Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!cmcl2!brl-adm!brl-smoke!gwyn
From: gwyn@brl-smoke.ARPA (Doug Gwyn )
Newsgroups: sci.crypt
Subject: Re: how do you tell encrytped data from random data?
Message-ID: <7037@brl-smoke.ARPA>
Date: 11 Jan 88 09:50:34 GMT
References: <660@bucket.UUCP> <6992@brl-smoke.ARPA> <1499@osiris.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 44

In article <1499@osiris.UUCP> mjr@osiris.UUCP (Marcus J. Ranum) writes:
>Another problem with Gywn's suggestion that the only way to tell if it
>is encrypted (decrypt it) - how can you tell that it is really "meaningful"?

It's not that hard in practice.  Witness the following, which have been
uniquely cryptanalyzed into plaintext which all involved agreed was
correct (and in cases where it could be checked with the encryptor,
its correctness was verified):
	(a)  the Chaucer transposition with spelling variations that
	     was posted to this newsgroup not long ago;
	(b)  "challenger" cryptograms
Some of the latter are extremely short, and have plaintext that nobody
in his right mind would really use.  I've personally broken quite short
messages, some 25 letters long, encrypted by some of the simpler
methods.  Usually, student cryppies would all get essentially the same
result (sometimes off in one or two letters) for cryptanalysis of
practice cryptograms, if they were able to crack them at all.

It is extremely hard to make up a cryptogram of modest length that
has two valid but different decryptions.  In fact, I don't know of
any of length more than a couple of dozen letters, although I can
imagine some ways to try to do this for somewhat longer messages.
This would make an interesting puzzle for someone.

On the other hand, there are people such as those who find Baconian
ciphers in Shakespeare's works, who put the message there themselves.
It's pretty easy to tell when this happens; either the decipherment
method is ambiguous, so the message could never have been
realistically expected to be read even by its intended recipient,
or so many "corrections" and otherwise unjustified special twists
on the decipherment scheme have been introduced that the
information content of the decipherment rivals that of the supposed
message being extracted.  It is interesting to note that William
F. Friedman, perhaps the greatest cryptanalyst of all time, and his
future wife got their start in the field by being employed at "Col."
Fabyan's Riverbank Labs, which was trying to find just such hidden
messages in Shakespeare.  The Friedmans showed how the same
methodology could be used to produce messages with precisely
opposite meaning, i.e. the methodology was making up the whole
content of the messages based on the analyst's preconceptions
about what was to be found.  On the other side of this issue,
several rum-runner codes (generally much more ambiguous to crack
than ciphers) were broken by the Friedmans, and they were able to
demonstrate their methodology satisfactorily to the federal courts.