secure fax (Re: email Crypto- third party)

[Tom.A.Parker@icl.com]

Paul Leyland wrote:
>If all you want is to keep nosy people out of your faxes, as opposed to the
>true professionals who are capable of black-bag and chequebook
>cryptanalysis, you could use Adi Shamir's technique of a random mask
>overlay.
>
>For those who haven't heard of this trick, Alice and Bob each have an
>identical random matrix of black and transparent squares printed onto an
>acetate sheet.  The transmitted fax is essentially the XOR of this mask and
>the original image.  Bob places the transparency on top of the received
>paper fax and jiggles the alignment until the image shows through in black
>and grey.  Remarkably effective and remarkably easy to use.   Of course,
the
>acetate mask has to be snail-mailed or couriered to the recipient
>beforehand.
>
>If the transparency is truly random and used only once, it is a true
>one-time-pad with all that entails.

I had occasion to try using transparencies to demonstrate one time pad
principles in a crypto lecture I gave some time ago. The aim was to use
overlays to show that choosing two different keys caused two different
apparent messages. However you can't do a straight overlay because it
doesn't give an XOR. 

Black + Black = Black 
Black + White = Black as well.

To solve this problem I had a rather complicated design of concentric black
and white circles. It worked, sort of, but I would be interested in more
details of the fax approach, which on the face of it doesn't seem to work.

Regards,

Tom Parker