Re: Basic tools of encryption: Transposition and substitution?

Doug <doug@null.com> writes:
> Peter Fairbrother wrote:
>
> > In lossless substitution the entropy remains unchanged. In lossless
> > compression the entropy also remains unchanged - no entropy is lost - the
> > message space shrinks.
>
> Hm. Not true:
>
> Eg.
> AABBBBAABBBB -> AA (compressed) Code: A = AABBBB
> AABBBBAABBBB -> BBCCCCBBCCCC (substitution)
>
> H(AA) != H(BBCCCCBBCCCC)
> H(x) = - Sum Pn * log(Pn)

Take care. If you change the meaning of your tokens, you need
to change the model by which you are measuring entropy.
In particular, your above example does not prove your 'Not
true' assertion.

Phil

-- 
What is it: is man only a blunder of God, or God only a blunder of man?
-- Friedrich Nietzsche (1844-1900), The Twilight of the Gods