Re: What is a "perfect secret" ?

From: Douglas A. Gwyn (DAGwyn_at_null.net)
Date: 09/30/04 

Date: Thu, 30 Sep 2004 02:42:05 -0400

Gustavo L. Fabro wrote:
> What is a "perfect secret"?

A perfect secret is one that can never be known to anybody.

Presumably you really want to know about "perfect secrecy"
instead.

> I saw one giving an example of a 'perfect secret' being a simple XOR on
> a plain text, using a key with the same size of the plain text. Something
> like this would be impossible to break since the same possibility would occur
> for,
> say, 'cat', 'rat', 'tap', 'dog' and so one.

You have several details wrong. The key *must* be
indistinguishable from white noise. It's probability
(or more accurately in this context, likelihood), not
possibility. And the possible plaintexts are *not*
all equally likely. However, their relative
likelihoods given the ciphertext is exactly the same
as their relative likelihoods without having the
ciphertext (which equals their relative probabilities
in the source language applies to the known context).

> something in some language? Like, in English, if 20 characters XORed with a
> brute force trying key matched "the dog is beautiful", and no else key did
> that, wouldn't the cipher be, in that case, successfully hacked?

No, because the actual plaintext (corresponding to a
different key) is "the cat is so hungry".