Extended Visual Secret Sharing Schemes: Improving the Shadow Image Quality

Visual secret sharing (VSS) scheme is a perfectly secure method to divide a secret image into several noise-like shadow images by splitting a secret pixel into black and white subpixels. Unlike other secret sharing schemes, the VSS scheme can be easily decoded by the human visual sight when staking shadow images. However, noise-like shadows are unusual and suspected by censors when delivered by e-mail or fax. Also, noise-like shadows are difficult to identify and manage when distributed. The problem was solved by adding the extended capability, a meaningful shadow image, in the so-called extended visual secret sharing (EVSS) scheme. In this paper, we present a new EVSS scheme by using gray and white subpixels to represent the secret pixel. Our proposed scheme still has the capability of visually revealing the secret image by stacking shadow images without the help of hardware and complex computation. When compared to the previous EVSS scheme, our new scheme has less number of subpixels to represent a secret pixel and the clearer shadow images.

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