From: "Mark M." <markm@voicenet.com>
Date: Sun, 2 Jun 1996 15:25:01 +0800
To: cypherpunks@toad.com
Subject: Re: Compressed data vulnerable to known-plaintext?
In-Reply-To: <199606020213.TAA00981@jobe.shell.portal.com>
Message-ID: <Pine.LNX.3.93.960602001039.2658A-100000@gak>
MIME-Version: 1.0
Content-Type: text/plain


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On Sat, 1 Jun 1996 anonymous-remailer@shell.portal.com wrote:

> But then you still have the problem of identifying the contents.  If there
> were no headers, one could not tell if the message was compressed using
> ZIP, LHA, StuffIt, tar*, compress, gzip, Alice's Magical Supercompressor,
> or even if it was left alone.  One could also not tell if the decryption
> happened successfully.

OK, I think I misunderstood.  You were talking about encrypting a file that
has already been compressed and I was talking about an encryption program that
would compress then encrypt.  In the latter case, headers would be completely
unnecessary, whereas in the former, they are necessary.  Generally, checksums
or parity bits are much better ways of checking if the decryption was
successful.  While this does make cryptanalysis easier, a known-plaintext will
not work.

- -- Mark

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markm@voicenet.com              | finger -l for PGP key 0xe3bf2169
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