of the three
senior tellers cannot obtain the permission. This problem
can be viewed as a general access structure scheme.In
),( nk
basic model, the secret image is decrypted by
stacking any
k
shares together. In a general access
structure, however, we can specify some qualified subsets
of shares that can decrypted the secret image, but other
forbidden subsets of shares have no information about the
it. For example, assume that there are four shares. Let
 
4321 ,,, ssss
be the set of all shares, and let

2
denotes the set of all subsets of

. Suppose that we want
to construct a VSSS such that the qualified sets are all
subsets of

containing at least one of the three sets
 
21,ss
,
 
32 ,ss
or
 
43 ,ss
. In order words,
1
s
and
2
s
together can decrypt the secret, so as
 
32 ,ss
and
 
43 ,ss
.
Hence, the family of qualified sets is

),,,(),,(),,(),,( 321433221 sssssssss
Qual 
),,,(),,,( 431421 ssssss

),,,(),,,( 4321432 sssssss
(3)
Let all remaining sets of

be the forbidden sets.
 
),(),,(,),(),(),(),(),( 4241314321 ssssssssss
Forb 
(4)

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The pair
),( ForbQual 
is called the access structure of
the scheme.
4. Black and White Visual Cryptography Schemes
4.1 Sharing Single Secret
Naor and Shamir’s [1] introduced encoding scheme to
share a binary image into two shares Share1 and Share2. If
pixel is white one of the above two rows of the table (2) is
chosen to generate Share1 and Share2. Similarly If pixel is
black one of the below two rows of the following table is
chosen to generate Share1 and Share2. Here each share
pixel pis encoded into two white and two black pixels
each share alone gives no clue about the pixel pwhether it
is white or black. Secret image is shown only when both
shares are superimposed.
Table 2: Naor and Shamir’s scheme for encoding to share a
binary image into two shares
Spatial-domain image hiding schemes to hide a binary
image into two meaningful shares was suggested by Chin-
Chen Chang et al. [13]. These two secret shares are
embedded into two graylevel cover images. At the
decryption side the hidden messages, embedding images
can be superimposed.Liguo Fang et al. [4] recommended a
(2, n) scheme based on combination to balancing the
performance between pixel expansion and contrast.
Threshold visual secret sharing schemes mixed XOR and
OR operation with reversing and based on binary linear
error correcting code was suggested by Xiao-qing and Tan
[14]. The disadvantage of the above schemes is that only
one set of confidential messages can be embedded, so to
share large amounts of confidential messages several
shares have to be generated.
4.2 Sharing Multiple Secrets
The visual cryptography schemes to share two secret
images in two shares presented first time by the researcher
Wu and Chen [15]. They hidden two secret binary images
into two random shares, namely A and B, such that the
first secret can be seen by stacking the two shares, denoted
by A , B, and the second secret can be obtained by first
rotating A Ө anti-clockwise. They designed the rotation
angle Ө to be 90◦. However, it is easy to obtain that Ө can
be 180◦ or 270◦. To overcome the angle restriction of Wu
and Chen’s scheme [15], Hsu et al. [16] proposed a
scheme to hide two secret images in two rectangular share
images with arbitrary rotating angles. Wu and Chang [17]
also refined the idea of Wu and Chen [15] by encoding
shares to be circles so that the restrictions to the rotating
angles (Ө = 90◦, 180◦ or 270◦) can be removed.
S J Shyu et al. [18] were first researchers to advise the
multiple secrets sharing in visual cryptography. This
scheme encodes a set of n .2 secrets into two circle shares.
The n secrets can be obtained one by one by stacking the
first share and the rotated second shares with n different
rotation angles. To encode unlimited shapes of image and
to remove the limitation of transparencies to be circular,
Fang[19] offered reversible visual cryptography scheme.
In this scheme two secret images which are encoded into
two shares; one secret image appears with just stacking
two shares and the other secret image appears with stack
two shares after reversing one of them. Jen-Bang Feng et
al. [20] developed a visual secret sharing scheme for
hiding multiple secret images into two shares. The
proposed scheme analyzes the secret pixels and the
corresponding share blocks to construct a stacking
relationship graph, in which the vertices denote the share
blocks and the edges denote two blocks stacked together at
the desired decryption angle. According to this graph and
the pre-defined visual pattern set, two shares are
generated.
To provide more randomness for generating the shares
Mustafa Ulutas et al.[3] advised secret sharing scheme
based on the rotation of shares. In this scheme shares are
rectangular in shape and are created in a fully random
manner. Stacking the two shares reconstructs the first
secret. Rotating the first share by 90• counterclockwise
and stacking it with the second share reconstructs the
second secret. Tzung-Her Chen et al. [21] offered the
multiple image encryption schemes by rotating random
grids, without any pixel expansion and codebook redesign.
A non-expansion reversible visual secret sharing method
that does not need to define the lookup table offered by
Fang [22]. To encode four secrets into two shares and
recovering the reconstructed images without distortions
Zhengxin Fu et al. [23] intended a rotation visual
cryptography scheme. Rotation visual cryptography
scheme construction was based on correlative matrices set
and random permutation, which can be used to encode
four secret images into two shares. Jonathan Weir et
al.[24] suggested sharing multiple secrets using visual
cryptography. A master key is generated for all the secrets;
correspondingly, secrets are shared using the master key
and multiple shares are obtained.
5. Gray Level Visual Cryptography Scheme
Previous works done in visual cryptography were
restricted to binary images which is insufficient in real
time applications. Visual cryptography for gray level
images by dithering techniques was suggested by Chang-
ChouLin, Wen-HsiangTsai [25]. Instead of using gray sub
pixels directly to constructed shares, a dithering technique
is used to convert gray level images into approximate
binary images. Then existing visual cryptography schemes
for binary images are applied to accomplish the work of
creating shares. The effect of this scheme is still
satisfactory in the aspects of increase in relative size and
decoded image quality, even when the number of gray
levels in the original image still reaches 256.

