[8379] in cryptography@c2.net mail archive
Re: Cryptographic Algorithm Metrics
daemon@ATHENA.MIT.EDU (Paul Crowley)
Wed Jan 3 22:24:58 2001
To: Peter Fairbrother <peter.fairbrother@ntlworld.com>
Cc: Greg Rose <ggr@qualcomm.com>, John Young <jya@pipeline.com>,
<cryptography@c2.net>
From: Paul Crowley <paul@cluefactory.org.uk>
Date: 04 Jan 2001 01:23:30 +0000
In-Reply-To: Peter Fairbrother's message of "Wed, 03 Jan 2001 22:38:00 +0000"
Message-ID: <87y9wst8j1.fsf@hedonism.subnet.hedonism.cluefactory.org.uk>
Peter Fairbrother <peter.fairbrother@ntlworld.com> writes:
> Not so. Perfect compression with encryption works too.
Er, does it? I get a 1k message from you, perfectly compressed and
then encrypted with some strong algorithm and a 128-bit key. As a
godlike being unhindered by constraints of computational power, I try
all 2^128 possible keys, and find due to the perfect compression that
each of the 2^128 plaintexts is equally likely. From an information
theoretic point of view, I'm much better off than I was before: I used
to be missing 8192 bits of entropy, but now I'm only missing 128 - the
space of possible messages has been vastly reduced. Put it this way,
if all I want to know is whether you're asking for a ticket to the
dance, I might well learn the answer since I might find that none of
the candidate messages include that request.
A 1k message encrypted with OTP, however, tells me nothing whatsoever.
--
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