Brilliant point. If you start thinking security by assuming there’s already a problem, you design for resiliency. The security measures you put into place won’t fail as soon as you do see a breach. (And if you currently have one, they don’t fail instantly.)
So, if someone is inside your security right now, what should you do?
For more advanced attackers, try the attached design. It’s version of the “All Band Reciever” which is much more sensitive to all signals: it will pick up and extract the audio from AM, FM, SSB(?), and even PM signals of just about any carrier frequency. It’s still pretty simple to build. (If you make it with SMD parts, microstrip construction, and maybe a faster diode it should be good up to quite a few GHz.)
The “Sinitsa” accomplishes this magic by using an idea invented by the Russians for a cold-war era covert-transmitter detector (http://cryptomuseum.com/df/sinitsa/index.htm): a 29kHz sine wave injected right before the detector diode. The detector diode therefore also becomes a mixer. The 29kHz local oscillator is just above hearing range, making it possible to hear absolutely everything that comes in the antenna because one mixing product is almost always within the audio band. (I suppose dropping the oscillator to 20kHz might improve things, but I didn’t notice much of a difference in tests.
As always, feel free to distribute as long as you keep my name off it. If you make improvements — and there are certainly a few places they could be made, I’m not a professional EE — please keep me posted!
“I would like to propose an additional way we can improve security, which doesn’t require a dime of investment, but does require a change in how we think.
Assume you’ve been hacked – and now map out your security policy.”