Cryptography Reference
In-Depth Information
line. In addition, if the 'PP' of UNTERGRUPPENFUEHRER happened to fall
in the position above the 'II' of DNIIEPROPETROWSK (such names occurred
frequently in radiograms on the eastern front), both 'PI' pairs ciphered into
identical digrams. There weren't that many possibilities of this sort that one
wouldn't have been able to try them all out — without the help of computers,
of course, but lots of intuition, enormous staffing, and huge time pressure. But
once the 'magic squares' were constructed, they could be used to decrypt all
messages encrypted in this way on the same day at one go. If a radio operator
inadvertently used the key of the previous day (which the British already knew)
and sent the same, unchanged message again, encrypted with the 'new' key,
the British jumped for joy.
2.4 Permanently Changing Tactics: Polyalphabetic
Substitutions
A major vulnerability of simple substitutions is the fact that they are reversible:
each character in the ciphertext always corresponds to the same plaintext char-
acter, no matter where exactly the ciphertext character stands within the text,
which means that characteristic patterns are preserved. For example, looking at
the encrypted word WLRWJXL and using an electronic dictionary, it shouldn't
be too hard to find out that the plaintext probably reads SEASIDE. (We have
already learned that word boundaries disappear since blanks are left out, but a
computer won't have any problem searching the text for certain patterns.) This
statement also holds true for digrams.
The idea behind
polyalphabetic substitution
is to make the substitution rule
dependent on the position in the text. Initial thoughts in this direction had been
expressed by Alberti in 1466. Some think this was the birth of modern cryptol-
ogy. Though polyalphabetic methods are broken by computers nowadays, they
are still much harder than simple substitution.
2.4.1 The Vigenere Cipher
It is easiest to go back to the above representation of the Caesar addition method
to describe the simplest case of polyalphabetic substitution:
c=a+smod26
