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.