[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. ....
Maybe I did not make my intent clear: I posted a comment that was 
intended to address the issue of whether the US put restrictions on 
receiving, as distinct from forms of transmission. Several posters had 
implied that listening was completely free, and I just meant to point 
out that was not so clear. Several have now emailed me to point out that 
the examples I gave (e.g. under many circumstances receiving equipment 
must not be capable of receiving cell 'phone transmissions) did not 
involve Tempest, which I certainly concede, but that was not what I was 
talking about. I have worked in "black" environments and do know a bit 
about Tempest, but I did not mean to involve that.

I am sorry if I did not emphasize that enough, and thus wasted bandwidth 
both by my original message and the replies it inspired.
Bob Wilson
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