Color Extended Visual Cryptography Using Error Diffusion
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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.
VISUAL CRYPTOGRAPHY (VC) is a type of secret
sharing scheme introduced by Naor and Shamir . 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.
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.