[Cryptography] Tempest and limits on receiving

[Jerry Leichter]

> On Apr 4, 2017, at 3:34 PM, Bob Wilson <wilson at math.wisc.edu> wrote:
> 
> 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.
There is not, to the best of my knowledge, any law preventing you from listening to any frequency you like.  (You aren't allowed to repeat anything you heard, or even acknowledge that you heard it, except if you heard it on bands specifically open to public/shared listening - broadcast bands, ham and CB radio bands, probably WiFi though there have been some arguments about what is permitted here and what amounts to "computer hacking".)

The limitations on receivers are enforced by a hack in the law:  Modern receivers are all likely to use a superheterodyne design, so are inherently transmitters as well.  To ensure they don't interfere with anyone else, they need to be approved by the FCC.  The FCC has written into its rules that receivers sold to consumers (or however they define the class) will not be approved unless they are hard-wired to block certain frequency ranges - particularly cell-phone and police bands.

(Digital radios - based on an A/D converter and signal processing in the digital domain - probably escape these regulations.  But they are not widely available in the mass markets the FCC cares about.)
                                                        -- Jerry