Cryptography Reference
In-Depth Information
tive color mixing scheme based on probabilities. This allows for a fixed pixel
expansion and improves on previous color secret sharing schemes. One prob-
lem with this scheme is that the overall contrast is reduced when the secrets
are revealed.
In most color visual cryptography schemes, when the shares are superim-
posed and the secret is recovered, the color image gets darker. This is due
to the fact that when two pixels of the same color are superimposed, the re-
sultant pixel gets darker. Cimato et al. [9] examine this color darkening by
proposing a scheme that has to guarantee that the reconstructed secret pixel
has the exact same color as the original. Optimal contrast is also achieved
as part of their scheme. This scheme differs from other color schemes in that
it considers only 3 colors when superimposing, black, white, or one pixel of
a given color. This allows for perfect reconstruction of a color pixel, because
no darkening occurs, either by adding a black pixel or by superimposing two
colors that are identical, that ultimately results in a final darker color.
A technique that enables visual cryptography to be used on color and
grayscale images is developed in progressive color visual cryptography [13].
Many current state-of-the-art visual cryptography techniques lead to the
degradation in the quality of the decoded images, which makes it unsuit-
able for digital media (image, video) sharing and protection. In [13], a series
of visual cryptography schemes have been proposed that not only support
gray-scale and color images, but also allow high quality images including that
of perfect (original) quality to be reconstructed.
The annoying presence of the loss of contrast makes traditional visual cryp-
tography schemes practical only when quality is not an issue which is relatively
rare. Therefore, the basic scheme is extended to allow visual cryptography to
be directly applied on grayscale and color images. Image halftoning is em-
ployed in order to transform the original image from the grayscale or color
space into the monochrome space which has proved to be quite effective. To
further improve the quality, artifacts introduced in the process of halftoning
have been reduced by inverse halftoning.
With the use of halftoning and a novel microblock encoding scheme, the
technique has a unique flexibility that enables a single encryption of a color
image but enables three types of decryptions on the same ciphertext. The
three different types of decryptions enable the recovery of the image of varying
qualities. The physical transparency stacking type of decryption enables the
recovery of the traditional VC quality image. An enhanced stacking technique
enables the decryption into a halftone quality image. A progressive mecha-
nism is established to share color images at multiple resolutions. Shares are
extracted from each resolution layer to construct a hierarchical structure; the
images of different resolutions can then be restored by stacking the different
shared images together.
The advantage is that this scheme allows for a single encryption, multi-
ple decryptions paradigm. In the scheme, secret images are encrypted/shared
once, and later, based on the shares, they can be decrypted/reconstructed in
 
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