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6. Color Visual Cryptography Scheme
Though some researches in field of visual cryptography
applied on the binary images, it left some limitations in the
quality of the decoded binary images, which makes it
inapplicable for protection of color image. AS a result of
these limitations, F.Liu, C.K. Wu X.J. Lin [26] introduced
a new approach on visual cryptography for colored
images. They suggested three approaches as follows:
1. The first approach to realize color VCS is to print the
colors in the secret image on the shares directly similar to
basic model. It uses larger pixel expansion which reduces
the quality of the decoded color image.
2. The second approach converts a color image into black
and white images on the three color channels (red, green,
blue or equivalently cyan, magenta, yellow), respectively,
and then apply the black and white VCS to each of the
color channels. This results in decrease of pixel expansion
but reduces the quality of the image due to halftone
process.
3. The third approach utilizes the binary representation of
the color of a pixel and encrypts the secret image at the
bit-level this results in better quality but requires devices
for decryption.
6.1 Sharing Single Secret
Visual cryptography schemes were applied to only black
and white images until the year 1997. First colored visual
cryptography scheme was developed by Verheul and Van
Tilborg ]27 [ .Colored secret images can be shared with the
concept of arcs to construct a coloredvisual cryptography
scheme. In c-colorful visual cryptography scheme one
pixel is expanded into msubpixels, and each subpixel is
divided into c color regions. In eachsubpixel, there is
exactly one color region colored, and all the other color
regions are black.The color of one pixel depends on the
interrelations between the stacked subpixels. For acolored
visual cryptography scheme with c colors, the pixel
expansion m is c×3. Yang andLaih [28] improved the
pixel expansion to c × 2 of Verheul and Van Tilborg [27].
But in bothof these schemes share generated were
meaningless.Chang and Tsai [29] anticipated color visual
cryptography scheme for sharing a secret color image and
also to generate the meaningful share to transmit secret
color image. For a secret color image two significant color
images are selected as cover images which are the same
size as the secret color image. Then according to a
predefined Color Index Table, the secret color image will
be hidden into two camouflage images. One disadvantage
of this scheme is that extra space is required to accumulate
the Color Index Table. In this scheme also number of
subpixels is in proportional to the number of colors in the
secret image as in Verheul and Van Tilborg [27] Yang and
Laih [28] schemes. When more colors are there in the
secret image the larger the size of shares will become. To
overcome this limitation Chin-Chen Chang et al. [30]
developed a secret color image sharing scheme based on
modified visual cryptography. This scheme provides a
more efficient way to hide a gray image in different
shares. In this scheme size of the shares is fixed; it does
not vary when the number of colors appearing in the secret
image differs. Scheme does not require any predefined
Color Index Table. Though pixel expansion is a fixed in
[30] this scheme is not suitable for true color secret image.
To share true-color image Lukac and Plataniotis [31]
introduced bit-level based scheme by operating directly on
S-bit planes of a secret image. Liu et al [25] developed a
colour visual cryptography scheme under the visual
cryptography model of Naor and Shamir with no pixel
expansion. In this scheme the increase in the number of
colors of recovered secret image does not increase pixel
expansion. Wei Qiao et al [32] suggested visual
cryptography scheme for color images based on halftone
technique. A secret image sharing scheme for true-color
secret images devised by Du-Shiau Tsai et al [33]. In the
proposed scheme through combination of neural networks
and variant visual secret sharing, the quality of the
reconstructed secret image and camouflage images are
visually the same as the corresponding original images.
6.2 Sharing Multiple Secrets
Tzung-Her Chen et al [34] anticipated a multi-secrets
visual cryptography which is extended from traditional
visual secret sharing. The codebook of traditional visual
secret sharing implemented to generate share images
macro block by macro block in such a way that multiple
secret images are turned into only two share images and
decode all the secrets one by one by stacking two of share
images in a way of shifting. This scheme can be used for
multiple binary, gray and color secret images with pixel
expansion of 4.Daoshun Wang et al [35] provided general
construction for extended visual cryptography schemes
using matrix extension algorithm. A general construction
method for single or multiple and binary, grayscale, color
secret images using matrix extension utilizing meaningful
shares was suggested. Using matrix extension algorithm,
any existing visual cryptography scheme with random-
looking shares can be easily modified to utilize
meaningful shares.
7. Halftone Visual Cryptography Scheme (HVC)
Mizuho NAKAJIMA and Yasushi YAMAGUCHI [36]
introduced extended visual cryptography which was of
poor quality of the generated meaningful shares which
again increases the suspicion of data encryption. Halftone
Visual Cryptography introduced in [37], [38], [39] which
increases the quality of the meaningful shares, it actually,
based upon the basis matrices collections available in
conventional visual cryptography. A secret binary pixel p
is encoded into an array of q=v1*v2 called a halftone cell,
in each of the n shares. The selection of the secret
information pixels (SIPs) in a halftone cell is important as
it affects the visual quality of the resultant halftone shares.
However, as long as the positions of the secret information
pixels are independent of the secret information, the
arrangement of the modified pixels satisfies the security
requirements. By using halftone cells with an appropriate
size, visually pleasing halftone shares can be obtained.
Also maintains contrast and security.
8. A probabilistic Visual Cryptography Scheme
Ito et al. [40] suggested the probabilistic model of the
VC scheme, where the scheme is based on the basis
matrices, but only one column of the matrices is chosen to
encode a binary secret pixel, rather than the traditional VC

