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.

Sounds like a homework problem. Presumably you're expected to
use the definition of "perfect secrecy" and some basic probability
theory in your proof. Once you have some experience with such
matters, it is easy to see that the ciphertext characters at the
1st and 5th positions are related through a constraint induced
by the key repetition, which means that they have less entropy
than would be the case in the absence of the constraint. Just
work that up in your theoretical framework and you will have a
proof.
Received on Tue Jan 17 16:50:20 2006