

You are correct (several times). TEMPEST is a codeword for a standard, shared between the NATO governments, to limit the inadvertent emission of information by either electromagnetic radiation or conduction. The limits set in the 
standard are classified but there is open source information to the effect 
that either the electrical or magnetic components of electromagnetism can be 
exploited. It is a basic rule of physics that there is an electromagnetic 
field associated with any path that conducts a flow of electrons.

Among other things, I drive a 1987 Korean built AT clone and an associated 24 pin dotmatrix printer. The major source of unintentional emission is the CRT. To that can be added the video driver card, the RS232 parallel cable and the 
printer head. The emissions from these are gross can be detected with the 
crudest of equipment. Were I to apply good test equipment and some 
intelligence to measuring emission levels, I would find many other potential 
sources of leaked information.

Where cryptography is used for serious purposes, poor TEMPEST protection 
becomes an important security hazard.

In the early eighties, the need to allow some commercial concerns, 
particularly financial institutions, a level of protection against TEMPEST 
threat led to a series of briefings to invited corporations and to the 
'release' of a cut-down TEMPEST stansard for commercial use. I say 'release' 
for while equipment to meet the standard is available (at a price and to 
approved customers) AFAIK even the commercial standard remains classified. If
you think about it, it would really have to wouldn't it? 
   
-- 