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scheme utilizing the whole basis matrices. The size of the
generated transparencies is identical to the secret image.
Yang [41] also introduced a probabilistic model of VC
scheme, and the two cases and are explicitly constructed to
achieve the optimal contrast. Based on Yang [41], Cimato
et al. [9] introduced a generalized VC scheme in which the
pixel expansion is between the probabilistic model of VC
scheme and the traditional VC scheme. Sian-Jheng Lin
and Wei-Ho Chung [42] proposed a probabilistic model of
(t,n) VC scheme with unlimited n. their contribution is that
the proposed scheme accommodates dynamic changes of
users in the group sharing a VC secret ,also their proposed
scheme allows changes of users without regeneration and
redistribution of VC transparencies, which reduce the
computing and communication resources in
accommodating user changes.
9. Hierarchical Visual Cryptography scheme
Pallavi V. Chavan and Mohammad Atique [43]
designed a Hierarchical visual cryptography as shown in
figure (4), which encrypts the secret in number of levels.
Initially the secret is divided into exactly two share called
share 1 and share 2. Each share is then encrypted
independently resulting in four shares: share 11, share 12,
share 21 and share 22. Later, among these four shares, any
three shares are chosen to generate the key share. The
superimposition of key share with the remaining share
reveals the secret information. The superimposition is
logically performed by the X-OR operation. As the level
of encryption in hierarchical visual cryptography
increases, the secrecy tends to increase. In their scheme
stated, a single pixel in original secret is represented by 2
pixels in the share. It gives raise to the expansion ration of
1:2. The benefit of reducing the expansion ratio is that the
shares require less storage space over the server, reducing
the time complexity of the authentication process. The
secret is chosen with size nxm. After encoding the size of
share becomes nX2m indicating the pixel expansion ratio
of 1:2 only.
Fig.4. Concept of hierarchical visual cryptography.
IV. INTERPRETATION (DISCUSSION)
In this section we summarize the visual cryptography
schemes based on our classification that has been
discussed in the previous section. The table (3) can
reflected brightly the various schemes suggested by the
authors mentioned respectively. This table can facilitate
the selecting of a proper scheme for specific application.
The visual cryptography schemes are more useful for
getting more the confidentiality, reliability, and integrity.
Using the visual cryptography schemes, the secret image is
encrypt into shares which are noise-like secure images
which can be transmitted or distributed over an untrusted
communication channel. In the decryption side, the
secretimage is decrypted without additional computation
and any knowledge of cryptography.
Table 3: Visual cryptography schemes with brief description focuses on the image format, Pixel Expansion, share
generated type and number of secret sharing
No.
Authors
Year
V. C. Scheme
Description
1
Shamir [10] and
Blakley [11]
1979
Traditional Secret
Sharing
Binary image format supported ,random share generated
2
Naor and Shamir [1]
1995
Visual Secret
Sharing
Binary image format supported ,random share generated,
each pixle expanded into 4 subpixle
3
G. Ateniese et al. [12]
1996
V. C. For General
Access Structure
Binary image format supported ,random share generated
4
Naor and Shamir’s[1]
1995
Black And White
V. C.
Binary image format supported ,random share generated, eachpixel
expanded into 4 subpixle, sharing single secret
5
Wu and Chen [15]
1998
Binary image format supported, random share generated, each pixel
expanded into 4 subpixle, sharing multiple secret
6
Hsu et al. [16]
2004
Binary image format supported, random share generated, each pixel
expanded into 4 subpixle, sharing multiple secret
7
Chin-Chen Chang et al.
[13]
2005
Binary image format supported ,meaningful share generated, eachpixel
expanded into 4 subpixle, sharing single secret
8
Wu and Chang [17]
2005
Binary image format supported, random share generated, each pixel
expanded into 4 subpixle, sharing multiple secret
9
Liguo Fang et al. [4]
2006
Binary image format supported ,random share generated, eachpixel
expanded into 2 subpixle, sharing single secret
10
S J Shyu et al. [18]
2007
Binary image format supported, random share generated, eachpixel
expanded into 2n subpixle, sharing multiple secret
11
Fang[19]
2007
Binary image format supported, random share generated, eachpixel
expanded into 9 subpixle, sharing multiple secret
12
Feng et al. [20]
2008
Binary image format supported, random share generated, eachpixel
expanded into 3n subpixle, sharing multiple secret
13
Mustafa Ulutas et al.[3]
2008
Binary image format supported, random share generated, eachpixel

