From: Bill Stewart <stewarts@ix.netcom.com>
Date: Tue, 29 Apr 1997 19:15:07 -0700 (PDT)
To: John Young <jya@pipeline.com>
Subject: Re: Fax Security Program
In-Reply-To: <1.5.4.32.19970427175927.0075b2f8@pop.pipeline.com>
Message-ID: <3.0.1.32.19970429184138.0063fb50@popd.ix.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


At 01:59 PM 4/27/97 -0400, John Young wrote:
>The TV show "Computer Chronicles" Saturday featured
>various comm security products, one of which used a fax 
>encoding system with unusual capabilities to transmit color
>images, condensed text (say, a 30-page doc encoded
>on a single fax page) or any other digital data as visual
>clutter.
...
>We missed the name of the product and would appreciate
>hearing from anyone who did, or who knows of the product.

There was a long discussion / flamewar about a product like this
a year or three ago; you can check the archives.
This system sounds a bit more interesting than the one
that someone was touting the last time, but essentially 
any system like this will have a subset of the following steps
0) Scan and digitize the original into a file, if it's not already online.
1) Compress the file (often combined with Step 0.)
2) Encrypt the file
3) Represent the file as a bunch of bits, possibly pixels (needs to add enough
	redundancy to recover from errors in Steps 4 and 5,
	and government-mandated anti-fax-spamming citizen-identification :-)
4) Transmit the pixels using fax or modem or Internet protocols
5) Print onto paper and rescan (if receiver isn't a computer)
6) Recover the encrypted compressed file from the pixel representation
7) Decrypt the file, if encrypted
8) Decompress the file
9) Print onto paper, if desired.

Regular fax machines, and many scanners, combine steps 0 and 1;
regular fax skips steps 2, 3, 5, 6, and 7.
If the original is already a file, or is a more complex picture than G3 fax,
most products will use some proprietary representation, or maybe G4.
One snake-oil product used "Visual Encryption", which meant that they
represented the file as blocks of pixels, shuffled the pixels in some
not-blazingly-secure manner that had enough redundancy for error recovery,
and transmitted that.  

Alternatives: 
1) If you wanted to, you could do the pixel-representation before the 
encryption, and use strong encryption, but then you'd either need
to transmit directly to a computer instead of a fax machine or risk
major lossage from printing and rescanning, and you'd be encrypting far more
bits than you really need to, for no particular benefit -
unless your pixel-representation is a simple black/white pixel per bit
(i.e. you skipped the error correction or did it on the raw data.)
2) You could use a fax or modem to transmit the data as analog sound,
digitize and encrypt the sound, and transmit that over a modem or internet.
This has the advantage that you can use a regular fax machine and
it doesn't need to know you're playing with the audio, though if you
do it crudely you need to transmit a lot more bits,
unless you're using an ISDN connection which was already bits.
2a) You could "digitize" the sound by recognizing it as fax/modem tones and
demodulating into the original pixel-representation bits.
This can save you bandwidth (especially if you transmit your vanilla-9600-bps
fax signal over a 28.8kbps modem.)  Of course, if you were going to pay
to do that anyway, you probably should have just encrypted the data
on a PC and transmitted it by modem anyway instead of messing with Faxes.

#			Thanks;  Bill
# Bill Stewart, +1-415-442-2215 stewarts@ix.netcom.com
# You can get PGP outside the US at ftp.ox.ac.uk/pub/crypto/pgp
#     (If this is a mailing list, please Cc: me on replies.  Thanks.)