Breaking A Vigenere Cipher

Vigenere cipher, how to crack the code...

The Vignere cipher was thought for a long time to be indecipherable - in other words, secure.

It did not seem to yield to frequency analysis as it is polyalphabetic and therefore there is a choice of alphabets being used to encode the message.

Therefore the most common letter in one cipher is not being coded with the same letter as in another one of the polyalphabetic substitute languages.

This seemed to make it very hard if not impossible to crack.

However as with most things in the end those who work in crpytanalysis managed to decipher the code.

Here is how it works.

You need a key word which you write above the plaintext to encipher it.

Thus if the key word is code and the message is how are you it might go:

COD ECO DEC
how are you

You then use what is known as the vignere square, a look up table with A to Z going down and across shifted once each time, to find out what the cipher is.

So you look down from the plaintext letter column headed with 'h' to where it meets 'C' to find out what the cipher for the first 'h' is and so on, so for instance how becomes:

VAG

Then the recipient just needs to know the keyword, then uses their vigenere square to reverse the process.

The key is that people tended to use fairly obvious or recognisable words or phrases as the keyword, and therefore with lots of effort and guesswork it could usually be broken due to that.

The simple step of using a meaningless key makes it much harder to crack.

Also there was the fact that many keywords were really quite short, and therefore if you guessed for instance that it was only three letters long or four letters long and the message was of a reasonable length, you could split the text into three different pieces, each using the third letter on.

The result would be, if you were right and the keyword was three letters long, three pieces of text each representing a single monoalphabetic substitution and you could then simply use frequency analysis to work out the letters of each and then recombine to find the plaintext original.

Author: Dan

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Last Updated: Oct 9th 2006

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