Only use your private key with Tinfoil Hat Linux on an offline air-gapped computer: http://tinfoilhat.shmoo.com/
I recommend disconnecting your monitor and only receiving output by having it blinked out at you through your capslock light on your keyboard. Bonus points if you can get your hands on some TEMPEST hardened hardware, and/or tamper-resistant hardware.
Anything less will leave you vulnerable to the black helicopters!
Note: I'm joking obviously, but this is something to take seriously.
The light reflected off your eyes from the capslock key is readable from high-res cameras. It's better to have leads hooked up to one of your toes and to toggle a 24V source so you can interpret the pulses in morse code.
Edit: obviously the 24V must come from a battery which is charged only at specific intervals -- otherwise they can interpret your messages by watching mains voltage variation.
Those leads are gonna generate magnetic distortions. You should only do this with your feet next to a giant 18" subwoofer while blasting dubstep in order to mask any electromagnetic fluctuations.
Bonus: Anyone surveilling you via audio bugs will need new ears.
I know this is all in good fun, but you all are uncomfortably close to describing things that will soon get added to the practical threat landscape.
As long as you have a flexible hardware platform that lets you crank up some of the voltage regulator outputs, gpios that can be attached to a long trace/external wire as a makeshift antenna and have a decently fast cpu clock you have all the ingredients for a crude but usable software defined radio. maybe not super fast if you can't repurpose a hardware phy or radio interface, but more than enough bandwidth to exfil a secret key or 10 for maybe a couple dozen meters.
Tools to do sdr utilizing only general purpose processors and no radio specific gear are already available here and there as research implementations, and code that uses gpus/audio dacs/ and re-purposed phys to make a radio interface with a different spec or broadcast frequency is already in production use (wifi phy using a dvb radio interface -> tv whitespace communicator).
Using an approach like that to exfil or bridge an air gap is just too tempting for it to not happen. Honestly, I'd be willing to bet there's already an example of that somewhere out there in the wild today.
Paranoid thinking is an extremely valuable asset for security researchers. The things we're all joking about are impractical for an average person, but in a spy vs. spy scenario, especially when each side is well funded, these are the kind of things that will actually get used.
Examples of genuine vulnerabilities that would make you look paranoid just by defending against:
* Make educated guesses about passwords from a microphone recording of the keypresses. Both the intervals between keypresses indicate the region of the keyboard being touched, and the sound of each key differs slightly. Given a statistically significant sample of typing, you could deduce which keys are which based on the frequency of their use. http://www.securityfocus.com/news/11318
Well, if you can do that (program a computer via a blinking keyboard LED in Morse code to avoid Van Eck phreaking,) you're well on your way to discovering the lost Nazi gold in the Philippines.
I recommend disconnecting your monitor and only receiving output by having it blinked out at you through your capslock light on your keyboard. Bonus points if you can get your hands on some TEMPEST hardened hardware, and/or tamper-resistant hardware.
Anything less will leave you vulnerable to the black helicopters!
Note: I'm joking obviously, but this is something to take seriously.