Copyright © 2014 IJECCE, All right reserved
418
International Journal of Electronics Communication and Computer Engineering
Volume 5, Issue 2, ISSN (Online): 2249–071X, ISSN (Print): 2278–4209
expanded into 4 subpixle, sharing multiple secret
14
Tzung-Her Chen et al.
[21]
2008
Binary image format supported, random share generated, eachpixel
expanded into 1 subpixle, sharing multiple secret
15
Xiao-qing and Tan [14]
2009
Binary image format supported, random share generated, eachpixel
expanded into 1 subpixle, sharing single secret
16
Fang [22]
2009
Binary image format supported, random share generated, eachpixel
expanded into 1 subpixle, sharing multiple secret
17
Zhengxin Fu et al. [23]
2009
Binary image format supported, random share generated, eachpixel
expanded into 9 subpixle, sharing multiple secret
18
. Jonathan Weir et
al.[24]
2009
Binary image format supported, random share generated, eachpixel
expanded into 4 subpixle, sharing multiple secret
19
Chang- ChouLin, Wen-
HsiangTsai [25]
2003
Gray Level V. C.
Gray image format supported, random share generated, using dithering
technique
20
Verheul and Van
Tilborg [27]
1997
Color V. C.
Color image format supported, random share generated, each pixel
expanded into c * 3 subpixle, sharing single secret
21
Yang and Laih [28]
2000
Color image format supported, random share generated, eachpixel
expanded into c *2 subpixle, sharing single secret
22
Chang and Tsai [29]
2000
Color image format supported, meaningful share generated, eachpixel
expanded into 592 subpixle, sharing single secret
23
Chin-Chen Chang et al.
[30]
2002
Color image format supported, meaningful share generated, eachpixel
expanded into 9 subpixle, sharing single secret
24
Lukac and Plataniotis
[31]
2005
Color image format supported, random share generated, eachpixel
expanded into 2 subpixle, sharing single secret
25
Tzung-Her Chen et al
[34]
2008
Color,binary,gray image format supported, random share generated, each
pixel expanded into 4 subpixle, sharing single secret
26
Wei Qiao et al [32]
2009
Color image format supported, random share generated, eachpixel
expanded into m subpixle, sharing single secret
27
Du-Shiau Tsai et al
[33].
2009
Color image format supported, meaningful share generated, eachpixel
expanded into 9 subpixle, sharing single secret
28
Daoshun Wang et al
[35]
2009
Color image format supported, meaningful share generated, using matrix
extension algorithm, sharing single secret
29
Z. Zhou et al.
[37][38][39]
2009-
2003-
2006
Halftone V. C.
Binary image format supported, meaningful share generated, secret binary
pixel is encoded into q=v1*v2, this scheme is increases the quality of
meaningful shares.
30
Ito et al. [40]
1999
Probabilistic V. C.
Binary image format supported, random share generated ,based on the basis
matrices, but only one column of the matrices is chosen to encode a binary
secret pixel
31
Yang [41]
2004
Binary image format supported, random share generated ,two cases and are
explicitly constructed to achieve the optimal contrast
32
Cimato et al. [9]
2006
Binary image format supported, random share generated , which the pixel
expansion is between the probabilistic model of VC scheme and the
traditional VC scheme
33
Sian-Jheng Lin and
Wei-Ho Chung [42]
2012
Binary image format supported, random share generated , proposed a
probabilistic model of (t,n) VC scheme with unlimited n.
34
Pallavi V. Chavan and
Mohammad Atique
[43]
2012
Hierarchical V. C.
Binary image format supported, random share generated, The shares
generated are expanded version of the original secret with expansion ration
of 1:4 .After that the expansion ratio is reduced to 1:2 their methodology of
key share generation is defined with a set of four shares of original secret.
The symbols used in visual cryptography schemes are m
refers to pixel expansion, crefers to number of colors, and
nrefers to number of shares in corresponding visual
cryptography schemes.
As a shown in table (3), for the pixel expansion a few of
schemes carry out minimum pixel expansion. The schemes
with m =1 preferably use for secure transmission over
limited bandwidth communication networks. But the
schemes with m > 1 required large storage space to store
and exchange the shares. To avoid attacks by hackers the
meaningful shares can be helpful. For applications in the
environment of multimedia, the schemes that supporting
color image format are useful. The schemes with sharing
multiple secret are useful to less overhead for storage and
transmission.To increasing the quality of meaningful
shares the halftone visual cryptography scheme are
useful.The probabilistic visual cryptography scheme is
useful to reduce the computing and communication
resources in accommodating user changes. The secrecy of
data tends to increase when the number of levels in
Hierarchical Visual Cryptography scheme is increase.
IV. CONCLUSION
In this paper, a literature and descriptive review have
been presented for state-of-the-art visual cryptography
with more focused on the concepts, and classifications of
schemes. We have classified the visual cryptography
schemes that have been extracted from our reading and
literature review. The mentioned classification of visual
cryptography schemes can be considered a contribution of
this paper through which one can select a proper scheme
for specific application. Finally, this paper introduces
interpretation on the base of some various criteria such as
pixel expansion, share generated, format of secret image
and number of secret images. Future work of this paper
can be introduced by developing and evaluating schemes
for visual cryptography.

