Re: [NEWBIE]vigenere

cicap wrote:
> How can I give a proof that Vigenere is not perfect, using Shannon theory?
> For example with a key length of 4, against a plain text of 8 characters.

   For perfect encryption, obtaining the ciphertext does not change your
a priori probabilities of various plaintexts. When you obtain the 8
character Vigenère ciphertext you speak of, though, a lot of plaintexts'
probabilities go to zero, namely those where the Vigenère key used for
the first four characters would be inconsistent with that used for the
second four characters.

--Mike Amling
Received on Tue Jan 17 16:50:20 2006