Applications of Visual Cryptography - Visual Cryptography and Secret Image Sharing - page 338

FIGURE 12.15
Each area of the slide (b) has fragments of lenses, which direct the view (c)
in a magnifying manor to one of the symbols on the encrypted text (a).
Figure 12.13. While producing fragments of lenses and prisms on the slide
might require expensive special machines, it will be much cheaper to produce
complete lenses. Since the lenses do not need to have a perfect shape, it would
be sucient to place drops of a transparent liquid that becomes hard on the
side. This can be done by either using a modified ink-printer or by spraying
the liquid using physical randomness (and Alice can scan it before sending
it to Bob). The use of such a slide is shown in Figure 12.14. Figure 12.15
shows an optical generalization of Cardano cryptography. The advantage to
Cardano cryptography is that the letters are magnified and can be read in a
more natural ordering. The advantage to pixel-based visual cryptography is
that much less precision is needed to position the slide. The disadvantage is
that, in order to achieve a sucient level of security, more than four choices
(like in Figure 12.15) would be required. A repeated use would be too insecure
as the slide contains only little information. Furthermore, a bigger distance of
slide and screen is required.
12.6 Technical Problems Concerning Adjustment
and Parallaxes
A disadvantage of pixel-based visual cryptography to Cardano cryptography
is that the slide has to be placed at an exact position. We propose to position
the slide at the (for example, left lower) corner of the screen making use of its
frame. Then use the mouse to position and stretch the encrypted image on the
screen accordingsly. The parallaxes is the shift of the pixel on the screen that
can be seen through one point of the slide, which is caused by looking at it from