Enhanced Visual Cryptography Using color Error
diffusion and Digital Watermarking
Tripta Deendayal
Computer Science Engineering
Linagaya’s University ,Faridabad Haryana
Tripta.deendayal @gmail.com
Ch.Sunitha
Senior Lecturer
Computer Science Engineering
sunithabvrit @gmail.com
Abstract
In this paper a new visual cryptography scheme
is proposed for hiding information in images
which divide secret images into multiple
shares.In order to provide more security to
existing schemes a new Technique called Digital
Watermarking is used,and to improve the quality
and size of images obtained color error diffusion
technique is used .secret information can be
retrieved by stacking any k number of decrypted
shares. which reduces the color sets that renders
the halftone image and chooses the color whose
brightness variation is minimal.
1. Introduction Visual cryptography Scheme (VCS) is an
emerging cryptography technology which uses
the characteristics of human vision system to
encrypt and decrypt images without the
requirement of complex computation. In order to
reduce the size and the distortion of decrypted
image we propose the visual cryptography for
color image using color error diffusion dithering
technique . This technique improves the quality
of decrypted image compared to other dithering
techniques, such as Adaptive order dither
technique and Flyod and steinberg error
diffusion . The two top images in fig 1
figure (a) and figure (b), share 1 and share2 are
two randomly generated images which carry the
secret information. If we print the two shares on
transparencies and superimpose them, as shown
in fig c below
.
Usage of XOR operation for superimposition of
share images leads to perfect reconstruction of
the image and the size of the decrypted image is
same as the secret image.
Each Image consists of pixels,Each pixel of a
32 bit digital color image are divided into four
channels, namely Alpha, Red, Green and Blue,.
Alpha part represents degree of transparency. To
add more security to this scheme we have
proposed a technique extended invisible digital
watermarking technique A 32 bit sample pixel is represented in the
following
11100111 11011001 11111101 00111110
Alpha red green blue
Structure of a 32 bit pixel
fig 1
Tripta Deendayal et al,Int.J.Computer Techology & Applications,Vol 3 (1), 261-264
IJCTA | JAN-FEB 2012 Available [email protected]
261
ISSN:2229-6093
.
2. Halftone Technology Halftoning is the process of transforming an
image with greater amplitude resolution to one
with lesser amplitude resolution. This has been
practiced for over a hundred years in the printing
industry: the solution for displaying continuous
tone images with only black or white dots. In
digital printing and display systems today the
problem is essentially the same:how to produce
the illusion of the original tonal quality of a
image by judicious placement of dots. The goal
of all halftoning techniques is to generate an
image with fewer amplitude levels that is
perceptually similar to the original.
2.1 Color Error Diffusion technique It is a
type of halftoning in which the quantization
residual is distributed to neighboring pixels that
have not yet been processed. Its main use is to
convert a multi-level image into a binary image
Unlike many other halftoning metho , error
diffusion is classified as an area operation,
because what the algorithm does at one location
influences what happens at other locations. This
means buffering is required, and complicates
parallel processing. Point operations, such as
ordered dither, do not have these
complications.Error Diffusion has the tendency
to enhance edges in an image. This can make
text in images more readable than in other
halftoning techniques.Error diffusion takes a
monochrome or color image and reduces the
number of quantization levels. A popular
application of error diffusion involves reducing
the number of quantization states to just two per
channel. This makes the image suitable for
printing on binary printers such as black and
white laser printers.
3.Overall Methodology Overall methodology is divided in to two
main parts
-Encryption
- Decryption
3.1 Algorithm for Encryption
a). Source image is divided into 4 different
channel images Ia,Ib,Ir,Ig.(alpha,blue,red,green)
b). Color error diffusion technique is applied on
each channel image to obtain halftone images.
c). VCS k,nis applied on each halftone image
to generate shares .
d). Each share is embedded into n different
evelope images using Digital
Watermarking(LSB Replacement).
e). Enveloped images are taken and original
image is produced using LSB Replacement with
XOR Operation.
