Cryptography Reference
In-Depth Information
is a helpful hint for cryptanalyzing it. I encourage you to try it using pen and
paper. You will get an idea as to how the cryptanalysts in World War I must
have felt.
However, it is shocking to find this ancient method still around. According to
[BauerMM], it was introduced to the Russian army in 1915 when it transpired
that harder methods had overstrained the top echelon. One could hardly have
given the cryptanalysts in Prussia and Austria a nicer present. (Meanwhile,
however, Russian cryptology has long been up to date.)
Quite another matter is ROT13, a method widely used in UNIX, which repre-
sents nothing more than a Caesar cipher with key '13'. ROT13 was not designed
to protect data by encrypting it; it serves to protect data against inadvertent
reading — just like newspapers often print the solution to a puzzle upside down
underneath the puzzle section. One simply had to think of something different
for computers.
There was a simple reason why 13 was chosen of all numbers. Encrypting the
ciphertext once more produces the plaintext:
ROT13(ROT13(Text)) = Text.
2.2 About Gold Bugs and Rhymes: Substitution
and Transposition
2.2.1 Simple Substitution
The Caesar cipher is a special case of a much more general method: simple
substitution. With this method, each letter of the alphabet is substituted by
any other letter. The only side condition is that two different characters must
not be substituted by the same letter (e.g., never substitute both A and X by
C); otherwise, the ciphertext cannot be decrypted unambiguously. This kind
of mapping the alphabet onto itself is called
permutation
(sort of rearranging
the alphabet). While the Caesar method uses 25 possible keys, the number of
theoretically possible substitutions is astronomically high, namely 26! or
403.291.461.126.605.635.584.000.000 (403 quadrillions).
Several possibilities can be disregarded though, because they leave exces-
sively large parts of the plaintext unchanged. That leaves perhaps only 400
