29-08-2014, 10:56 AM
Visual Cryptography Scheme for Color Image Using Random Number
with Enveloping by Digital Watermarking
Visual Cryptography.pdf (Size: 899.96 KB / Downloads: 62)
Abstract
Visual Cryptography is a special type of encryption technique to
obscure image-based secret information which can be decrypted
by Human Visual System (HVS). This cryptographic system
encrypts the secret image by dividing it into n number of shares
and decryption is done by superimposing a certain number of
shares(k) or more. Simple visual cryptography is insecure
because of the decryption process done by human visual system.
The secret information can be retrieved by anyone if the person
gets at least k number of shares. Watermarking is a technique to
put a signature of the owner within the creation.
In this current work we have proposed Visual Cryptographic
Scheme for color images where the divided shares are enveloped
in other images using invisible digital watermarking. The shares
are generated using Random Number.
Introduction
Visual cryptography is a cryptographic technique where
visual information (Image, text, etc) gets encrypted in
such a way that the decryption can be performed by the
human visual system without aid of computers [1].
Like other multimedia components, image is sensed by
human. Pixel is the smallest unit constructing a digital
image. Each pixel of a 32 bit digital color image are
divided into four parts, namely Alpha, Red, Green and
Blue; each with 8 bits. Alpha part represents degree of
transparency.
Enveloping Using Invisible Digital
Watermarking
Using this step the divided shares of the original image
are enveloped within other image. Least Significant Bit
(LSB) replacement digital watermarking is used for this
enveloping process. It is already discussed that a 32 bit
digital image pixel is divided into four parts namely
alpha, red, green and blue; each with 8 bits. Experiment
shows that if the last two bits of each of these parts are
changed; the changed color effect is not sensed by human
eye[6]. This process is known as invisible digital
watermarking [7]. For embedding 32 bits of a pixel of a
divided share, 4 pixels of the envelope image is
necessary. It means to envelope a share with resolution w
X h; we need an envelope image with w X h X 4 pixels.
Here we have taken each envelope of size 4w X h.
Decryption Process
In this step at least k numbers of enveloped images are
taken as input. From each of these images for each pixel,
the last two bits of alpha, red, green and blue are retrieved
and OR operation is performed to generate the original
image. It is already discussed that human visual system
acts as an OR function. For computer generated process;
OR function can be used for the case of stacking k
number of enveloped images out of n.
. Conclusion
Decryption part of visual cryptography is based on OR
operation, so if a person gets sufficient k number of
shares; the image can be easily decrypted. In this current
work, with well known k-n secret sharing visual
cryptography scheme an enveloping technique is
proposed where the secret shares are enveloped within
apparently innocent covers of digital pictures using LSB
replacement digital watermarking. This adds security to
visual cryptography technique from illicit attack as it
befools the hackers’ eye.
The division of an image into n number of shares is done
by using random number generator, which is a new
technique not available till date. This technique needs
very less mathematical calculation compare with other
existing techniques of visual cryptography on color
images [10][11][12][13]. This technique only checks ‘1’
at the bit position and divide that ‘1’ into (n-k+1) shares
using random numbers. A comparison is made with the
proposed scheme with some other schemes to prove the
novelty of the scheme.