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Halftoning Based Visual Secret Sharing (VSS)

image decrypted halftone visually

Definition: In Visual Secret Sharing based on halftoning, the continuous-tone image is first transformed into a halftone image, and then the VSS is applied.

Since VSS-schemes require binary inputs , the continuous-tone image is first transformed into a halftone image using the density of the net dots to simulate the intensity levels. In the sequence, the halftone version of the secret image is encryptedinto the n shares using the conventional ( k,n ) visual cryptography based encryption process. The secret image is visually revealed if at least k shares are available for decryption and stacked together on an overhead projector or specialized display.

Due to the nature of visual cryptography, and the expansion and contrast properties of ( k,n ) -VSS schemes, both binary (conventional) and halftoning-based VSS schemes usually produce shares and decrypted images with expanded dimensions compared to those of the original input (secret) image. In addition, VSS schemes introduce various visual impairments to the output image. Typically, a visually decrypted image is darker compared to the secret (input) image and has a halftone-like appearance (Figure 1).

To avoid the requirement for external hardware (overhead projector, display) or manual intervention, and obtain the decrypted output in digital form, VSS decryption can be realized using logical operations by a personal computer. The decrypted pixel visually appears as transparent (or white) if and only if all share pixels with the identical coordinates are transparent. If any pixel corresponding to the same spatial location of stacked shares has frosted (black) representation, then the decrypted pixel is visually revealed as black. To recover the continuous image from the decrypted halftone-like image, expensive inverse halftoning operations are needed. This feature in conjunction with impairments introduced by the VSS-decryption process make the approach disadvantageous for real-time multimedia applications.

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