Cryptography Reference
In-Depth Information
this case the carrier of the information. What the eavesdropper can measure,
and how, depends exclusively on the laws of physics. Using quantum phe-
nomena, we can design and implement a communication system that can
always detect eavesdropping. This is because measurements on the quantum
carrier of information disturb it and so leave traces. What follows is a brief
overview of the quest for constructing unbreakable ciphers, from classical to
quantum.
1.1 Classical Origins
Human desire to communicate secretly is at least as old as writing itself and
goes back to the beginnings of civilization. Methods of secret communication
were developed by many ancient societies, including those of Mesopotamia,
Egypt, India, China, and Japan, but details regarding the origins of cryptology,
i.e., the science and art of secure communication, remain unknown.
We know that it was the Spartans, the most warlike of the Greeks, who
pioneered cryptography in Europe. Around 400
B
.
C
. they employed a device
known as the scytale (Figure 1.1). The device, used for communication be-
tween military commanders, consisted of a tapered baton around which was
wrapped a spiral strip of parchment or leather containing the message. Words
were then written lengthwise along the baton, one letter on each revolution
of the strip. When unwrapped, the letters of the message appeared scrambled
Figure 1.1
Scytale (top) and Alberti's disk (bottom) were the first cryptographic
devices implementing permutations and substitutions, respectively.
