Color Extended Visual Cryptography Using Error Diffusion
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Abstract
Some methods for color visual cryptography are not satisfactory

in terms of producing either meaningless shares or meaningful

shares with low visual quality, leading to suspicion of encryption.

This paper introduces the concept of visual information pixel

(VIP) synchronization and error diffusion to attain a color visual

cryptography encryption method that produces meaningful color

shares with high visual quality. VIP synchronization retains the

positions of pixels carrying visual information of original images

throughout the color channels and error diffusion generates shares

pleasant to human eyes. Comparisons with previous approaches

show the superior performance of the new method.

INTRODUCTION
VISUAL CRYPTOGRAPHY (VC) is a type of secret

sharing scheme introduced by Naor and Shamir [1]. In

a -out-of- scheme of VC, a secret binary image is cryptographically

encoded into shares of random binary patterns.

The shares are xeroxed onto transparencies, respectively,

and distributed amongst participants, one for each participant.

No participant knows the share given to another participant.

Any or more participants can visually reveal the secret image

by superimposing any transparencies together. The secret

cannot be decoded by any or fewer participants, even if

infinite computational power is available to them.

COLOR VC ENCRYPTION BASED UPON PIXEL

SYNCHRONIZATION AND ERROR DIFFUSION
In this section, we describe the encryption method for color

meaningful shares with a VIP synchronization and error diffusion.

First, we describe the VC matrix derivation method for

VIP synchronization from a set of standard VC matrices. We

then introduce an error diffusion process to produce the final

shares. The halftone process is independently applied to each

cyan ©, magenta (M), and yellow (Y) color channel so each

has only one bit per pixel to reveal colors of original images. A

secret message is halftoned ahead of the encryption stage.

Share Generation via Error Diffusion
Once the distribution of the basis matrices is completed, a

halftoning algorithm is applied to produce the final encrypted

shares. Error diffusion is used in our scheme as it is simple and

effective. The quantization error at each pixel is filtered and fed

back to future inputs. Fig. 5(a) shows a binary error diffusion

diagram designed for our scheme. To produce the th halftone

share, each of the three color layers are fed into the input.

CONCLUSION
This paper develops an encryption method to construct color

EVC scheme with VIP synchronization and error diffusion for

visual quality improvement. VIPs synchronize the positions of

pixels that carry visual information of original images across

the color channels so as to retain the original pixel values the

same before and after encryption. Error diffusion is used to construct

the shares such that the noise introduced by the preset

pixels are diffused away to neighbors when encrypted shares

are generated. It is obvious that there is a tradeoff between contrast

of encryption shares and the decryption share, however,

we can recognize the colorful secret messages having even low

contrast. Either VIP synchronization or error diffusion can be

broadly used in many VC schemes for color images.