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VMs: Cardan grille
I was just informed by one cryptographer-lecturer about one thing I did not know -
actually he found that on my page. You might be aware of it, but it may require further
study and brigs rather bad news, I quote:
"Cardan grill is different from special grill called Fleissner-Wustrovitz, where the grill is
designed such way it can be rotated and eventually all places are filled with valid
letters, therefore it is a kind of transposition cipher.
Cardan grille is a seismographical method that brakes the mutual bonds and deforms the
statistics. We take say the matrix 15x15 and cut-out randomly not more then quarter
openings. The grill is then used such way that we write plaintext into the openings (in
forward or backward direction). It is not rotated but put away and remaining spaces
are filled with nulls (random, not valid letters). That makes all statistics and cracking
methods useless, including bigraphs. The solution is possible only when the same grill
is used many times and the text is long enough."
It is my impression that Rugg is not using the true Cardan grill, since he is just juggling
pre-, mid- a suffixes, not individual letters. It also brings for us very difficult point to
overcome: using of the true Cardan grill will provide the text which is 75 percent
"gibberish"! Such text cannot be handled by statistical nor by grammatical methods.
The repetitions of signs are irregular and I wonder what can be said about Zip's laws
and second entropy of such text, provided that even the spaces between words ( or
rather "packets") are put in by random.
I even suspect Cardan designed his grill as "almost unbreakable" cipher and may even
warn its users against repeated use of it. The author of the VM could know that and use
different grill for each page - the key being hidden in the picture on the same page. That
would make solution extremely difficult, but still possible - if we find those keys :-). On
the other hand, that would explain few points: the lack of longer words, difficulties of
our linguists as well as cryptographers, the strangeness of the pictures, etc.
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