Copyright © 2014 IJECCE, All right reserved
419
International Journal of Electronics Communication and Computer Engineering
Volume 5, Issue 2, ISSN (Online): 2249–071X, ISSN (Print): 2278–4209
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Volume 5, Issue 2, ISSN (Online): 2249–071X, ISSN (Print): 2278–4209
AUTHOR’SPROFILE
Mahmoud E. Hodeish
Currently is a Ph.D. candidate in the School of
Technology, at Swami Ramanand TeerthMarathwada
University, Nanded, Sub-center, latur, India .He
received his M.Sc. degree in Computer networking
from the School of Computational Science at Swami
Ramanand Teerth Marathwada University, in 2013,
Nanded, India .He is currently working on Secret Sharing Scheme for
Visual Cryptography.
Dr. Vikas T. Humbe
He has done his M. Sc. (Computer Science) and Ph.
D. (Computer Science) from Department of
Computer Science and Information Technology, Dr.
Babasaheb Ambedkar Marathwada University,
Aurangabad in 2003 and 2008 respectively. He has
published 45 research papers in various National and
International Journals and conferences. He is Editorial Member for
International Journal on Computer Science and IT, also working as
reviewer for various International and National Journals and Conferences
like Eleswer’s Pattern Recognition Letters, Journal on Machine Vision
and Applications, IEEE IJCNN 07 and 09, ACVIT-09 etc. He has been
worked as Technical Committee Member for National and International
conferences and Journals. He is Life Member of Indian Science
Congress, Kolkata, Member IEEE, USA, Member of ACM, New York,
Member of International Association of Computer Science and
Information Technology, Singapore, Member of International
Association for Engineer’s, Hong Kong, and Member of Computer
Science Teacher’s Association, USA. Presently he is working as
Assistant Professor in School of Technology, SRTM University, Nanded
(Sub-campus, Latur) and his area of research interest are Biometrics,
Image Processing, Computer Vision and Video Processing.