XOR operation for stacking share images is
used to get less distorted decrypted image and
subsampling 2X2 block into single pixel
produces a decrypted image with same size as
original image.
3.2 Proposed Model of the scheme
3.3 encryption
3.3 Description of the model
1. Color images to be sent as secret is taken as
input.
2. Image is divided into channels like alpha
channel ,red channel,green channel,red channel.
Alpha channels are masks through which you
can display images. The alpha channel is an 8-bit
channel, which means it has 256 levels of gray
from 0 (black) to 255 (white). White acts as the
visible area; black acts as the transparent area .
The level of gray in between determines the level
of visibility. For example, 50 percent gray allows
for 50 percent visibility. The resulting image is
called RGBA.
Color images
Separate images into 4 different channel images
Ir Ig Ib
Apply color error diffusion technique to each channel to get halftone image
I rhft I ghft I bhft
Apply VCS k,nto each haftone image
Produce Shares : (Sbo,Sb1) Sa0,Sa1 )(Sro,Sr1 ),( Sgo,Sg1 ),
Apply Digital Watermarking
Enveloped images
Image is Encrypted
For Encryption
Ia
Iahft
Tripta Deendayal et al,Int.J.Computer Techology & Applications,Vol 3 (1), 261-264
IJCTA | JAN-FEB 2012 Available [email protected]
262
ISSN:2229-6093
fig 2
3. Then Color error diffusion dot dether
technique is applied to these channels to obtain
halftone images for example output of halftoning
process looks like below image
Original image halftone image
4. Each channel image is divided into multiple
shares say n according to the input given by user
earlier developed using VCS k,n scheme . In
XOR based VCS scheme
where the share images are superimposed using
XOR operation which results in perfect
reconstruction of both Black and white pixels.
Fig 3
Fig 4
Every secret pixel is turned into two shares, and
each share belongs to the corresponding share
image. In the decryption process the two
corresponding shares are stacked together
(using XOR operation) to recover the secret
pixel. Two share of a white secret pixel are of the
same while those of a black secret pixel are
complementary as shown in Figure 3, 4 above.
Similary, a white secret pixel is recovered by a
share with the stacked result of half white sub-
pixels and a black secret pixel is recovered
by all black.
5. In this step each share is enveloped in to
a different image using Digital
Watermarking(LSB Replacement). That
is,for n shares n enveloping images are
required.
For example:-
Fig 5
+
One share
Enveloping image
Enveloped image
Tripta Deendayal et al,Int.J.Computer Techology & Applications,Vol 3 (1), 261-264
IJCTA | JAN-FEB 2012 Available [email protected]
263
ISSN:2229-6093
LSB Replacement Fig 6
Each two bits of each pixel of share is
replaced with last two bit of each pixel of
eveloping image as shown above in figure,In
this way all the pixels of a share is
embedded into enveloping image,such that
original is hided and enveloped image
obtained looks similar to original image.
In this way each 8 bit pixel of a share is
embed into 32 bit pixel of envelope images.
For enveloping n shares n different images
are required
4 Decryption
Fig 7
In the decryption algorithm the color image
channels are reconstructed by stacking the shares
of channels.These color image channels are
combined to get the secret color image.
5. Parameters for measuring
performance
Quality evaluation for decrypted images is done
using parameters such as
1. Peak Signal to Noise Ratio (PSNR)
2. Mean Square Error (MSE)
3. Normalized Cross correlation (NC) is
calculated between decrypted color image and
original color image
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Image using Color Error Diffusion ,IGCST –GVIP
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McGraw Hill, 2006.
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Sharing Visual Cryptography Scheme For Color
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Technique with
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RRRRRRRR RRRRRRRR RRRRRRRR RRRRRRRR
RRRRRRRR
AAAAAAAA RRRRRRRR GGGGGGGG BBBBBBBB
Pixel of envelope
N different images are required to envelope n shares.
Tripta Deendayal et al,Int.J.Computer Techology & Applications,Vol 3 (1), 261-264
IJCTA | JAN-FEB 2012 Available [email protected]
264
ISSN:2229-6093
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