28-09-2012, 01:31 PM
Visual Cryptography
Visual Cryptography.ppt (Size: 283 KB / Downloads: 77)
Terminology:
Pixel—Picture element
Grey Level: The brightness value assigned to a pixel; values range from black, through gray, to white.
Hamming Weight (H(V)): The number of non-zero symbols in a symbol sequence
V- Vector of 1 and 1 of any length
A qualified set of participants is a subset of Ρ whose shares visually reveal the 'secret' image when stacked together.
A forbidden set of participants is a subset of Ρ whose shares reveal absolutely no information about the 'secret' image when stacked together.
The relative contrast (also called relative difference) of a VCS is the ratio of the maximum number of black sub pixels in a reconstructed (secret) white pixel to the minimum number of black sub pixels in a reconstructed (secret) black pixel. So, the lower the relative contrast in a scheme, the better. Note: the smallest relative contrast attainable in a VCS is 1/2, which is only achieved in a (2,2)-threshold VCS
The contrast of a VCS is the difference between the minimum number of black sub pixels in a reconstructed (secret) black pixel and the maximum number of black sub pixels in a reconstructed (secret) white pixel.
Visual Cryptography
Visual Cryptography is a secret-sharing method that encrypts a secret image into several shares but requires neither computer nor calculations to decrypt the secret image. Instead, the secret image is reconstructed visually: simply by overlaying the encrypted shares the secret image becomes clearly visible
A Visual Cryptography Scheme (VCS) on a set Ρ of n participants is a method of encoding a 'secret' image into n shares such that original image is obtained only by stacking specific combinations of the shares onto each other.
The Model
A solution to the k out of n visual secret sharing scheme consists of two collections of n x m Boolean (Basis) matrices S0 and S1. To share a white pixel, the dealer randomly chooses one of the matrices in S0 , and to share a black pixel, the dealer randomly chooses one of the matrices in S1. The chosen matrix defines the color of the m sub pixels in each one of the n transparencies for a original pixel.
Basis Matrices
Basis matrices are binary n x m used to encrypt each pixel in the secret image, where n is the number of participants in the scheme and m is the pixel expansion. The following algorithm is used to implement a VCS using basis matrices:
If the n x m basis matrices S1 (used to encrypt black pixels) and S0 (used to encrypt white pixels) for any VCS are given, the secret image SI is encrypted as follows:
Visual Cryptography.ppt (Size: 283 KB / Downloads: 77)
Terminology:
Pixel—Picture element
Grey Level: The brightness value assigned to a pixel; values range from black, through gray, to white.
Hamming Weight (H(V)): The number of non-zero symbols in a symbol sequence
V- Vector of 1 and 1 of any length
A qualified set of participants is a subset of Ρ whose shares visually reveal the 'secret' image when stacked together.
A forbidden set of participants is a subset of Ρ whose shares reveal absolutely no information about the 'secret' image when stacked together.
The relative contrast (also called relative difference) of a VCS is the ratio of the maximum number of black sub pixels in a reconstructed (secret) white pixel to the minimum number of black sub pixels in a reconstructed (secret) black pixel. So, the lower the relative contrast in a scheme, the better. Note: the smallest relative contrast attainable in a VCS is 1/2, which is only achieved in a (2,2)-threshold VCS
The contrast of a VCS is the difference between the minimum number of black sub pixels in a reconstructed (secret) black pixel and the maximum number of black sub pixels in a reconstructed (secret) white pixel.
Visual Cryptography
Visual Cryptography is a secret-sharing method that encrypts a secret image into several shares but requires neither computer nor calculations to decrypt the secret image. Instead, the secret image is reconstructed visually: simply by overlaying the encrypted shares the secret image becomes clearly visible
A Visual Cryptography Scheme (VCS) on a set Ρ of n participants is a method of encoding a 'secret' image into n shares such that original image is obtained only by stacking specific combinations of the shares onto each other.
The Model
A solution to the k out of n visual secret sharing scheme consists of two collections of n x m Boolean (Basis) matrices S0 and S1. To share a white pixel, the dealer randomly chooses one of the matrices in S0 , and to share a black pixel, the dealer randomly chooses one of the matrices in S1. The chosen matrix defines the color of the m sub pixels in each one of the n transparencies for a original pixel.
Basis Matrices
Basis matrices are binary n x m used to encrypt each pixel in the secret image, where n is the number of participants in the scheme and m is the pixel expansion. The following algorithm is used to implement a VCS using basis matrices:
If the n x m basis matrices S1 (used to encrypt black pixels) and S0 (used to encrypt white pixels) for any VCS are given, the secret image SI is encrypted as follows: