The Da Vinci Code, Enigma, the key to the battle of Midway - codes and encryption have been with us in fact and fiction since before the time of Caesar.
Control of a battlefield, manipulation of strategic events, knowledge of an enemy's capabilities and vulnerabilities, all require communication between the leader and those carrying out the leader's orders and providing information upon which the leaders bases decisions. Equally important is the denial of a foe that same information, direction and control process. These communications often are too detailed or extensive - or important - to trust to a verbal exchange.
I expect that such needs were the second reason why man developed the various forms of writing. The first was probably historical, a way to record the deeds of a ruler - to ensure history was interpreted the way the ruler intended. Cryptography probably dates from the time of the first written communiqué. Certainly we know cryptography was known to Arab, Greek and Roman scholars, and was in common use at the time of Christ.
Doug Stinson, of the University of Nebraska at Lincoln, is the author of extensive lecture notes, information available in many basic texts on cryptology, and as further explained in his textbook. In his notes on substitution codes Stinson says:"In a substitution cipher, the letters are systematically replaced by other letters or symbols.
Caesar's Cipher (Simple shift - monoalphabetic) In this classical cipher, each letter is replaced by the letter that is 3 positions further along in the usual lexographical ordering. Thus, "A" is replaced by "D", "B" is replaced by "E", and so on. In general, a shift cipher replaces the letters by some cyclic shift of the alphabet. This is most easily done by assigning the letters numbers from 0 to 25. Each letter of the clear message is replaced by the letter whose number is obtained by adding the key (a number from 0 to 25) to the letter's number modulo 26. In the Caesar cipher the key is 3."
Our riddle - The Image of Christ - is a simple form of the monoalphabetic shift. How is it broken? Knowledge of (or a series of guesses) concerning the probability of occurrence of the letters used by the particular language in which the code is written and a simple analysis of frequency provides the starting point.
Example of the frequency of occurrence in a 135 letter English paragraph provided in the lecture notes:
So you can see that when you look at this particular example "e" is the most likely letter, followed by "t", with "y", "b" and "g" as the least likely. A statistical analysis of large blocks of English text will provide similar results, varying only in minor placements of some of the lower frequencies if you happen to pick a "peculiar" block of text. The "e", "t", "a," and "o" frequencies are usually enough to get you started picking out words.