From: jim bell <jimbell@pacifier.com>
Date: Thu, 5 Dec 1996 17:31:16 -0800 (PST)
To: Joel McNamara <joelm@eskimo.com>
Subject: Re: Laptops and TEMPEST
Message-ID: <199612060131.RAA19977@mail.pacifier.com>
MIME-Version: 1.0
Content-Type: text/plain


At 02:18 PM 12/5/96 -0800, Joel McNamara wrote:
>Thinking that LCD screens reduce the risks of emanation monitoring is a
>dangerous misconception (at least under a high threat model).  

I disagree that it's a "dangerous misconception" to believe that "LCD 
screens reduce the risks of emanation."

While it might be correct to say that it would be dangerous to believe that 
they ELIMINATE the risks, LCD displays would make it exceedingly difficult 
to read the screen remotely.  The main reason is that the LCD segments are 
activated in parallel, which means that the electronic noise associated with 
a particular row is a smeared product of each portion of the display.  This 
is quite unlike CRT's, in which individual pixel information is completely 
(well, except for color) demultiplexed by time.


> However, the gotcha with current laptops is their
>backlighting.  Electric and magnetic fields are considerably higher
>compared with a low-res/contrast device.

What does the backlighting have to do with anything?  The backlight and its 
noise is not modulated (at least, not in anything less than the optical 
band) by the imformation provided on the screen.


Jim Bell
jimbell@pacifier.com