Cryptography Reference
In-Depth Information
Figure 4.5:
The transformation apparently produces the original image after
72 rounds.
'chaos' cyclically. However, you will see a clear structure again after 64 rounds,
which becomes gradually clearer to eventually peak in the original image after
72 iterations to our surprise (Figure 4.5).
How is this possible? A closer look at this method reveals that there should
actually be chaos — a totally mixed-up image — after 72 rounds. Well, almost
totally mixed — except for a raster with 216
×
216 dots, on which the origi-
nal image forms again! Mathematically, the image is gray almost everywhere,
except in the raster dots, and exactly these dots are represented on the screen.
The image is no longer there, but we still see it!
Only thanks to the special property of the human brain to be able to construct
an image from adjacent dots can we recognize the risk that could arise out of
the use of this type of encryption mechanism. If we had transformed the bits of
a text, we would have been deceived by an illusion. When bits within a close
neighborhood are strongly correlated (i.e., when only very few randomly depend
on one another), then it might be possible to reconstruct the original text
...
