From: stewarts@ix.netcom.com
Date: Thu, 5 Dec 1996 01:47:02 -0800 (PST)
To: cypherpunks@toad.com
Subject: Laptops and TEMPEST
Message-ID: <1.5.4.32.19961205094443.003ac018@popd.ix.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


>Not to mention the point that an external attacker--say, the NSA van parked
>across the street--will under no circumstances be able to measure "the"
>spectrum: his antennas cannot possibly measure the signals (at the lower
>bits) seen by the FM receiver, noise source local to the computer, whatever.

Tim was talking about the Bad Guys setting your radio noise generator,
but the other side of the coin is TEMPEST - making sure your computers
don't emit enough radiation for Bad Guys to read it.  CRTs are well known
as emitters of easily decoded signal, but people have occasionally suggested
on this list that laptop LCD screens are much quieter.  I now have a data
point on this one, and basically, it ain't so.

Take a basic television with big rabbit-ear antennas.
Tune to the football game on Channel 6.
Take your AT&T Globalyst 250P (which is a gray NEC Versa with a Death Star),
with the 16-million-color 640x480 screen in 65536-color mode, 
and pop up a DOS command window in white-on-black.
Type a few lines of text, then look at the TV.

The sync wasn't quite right, but there were about three copies of
my DOS window.  It may have been scrolling slowly vertically or horizontally,
but it was relatively readable given the lack of resolution of the screen.
A good receiver run by a Bad Guy ought to be able to set its scan rates
correctly to pick up the screen at better resolution.
There are obviously more variables to be explored, but other people
who were present at the time considered the football game to be
more important :-)

#			Thanks;  Bill
# Bill Stewart, +1-415-442-2215 stewarts@ix.netcom.com
# You can get PGP outside the US at ftp.ox.ac.uk
#     (If this is posted to cypherpunks, I'm currently lurking from fcpunx,
#     so please Cc: me on replies.  Thanks.)