The Vigenere encryption was the creation of the French diplomat, Blaise de Vigenere, 1523-1596. Like Caesar and all the cryptographers that followed, he did not visualize the cipher in modular arithmetical terms. Rather he viewed the cypher as a substitution cipher where a different alphabet was used for the next letter of the message, with the alphabets repeating periodically --- according to some key. Rather than setting several different alphabets, the cryptographer would use the Vigenere square.

Here's the idea. For the given key word "FIRST", encrypt each letter of the message taken in the left-most column to the letter in the keyword-letter column. Thus, the first five letters of the message use the alphabets corresponding the the "F", "I", "R", "S", and "T" columns. So, the Vigenere code with this keyword is really five Caesar shifts used in a cyclical fashion. Decription is carried out working backwards from the keyword-columns to the left-most column. Because we are really using five alphabets, the Vigenere encryption is sometimes called a polyalphabetic (many + alphbets) code.

This encryption never really caught on, partly because it was difficult to use for the encryption and decryption of hundreds of messages daily as required by the military.

It was broken for the first time by a Prussian major named Kasiski in 1863. He proposed a method for breaking a Vigenere cipher that consisted of finding the length of the keyword and then dividing the message into that many simple substitution cryptograms. Apply frequency analysis to decode the multiple cryptograms. Another to break the code was computing engine pioneer Charles Babbage more than three hundred years later. Babbage applied a very careful analysis on the structure of groups of letters and a great deal of hard work. The Vigenere cypher today is just moderately good. No serious cryptologist would use it for secure information transmission.