From: jrochkin@cs.oberlin.edu (Jonathan Rochkind)
Date: Wed, 11 Jan 95 22:00:26 PST
To: cypherpunks@toad.com
Subject: Re: How do I know if its encrypted?
Message-ID: <ab3a75370402100421a8@[132.162.201.201]>
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At 12:40 AM 01/12/95, Dale Harrison (AEGIS wrote:
>Won't work!  You can always embed an encrypted message in what 'looks'
>like plaintext.  A trivial example: Encrypt a message with a caesar
>cypher, then build a story where the first char of each word maps to
>each subsequent char from the encrypted text.  At the cost of expanding
>the size of the message by a factor of 5 to 10 you've hidden the
>encrypted message in what looks like a letter to your mother (or a news
>story in the NY Times, etc.)  This is old technique.

The context this was being discussed in, was trying to make _plaintext_
look like _ciphertext_. The operator of a data haven or remailer might
hypothetically want to ensure that all text he dealt with was encrypted.
So your method wouldn't do anything in that area.  Unless you can think of
a way to embed plaintext in ciphertext in such a  way that it looks like
ciphertext, and my guess is that any method that did that well would be
sufficiently obscure as to be analagous to encryption for our purposes.
Really bad encryption, with very little point, but still hidden text. Which
is the real point, the operator doesn't want to deal with any text  that
isn't "hidden" in some way.

Of course, we're not just dealing with text.  So the scheme has got to be
changed a bit so as to be able to detect unencrypted GIFs, and mu-law
files, and as yet to be determined unknown files.  I don't know enough
about what's being talked about to know if this entropy detecting stuff
will generalize to non text files. Cause we want to catch unencrypted GIFs
too.  [And doesn't compression alone do similar things to the entropy as
encryption does, anyhow? If someone compresses their file with a good
compression algorithm, as I understand it the non-randomness left will be
pretty low. But it won't meet the needs we're discussing, I don't think.]