From: mathias@solomon.technet.sg (Mathias Koerber)
Subject: Re: Fax with encryption?
X-Newsreader: TIN [version 1.1 PL8]
Date: Tue, 12 Jan 1993 01:20:38 GMT

Thomas Sippel - Dau (vulture@imperial.ac.uk) wrote:
: In article <C0oMzE.LD6@newsserver.technet.sg>, mathias@solomon.technet.sg (Mathias Koerber) writes:

: You can either encrypt the TEXT of the fax message, which the receiving
: fax machine will then print out as a faithful rendering of the garbage
: that comes out of the crypter.

This is hard for casual use. A) People would have to know about encryption,
and B) they would have to transcribe the message into another decryption
machine or so.

: Or you can view the bitstream that represents the pixels on a sheet of
: paper as a message, and encrypt that. This would need a configurable lookup
: table in the receiving fax machine, and a keyword exchange mechanism, 
: presumably a public key system. For this it would probably be easier
: to use a different modulation scheme - i.e. do the encryption at the
: physical/data link interface than at the transport or presentation
: layer. 

No, no. What I'm thinking of is just an extension to the existing protocol.
Like now the faxes can negotiate whether a fax is in normal or fine mode,
there should be bit that tells the receiving fax that the message was 
encrypted. The key does not even get passed, nor does a part of it.
Both sides will have to agree on a key to use, which is then keyed into the
fax-machine.

Picture this:

	- A goes to his fax machine with a confidential message for B
	- He selects "encrypt" plus all the other options
	- The fax machine prompts him for a key (preferably some big
	  number)
	- The faxmachine scans in A's message, line by line. Every line
	  is encrypted using a well-known algorithm with the entered bignum
	  as key. the encrypted line is then sent over to B's fax machine

	- While negotiating resolution etc, B's fax machine was told that
	  the message is encrypted. It reacts to this by not printing out
	  the message, but storing it in memory and just printing a
	  short reminder that an encrypted fax from number xxx has arrived.
	- The recipient is notified, comes to the fax-machine and types in
	  the command to print-out the stored fax. The fax prompts him 
	  for the key, which is then applied to all lines of the fax 
	  before it is printed out. The message in memory is in fact *not*
	  decrypted, only the printout is.


The whole scheme is just intended to make it difficult for unauthorised
people to read confidential incoming faxes. It sure will be possible to
listen to the telecom-line, grab the fax off and apply all combinations
of bignums to it, until a nice picture appears, but guarding against that
would be overkill for the normal user.

Another scenario is that it would be easy for anyone to impose when faxes
are stored for retrieval by a remote machine. Since not all telecoms
systems have caller-identification, and the current fax-protocol relies
on the fax-machine to send identification information, plus the fact
that currently the passwords used for allowing a remote fax to retrieve
faxes at a later time are rather poor (They are short, only numbers and
given out to almost everyone who might want to retrieve your fax, plus
the fact that there is only password per fax-machine instead of message),
it would be a good thing to have this additional security.



: But since you would give up the unversality of fax for security, you 
: might as well go the whole hog and use encrypted email to send your
: message - including scans of handwritten text and drawings. Probably 
: much cheaper in the long run.

:                                  Thomas

Again, too hard for casual users or secretaries who just want to get that
confidential drawing over to the other side.


regards, Mathias -- 
Xref: sparky comp.dcom.fax:2417 comp.dcom.modems:19301