secure fax (Re: email Crypto- third party)

[Paul Leyland]

> From: Pete Chown [mailto:Pete.Chown@skygate.co.uk]
> Tom.A.Parker@icl.com wrote:
> 
> > However you can't do a straight overlay because it
> > doesn't give an XOR.
> > 
> > Black + Black = Black
> > Black + White = Black as well.
> 
> If you don't mind halving the resolution, you could use two 
> dots on the
> fax for each dot in the real image.  Then you get the XOR like this:
> 
> 0 xor 0 = 0      [*-] on transparency [*-] on fax gives [*-]: grey
> 1 xor 0 = 1      [-*] on transparency [*-] on fax gives [**]: black
> 0 xor 1 = 1      [*-] on transparency [-*] on fax gives [**]: black
> 1 xor 1 = 0      [-*] on transparency [-*] on fax gives [-*]: grey

An admirable summary, except the original proposal used a 2x2 patch of
pixels to give the same resolution in both axes.

It really works very well, and has exactly the same benefits and
disadvantages of conventional stream ciphers, including the problems of
using a mask (equivalent to keystream) for more than one message.

Paul