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 online@www.ijcta.com

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 online@www.ijcta.com

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 online@www.ijcta.com

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

6. References [1]. Nagaraj V. Dharwadkar, B. B. Amberker, Sushil Raj Joshi “Visual Cryptography for Color

Image using Color Error Diffusion ,IGCST –GVIP

Journal,volume-10, issue 1.feb 2010 [2] M. Naor and A. Shamir, “Visual cryptography,”

Advances in Cryptology-Eurocrypt’94, 1995, pp. 1–12.

[3] P. Ranjan, “Principles of Multimedia”, Tata

McGraw Hill, 2006.

[4] John F Koegel Buford, Multimedia Systems, Addison Wesley, 2000.

[5] Kandar Shyamalendu, Maiti Arnab, “K-N Secret

Sharing Visual Cryptography Scheme For Color

Image Using Random Number” International Journal of

Engineering Science and Technology, Vol 3, No. 3,

2011, pp. 1851-1857.

[6] Naskar P., Chaudhuri A, Chaudhuri Atal, Image Secret Sharing using a Novel Secret Sharing

Technique with

Steganography, IEEE CASCOM, Jadavpur University,

2010, pp 62-65. [7] Hartung F., Kuttter M., “Multimedia

Watermarking Techniques”, IEEE, 1999.

[8] S. Craver, N. Memon, B. L. Yeo, and M. M.

Yeung. Resolving Rightful Ownerships with Invisible Watermarking Techniques: Limitations, Attacks and

Implications. IEEE Journal on Selected Areas in

Communications, Vol16, No.4 May 1998, pp.573–

586,. [9] Schildt, H. The Complete Reference Java 2, Fifth

Ed. TMH, Pp 799-839 [10] Krishmoorthy R, Prabhu

S, Internet & Java Programming, New Age

International, pp 234. [11] F. Liu1, C.K. Wu1, X.J. Lin, Colour visual

cryptography schemes, IET Information Security, July

2008. [12] Kang InKoo el. at., Color Extended Visual

Cryptography using Error Diffusion, IEEE 2010.

[13] SaiChandana B., Anuradha S., A New Visual

Cryptography Scheme for Color Images, International Journal of

Engineering Science and Technology, Vol 2 (6), 2010.

[14] Li Bai , A Reliable (k,n) Image Secret Sharing

Scheme by, IEEE,2006.

[14] Shyamalendu Kandar1, Arnab Maiti2, Bibhas

Chandra Dhara3 IJCSI International Journal of

Computer Science Issues, Vol. 8, Issue 3, No. 1, May 2011

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 online@www.ijcta.com

264

ISSN:2229-6093