> The tempest emissions can be handles by RF white noise generators > which are set up to generate RF in the same range and distribution of > frequencies and amplitudes as the RF generated by the box and the > monitor. As for the encryption, one should assume that the NSA could
White noise generators.. You are dealing with an analog signal. Also, finding a source of entropy for your white noise generator would be difficult. The first problem is you need to not only cover the primary frequency, but also each harmonic frequency. Let's say you're trying to block 20khz, ok? Here's the other frequencies you need to block as well: ^^^^^^^^^^^^^ 160000 8/1 80000 4/1 40000 2/1 20000 <--- Fundamental 10000 1/2 5000 1/4 2500 1/8 1250 1/16 625 1/32 312 1/64 vvvvvvvvvvvvvv In the US, it is highly unlikely that all of those frequencies would be unused. That means that you would be intentionally causing destructive inteference. The FCC would shut you down. In most contries, especially those with internet access, the airwaves are controlled. But let's assume those legal obstacles don't exist and continue for now. Also, because we are dealing with an analog signal, it will be largely impossible to completely remove the signal. How to counter-act your "white noise" generator would depend on two things: the source of entropy, its distribution, and the amplitude. What you are doing is not destroying the original wave, but infact adding two waves and hoping that when the signal is passed back through an amplifier it will heterodyne. This means when when two sine waves pass through the amplifier it will sum them and you will get a resultant signal. This is how white noise generators which function in the audio range work.. when that signal is picked up by a microphone it is usually passed through a preamp, and finally a power amplifier, at which point the signal is written out to tape. The original signals have been combined and if one of those waveforms was randomly created, the output will likewise be randomized to a degree. But have you noticed that even though your microphone picks up static, you can still hear everything that goes on in that room - the person in question speaking, as well as yourself, and you can both hear each other. Why is this? Because the brain doesn't have an amplifier. :) Enter digital signal processing. I don't want to give a technical overview of how it works in detail, but suffice it to say it doesn't use an amplifier, and is not as vulnerable to heterodyning. It can then seperate out the random noise from the original signal. There is one catch though. Typically the signal needs to be slightly above the threshold that the "random" signal operates on at some point. The more points, the better your reception, and the easier it is to reconstruct the original signal. Past a certain point, it will no longer be possible to recover that signal, but that requires a very powerful white noise generator. Which brings us to the next feasibility problem: cost. It would be far cheaper to simply shield the equipment than it would be to build a white noise generator which would consume, likely, a factor of 50 times more energy than the monitor's EM output. In short, a white noise generator is impractical. It is far more practical to attenuate the signal to the point where reception would be difficult beyond 20-30 feet. At that point, if you felt like it, a white noise generator could be operated on at a lower power consumption rate and be more effective. Twice nothing is still nothing, however...... ~ Signal 11 _______________________________________________ Freenet-dev mailing list Freenet-dev at lists.sourceforge.net http://lists.sourceforge.net/mailman/listinfo/freenet-dev
