I wanted to use Vigenere as the cipher for the 3rd message on the competition to win a domain name, but changed my mind due to the number of online tools that might help decode this cipher easily even if you know nothing about how Vigenere works.

Before you start reading about how deciphering Veginere works, I invite you to take a look how Vigenere is used to cipher messages from the Wikipedia article here. In fact, Vigenere was called “le chiffre indéchiffrable” or the undecipherable cipher, because any means of cryptanalysis invented before it was defeated, till Charles Babbage found a clever, very clever, way to crack it.

After finishing reading the Code Book the last week, I started decoding the different enigmas proposed at the end of the book; and yesterday I started deciphering the 4th enigma which is a Vigenere cipher (that I finished yesterday too), and I find it pretty amazing for starters to try. I must mention that this short article will contain the solution to the enigma.

I chose to go old way while deciphering this, finding the key by hand, then decoding the message step by step.

Babbage’s method for finding the key length was that the repeated text in the ciphered message is probably produced from the same repeated letters from the clear message and which are ciphered with the same part of the key. Hence, finding an estimate between the repeated series of letters in the ciphered message would give us a good hint about the length of the key.

As you can see the series of letters T, H, I is ciphered to the same series V, V, L, because they were situated at the same relative position according the key; notice also that E, A is ciphered to the same S, D.

The key length is at worst the spacing between these repetitions in the ciphered text, and usually it is the greatest common divisor of the spacing of (most) the repetitions (why? give it a second thought.).