The keyword is then repeated to create a string the same length as the plaintext. The plaintext is stripped of all characters except the letters a-z and A-Z, and then converted to upper case. For this example I will use " Imagination is more important than knowledge." as plaintext and " orchestra" as a keyword. This can be anything but ideally should be of a reasonable length. To encipher a string of text you need a keyword. The processes of both encipherement and decipherement involve the use of a table of letters, variously know as a Vigenère Square, Vigenère Table of Tabula Recta, which looks like this. It may even be encrypted as itself which the famous German WW2 Enigma machine could not do, providing the Allies with a way to crack the code. The Vigenère Cipher is more sophisticated in that a letter may be encrypted as any other letter depending on its position in the plaintext. The problem with the Caesar Shift Cipher is that each letter is always enciphered the same way, so for example if E, the most common letter, is always enciphered as S then simple frequency analysis will reveal the shift if you have a reasonably long piece of encrypted text.
#SHIFT CIPHER FULL#
If you want to read up on it in full check out the Wikipedia article It was not broken until 1863 although these days it is highly insecure and is now only useful as an interesting programming exercise. The Vigenère Cipher was invented in 1553 by the Italian Giovan Battista Bellaso but is now erroniously named after the Frenchman Blaise de Vigenère. I will now expand on the theme by implementing the Vigenère Cipher. A while ago I wrote a post on implementing the Caesar Shift Cipher in Python.
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