Basic Cryptography

The aim of cryptography is to develop systems that can encrypt plaintext into ciphertext that is indistinguishable from a purely random collection of data. This implies that all of the possible decrypted versions of the data will be hopelessly ambiguous, with none more likely to be correct than any of the others. One of the simplest ways to create ciphertext is to represent each character or word in the plaintext by a different character or word in the ciphertext, such that there is no immediately apparent relationship between the two versions of the same text.

Early Ciphers
To see basic encryption in action, and understand the evolution of today’s cryptography, consider a cipher used by Julius Caesar, illustrated in the following table. Note that the letters of the alphabet are simply shifted several places.
Plaintext: a b c d e f g h i j k l m n o p q r s t u v w x y z
Ciphertext: E F G H I J K L M N O P Q R S T U V W X Y Z A B C D

To encrypt a message, the sender finds each letter of the message in the plaintext alphabet and uses the letter below it in the ciphertext alphabet. Thus the clear message:

Plaintext: beware the ides of march
Is transformed into the encrypted message:
Ciphertext: FIAEVI XLI MHIW SJ QEVGL

This type of cipher is known as a substitution cipher. Although the Caesar cipher is relatively simple, substitution ciphers can be very powerful. Note that most examples of the Caesar cipher shift the alphabet three places so that the ciphertext line begins with D, but some authors suggest Caesar might have used other numbers, so the term Caesar cipher is used for all ciphers that conform to this algorithm (an algorithm being a formula or recipe for solving a problem).
This level of encryption might seem rudimentary, but it is an important starting point for much that follows. For example, one way to visualize the Caesar cipher is a pair of rings, one inside the other, as shown in the following figure.

Both circles contain the letters of the alphabet. If one is rotated relative to the other the result is a cipher wheel, something well suited to automation. Eventually this happened, at first mechanically, then electrically, and today digitally. Automation facilitates repetition and messages encrypted with a substitution cipher can be a lot harder to decipher if multiple different substitutions are used. Thus the code wheel earned a place in the seal of the NSA, the US government agency most influential in the development of encryption.