At first glance the Vigenère Cipher appears to be unbreakable, due to its use of up to 26 different cipher alphabets. Ciphers like this, which use more than one cipher alphabet are known as Polyalphabetic Ciphers. These can be incredibly difficult to decipher, because of their resistance to letter frequency analysis. Indeed, over time, the Vigenère cipher became known as 'Le Chiffre Undechiffrable', or 'The Unbreakable Cipher'.
It wasn't until 1854, over two hundred years later, that the Vigenère Cipher was finally cracked by the British cryptographer Charles Babbage. Babbage employed a mix of cryptographic genius, intuition and sheer cunning to break the Vigenère Cipher. Amazingly, his work was never published in his lifetime, and it was over a hundred years later, in the 1970's, that his technique was finally made public.
By linking to the pages in the left hand menu bar you can see for yourself how the Vigenère cipher was cracked. This section also contains a tool to allow you to crack the Vigenère Cipher for yourself.