[Cryptography] Tempest and limits on receiving

[Bob Wilson]

Several posters seem to think that in the USA there are no restrictions 
on just listening, e.g.
> In the US laws only prohibit active interference, not passive shielding.
Have you tried to buy a shortwave radio that covers all of the 
frequencies from, say, 100MHz to 10GHz? There are quite a few receivers 
that say they do that, until you look at the footnote saying that 
reception of certain frequencies is disabled, because the disabled 
frequencies are used by some cell 'phones. I admit I don't know where 
the law might be, but something is certainly causing all those 
manufacturers to remove that capability from their receivers. This 
applies to units sold in the USA but built elsewhere as well as those 
built here.

Since we don't have vans on the streets looking for radiation from 
receivers (the way Britain for a long time enforced the requirement to 
buy a radio receiving license), the easy way to enforce such a 
restriction is to place it on all equipment made for sale. You could 
presumably build your own receiver with whatever reception frequencies 
and modes you wanted, and wait to see if any such restrictions are being 
enforced in some other way.

(Note that I am specifically saying "receivers". Many of them are built 
into "transceivers" that combine a receiver and a transmitter, 
frequently designed for ham radio use, but the (a) limitation is on 
reception and (b) if the company makes a separate receive-only device 
for those frequencies, the same restriction will be included.)

Bob Wilson