Cryptography Reference
In-Depth Information
FIGURE 12.3
The main method is also applicable to mobile banking.
In the original purpose of secret sharing in [14], the order of the slides was
not relevant. Later work [15] showed that a better contrast can be achieved
with colors if the first slide can have non-transparent colors. Furthermore,
practical applications will work in the way that the (first) slide is sent first
from Alice to Bob over a secure channel (i.e., by surface mail) and used later
as a key to decrypt an image received over an insecure channel.
We regard visual cryptography as a special case of the Cardano grille,
which works on pixels instead of letters. In both cases we can describe the slide
(grille) as a 2-dimensional array over f0; 1g, where 0 stands for "transparent"
and 1 for "black." We describe the encrypted image as a 2-dimensional array
over = f0; 1g or = f0; 1;red;green;:::g or = f0;a;b;c;d;:::g, where
0 stands for "white" and 1 for "black." Colors are used for pixel-oriented
applications and in case the areas are big enough, any alphabet of symbols
can be used, which the receiver of the image can read through a transparent
area.
A compromise between pixel- and symbol-orientation is the segment-based
method described in [2], which works as demonstrated in Figure 12.5. It is
applicable whenever the message consists of symbols that can be represented
by a segment code, for example the 10 digits by the well-known 7-segment
code. The encryption method is basically the same: Instead of pixels, longish
and larger segments are encoded via two possible parallel positions.
Outline: Based on the above idea in [13], we describe techniques in Section
12.2 where the user can confirm a transaction as shown in Figure 12.1. Section
12.3 describes similar techniques that allow the user who received a slide
 
 
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