>>>Well if they don't know the key length, then using somewhere between
>>> 1/3 and 2/3 should make it unbreakable.
>
>> No, using the full length is necessary. Using a key length less than the
>> full length of the message is very dangerous.
>Hmm, as far as I'm aware, you can't do cryptanalysis on 2 characters.
>Marxos
A factoid which is irrelevant in this context.
If you only wanted to encrypt two characters, once, you could use ROT-13 and nobody would be the wiser.