From: wcs@anchor.ho.att.com
Date: Wed, 19 May 93 18:12:27 PDT
To: cypherpunks@toad.com
Subject: TEMPEST
Message-ID: <9305200112.AA02467@anchor.ho.att.com>
MIME-Version: 1.0
Content-Type: text


>   Where can I find more info about tempest?  Is it a roomwide thing, is it
>   implemented in the cables and housings, or what?  I understand that the
>   purpose of Tempest is to prevent people from spying on you via electronic
>   emissions detection.  Is this just a glorified Faraday cage?

Both techniques are used.  You can either buy TEMPEST-designed equipment,
which is designed for low emissions, separation of signals between classified
and unclassified components, shielded cables, etc., or you can build a 
shielded box or room and use special filtered power supplies, fiber optics, etc.

The exact standards are classified, but they're a lot stricter than FCC Class A or B.
The shielded-room vendors out there also sell to the electromagnetic-compatibility-
testing market, who want to have nice quiet rooms to measure emissions from their
equipment in.  Last time I saw one of these rooms built, about 5 years ago,
typical construction used plywood sheets with thick sheet metal 
on each side, fancy connectors between plywoods, copper-wool crammed in any cracks,
and special waveguide meshes for air vents and fiber-optic communication cables,
and gives about 100-120 dB shielding for frequencies up to about 1-10GHz.
Twenty years ago, typical construction used copper screening and was good to ~60dB.

About 3-4 years ago, the typical cost for a TEMPEST PC was ~$4000 more than
the non-TEMPEST equivalent, and the equipment was maybe 1 year behind the
commercial models due to integration and testing time.
TEMPEST mini-computers, if they were small enough, generally took the approach of 
putting the standard versions of the machine in a box built like the TEMPEST rooms;
TEMPEST PCs had a somewhat more integrated design, though they were starting to use
commercial motherboards.

			Bill Stewart