08-12-2012, 06:44 PM
VISUAL CRYPTOGRAPHY
Visual-Cryptography-1.ppt (Size: 2.73 MB / Downloads: 70)
What is Visual Cryptography ?
Visual cryptography is a cryptographic technique which allows visual information (pictures, text, etc.) to be encrypted in such a way that the decryption can be performed by the human visual system.
Visual cryptography was pioneered by Moni Naor and Adi Shamir in 1994
Basis matrices
The two matrices S0,S1 are called basis matrices, if the two collections C0,C1 as defines in [1] are obtained by rearranging the columns of S0,S1 satisfy the following condition:
the row vectors V0,V1 obtained by performing
OR operation on rows i1,i2,…..iv of S0,S1 respectively, satisfy
(V0) ≤ tX - (m) m and (V1) ≥ tX
A pixel P is split into two sub pixels in each of the two shares.
If P is white, then a coin toss is used to randomly choose one of the first two rows in the figure above.
If P is black, then a coin toss is used to randomly choose one of the last two rows in the figure above.
Then the pixel P is encrypted as two sub pixels in each of the two shares, as determined by the chosen row in the figure. Every pixel is encrypted using a new coin toss.
Now let's consider what happens when we superimpose the two shares.
If P is black, then we get two black sub pixels when we superimpose the two shares;
Halftone visual cryptography
A halftone image is made up of a series of dots rather than a continuous tone.
These dots can be different sizes, different colors, and sometimes even different shapes.
Larger dots are used to represent darker, more dense areas of the image, while smaller dots are used for lighter areas.