Chapter 1: Julius Caesar's Shift Code
Julius Caesar devised a method to send messages to his commanders that could not be read by his enemies. Each letter is replaced by a letter three down in the alphabet. For example the English phrase ATTACK AT DAWN would be written as DWWAFO DW GDZQ. The ROT13 cipher is similar except you shift 13 instead of 3.
Such ciphers are very easy to break. Luckily most of Caesar's enemies couldn't read.