Progressive Visual Cryptography with Friendly and Size
Invariant Shares
Young-Chang Hou1, Zen-Yu Quan2, and
Chih-Fong Tsai2
1Department of Information
Management,
2Department of Information Management,
National Central University, Taiwan
Abstract: Visual cryptography is
an important data encoding method, where a secret image is encoded into n
pieces of noise-like shares. As long as there are over k shares stacked out of
n shares, the secret image can be directly decoded by the human naked eye; this
cannot be done if less than k shares are available. This is called the (k, n)-threshold
Visual Secret Sharing Scheme (VSS). Progressive Visual Cryptography (PVC)
differs from the traditional VSS, in that the hidden image is gradually decoded
by superimposing two or more shares. As more and more shares are stacked, the
outline of the hidden image becomes clearer. In this study, we develop an
image sharing method based on the theory of PVC, which utilizes meaningful
non-expanded shares. Using four elementary matrices (C0-C3) as the
building blocks, our dispatching matrices (M0 - M3) are designed to be expandable so that the
contrast in both the shares and the restored image can be adjusted based on
user needs. In addition, the recovered pixels in the black region of the secret
image are guaranteed to be black, which improves the display quality of the
restored image. The image content can thus be displayed more clearly than that
by previous methods.
Keywords: Visual cryptography, progressive visual
cryptography, secret sharing, unexpanded share, meaningful (Friendly) share.
Received April 8, 2015; accepted October 7, 2015
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