Caesar Cipher
First century B.C.: Julius Caesar introduced a simple encryption technique in which he replaced each letter in the plain text uniformly with a letter three positions down the alphabet. Thus 'A' would become 'D', 'Z' would become 'C' and so on. Caeser's use of this technique was recorded by Suetonius, a Roman historian during Caesar's period, in his work titled, "Vita Divi Juli" (The Life of Julius Caesar). This technique is now called Caesar's Shift or Caesar's Cipher. Though Julius Caesar used a transposition 'key' of three, later variations of the Caesar's Shift employed other keys as well.
The strength of this cipher may have been adequate during Caesar's period - a time when literacy levels were very low. To a person who does not know to read and write, the plain text would make no more sense than the encoded message. Nevertheless, Caesar employed this method to protect his messages from prying eyes. However, given today's advances in cryptanalysis, deciphering a message encoded using the Caesar's Shift should be a walk in the park, even for the amateur cryptanalyst. A simple brute force attack would reveal the encoded message, as there are only twenty-five possible keys: probably a 10 nanosecond job for a computer. Even if the cipher system used to encode the message is not known, a frequency analysis on the cipher text would easily reveal whether the Caeser's shift was used.