Cryptography Reference
In-Depth Information
•
We add more and more sections (periods) of the ciphertext as we continue
revealing the correct choice for as many disks as there are letters in the
probable word.
•
In every period, we decrypt the part of the ciphertext determined by
the disks we already know. We will hit scraps of words like ORDE,
ANNABI, MITTANC, or XPOS, and completing them shouldn't pose a
major problem. Having revealed yet another piece of plaintext, we can
start all over again, luckily from a better starting position.
•
Step by step and piece by piece, we will end up knowing all disks.
The interesting part of this approach is that even homophony — ambiguity in
the cipher — does not represent an insurmountable obstacle. Of course, my
representation refers to the way we'd have worked before the computer era.
Humans are still better than computers when it comes to forming sentences
from scraps of words. But when you use a computer you'd proceed differently
anyway.
3.4.2 The Viaris Method
The method developed by Viaris represents a refinement of the cryptanalysis
discussed above. Again, we use a probable word, except that this time we
improve the negative pattern search.
To this end, we hold the row from which the ciphertext had been read (the
so-called
generatrix
) on the cylinder for a moment. Under this prerequisite,
we analyze the letters that could form at all from the letters of the probable
word for all disks. We appropriately build ourselves a table (
matrix
) for this
purpose. Each row in the table corresponds to a disk, and each column to a
letter of the probable word (see Figure 3.8).
As before, we move the probable word along underneath the ciphertext. We'll
know we have hit the correct position when each character of the ciphertext
above the word appears at least once in the corresponding matrix column.
Positions with coincidences (character matches like above) fall out automat-
ically, but generally other cases do too. If we find no possible position, we
have to try all over again using a different generatrix. Notice that the number
of possibilities to be analyzed is slightly smaller with this method. Givierge
refined the method once more by doing without probable words and using only
digram and trigram frequencies. You will find more details and references in
[BauerMM, 14.3].
