[Cryptography] Radios vs. computers

Bob Wilson rwilson at wisc.edu
Thu May 26 22:37:25 EDT 2016


> Thanks for the info, but this device isn't a *radio*, it's a *computer*,
> and a really low power one at that.  So far as I know, there's no antenna
> anywhere on this device.  I'm only interested in hacking its f'ing clock!
>
> If this device is "transmitting" anything at all, then there's something
> dreadfully wrong with it, and/or someone else has already hacked it.
There is no computer that is not a radio transmitter. For a personal 
computer to be sold in the US, all that fancy metal stuff around the 
cabinet edges, the conductive layers put on a plastic case, etc., are 
all intended to try to control what it radiates so as to meet FCC 
specifications. Your computer will have been certified to meet certain 
requirements, differing depending on the expected use environment. Even 
a little "wall wart" power supply that you plug into the wall to charge 
a cell phone or to run a small appliance probably has enough 
high-transient switching going on that it is radiating fairly strong 
radio signals. (Older linear supplies don't radiate much but they use 
more expensive copper and iron so they are disappearing.)

I can remember hearing music played by the ORACLE at Oak Ridge in 1953, 
I think it was, by turning on a nearby radio and running appropriate 
code, and a decade later similarly for the little IBM 1620. What you 
want to do may not depend on your computer transmitting, but someone 
else wanting to hear what you are doing might well take advantage of it. 
Try putting a radio near your computer(s) and running a variety of 
programs. The radio will definitely pick up signals, but whether it puts 
out sounds that you can hear depends on how it is choosing to demodulate 
those signals as well as what frequencies it detects. AM will more 
likely produce a sound you can hear than FM would, but the signals are 
there even if they don't get demodulated in a way your ears pick them up.

Bob Wilson


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