Cryptography Reference
In-Depth Information
paper containing lists of random numbers to be added to diplomatic code-
books, with each sheet of the pad torn off after use, never to be reused
again (thus the name
one-time pad
).
In the later stages of World War II, the Germans combined the Enigma
and the Vernam designs into a teleprinter-based cryptographic apparatus,
the Lorenz cipher. Each letter of the plaintext was encrypted individually,
using a random key generated by a system of stepping wheels similar to
the Enigma's rotors. By setting the wheels of the encryption and decryp-
tion apparatus to the same initial positions, an identical pseudorandom
key sequence could be generated at both ends.
41
The system proved no
more resistant to the Allies than the Enigma. Taking advantage of operator
blunders, Bletchley Park's codebreakers eventually deduced the design of
the wheels and went on to build the famous “Colossus” electromechanical
computer to automate cryptanalysis of the intercepts.
42
It would take Claude Shannon to turn Vernam's original intuition into
a full-fledged mathematical theory of cryptography. In his 1945 paper
“Communication Theory of Secrecy Systems,” Shannon enquired: “How
secure is a system against cryptanalysis when the enemy has unlimited
time and manpower available for the analysis of the intercepted crypto-
grams?”
43
He proved the Vernam one-time pad to be, in fact, unbreakable
even by such an enemy if the key was indeed as long as the plaintext,
genuinely random, and used only once. It was the first mathematical result
of its kind in cryptography, the proof of a cryptosystem's efficacy
regardless
of the adversary's computational resources
. Shannon distinguished such “the-
oretical” proofs from those obtaining in the case of “practical secrecy,” in
which the goal is to build systems “which will require a large amount of
work to solve.” The distinction was a conceptual milestone, but one that
also highlighted the gap between cryptographic theory and practice,
between the laboratory and the field, with the one-time pad relegated to
niche applications such as the Washington-Moscow hotline.
44
Shannon's mathematical theory also firmly situated steganography
outside of cryptography proper:
There are three general types of secrecy system: (1) concealment systems, including
such methods as invisible ink, concealing a message in an innocent text, or in a
fake covering cryptogram, or other methods in which the existence of the message
is concealed from the enemy; (2) privacy systems, for example speech inversion,
in which special equipment is required to recover the message; (3) “true” secrecy
