Cryptography Reference
In-Depth Information
since a 'W' is above the left 'R', and a 'Z' above the right 'R'. This can't be
the right position. Similarly, we find the letters 'K' and 'H', 'D' and 'L' as
well as 'Z' and 'V' above the 'R' in the second, third, and fourth positions.
They are out of the question, too. The last position remains, where 'R' is
always converted into 'U'. We try to determine a shift (here 3) and, running a
deciphering attempt, obtain the following:
DIESERTEXTENTHAELTEINWAHRSCHEINLICHESWORT
All right, so this text was 'originally' Caesar-encrypted (i.e., using shift 3).
Of course, there are other methods we could have used to cryptanalyze this
example. For instance, the letter 'H' occurs in the ciphertext with a frequency
of 19 %. Assuming that 'H' corresponds to the most frequent letter, 'E', we
would also have found the solution. This is actually the way for ciphertext
attacks. However, using the negative pattern search on a probable word meant
that we didn't have to count up anything. It led to success almost effort-
lessly.
Approach for Ciphering Cylinders
The approach for ciphering cylinders is similar: the negative pattern search
supplies us with a few possible positions of the plaintext for a starter. What
makes this approach more difficult, however, is that the probable word could
be torn, and a careful code writer may have selected a different cylinder line
for each period. But we're not interested in this right now.
We start a plaintext attack by exploiting each possible position of the probable
word as follows:
•
For every letter, we know the character it will be transformed to in
the ciphertext. This heavily limits the selection of the disks, and even
homophony cannot change that (i.e., the fact that we don't know from
what row the ciphertext was read), which increases our effort by 26-fold.
•
For each assumed disk choice, we look up another period of the cipher-
text to see whether or not this choice will produce a fragment of some
meaningful plaintext in a row. Only a few possibilities will generally
remain.
