Excerpted from:
Survival Skills for the Modern Spy, 2nd Edition
By Giacomo Fleming, Belle Maclean, and S. Gromonowitz
Basic
Caesar Cipher or Caesar’s Code
Codes have been in use as long as people have had to communicate something they didn’t want the whole ruddy world to know. One of the oldest and most widely used is the Caesar Cipher, named after Roman Emperor Julius Caesar, who used it in his private letters.
This simple encryption technique relies on substitution. Each letter in the plaintext (original message) is replaced by a letter some fixed number of positions down (or up) the alphabet. For instance, with a shift of 2, A would be replaced by C, B would become D, and so forth.
This code is easily broken, especially in our computer age. However, in Caesar’s day, when many people could barely read, it proved effective enough in protecting his military messages from prying eyes. This is an excellent cipher for the beginner to start building his or her experience of cryptology.
EXAMPLE:
To encode a message in Caesar Cipher, first align two alphabets; the cipher alphabet being the plain alphabet shifted left or right by a designated number of spaces. For instance, here is a Caesar Cipher using a right rotation of two places:
When you’re encrypting a message, look up each letter in the “plain” line and replace it with the corresponding letter in the cipher line, like so:
To enable deciphering, tell your contact person the key—the number of spaces shifted left or right—and they will perform the process in reverse.