7 expanded posts ยท every known parent and child
>1.C How about if we take the hiss that you find between stations on an >FM receiver, and digitize them through a PC soundcard? Hmm. And if an attacker discovers you're using this method, and decides to send out a signal on the same frequency? You might find your "random" numbers are suddenly all zeros... --- Jef
> >1.C How about if we take the hiss that you find between stations on an > >FM receiver, and digitize them through a PC soundcard? > > Hmm. And if an attacker discovers you're using this method, and > decides to send out a signal on the same frequency? You might find > your "random" numbers are suddenly all zeros... > --- > Jef > Band hop. Build a variable modulo counter and use its output to reset its modulo and hop the band around pseudo-randomly. Unless they know or stumble on the particular design you are using it will be very difficult to track. Since music, energy wise, approaches 1/f^2 it might be ok to even use the actual stations.
>> Hmm. And if an attacker discovers you're using this method, and >> decides to send out a signal on the same frequency? You might find >> your "random" numbers are suddenly all zeros... >Band hop. Build a variable modulo counter and use its output to reset its >modulo and hop the band around pseudo-randomly. Unless they know or stumble >on the particular design you are using it will be very difficult to track. An even better idea: disconnect the antenna. Most of the noise comes from the front end amplifier, not the galactic and cosmic background, at least in your average consumer grade receiver. And this is a quantum process that someone else definitely can't predict or copy. Phil
> An even better idea: disconnect the antenna. Most of the noise comes > from the front end amplifier, not the galactic and cosmic background, > at least in your average consumer grade receiver. And this is a quantum > process that someone else definitely can't predict or copy. And if that doesn't work, crawl up the spectrum a bit. The higher in frequency you go, the more thermal noise you'll see. Gee, it's *good* to be able to write something intelligent in response to one of Phil's posts... :) -- Ed Carp, N7EKG/VE3 ecarp@netcom.com, Ed.Carp@linux.org "What's the sense of trying hard to find your dreams without someone to share it with, tell me, what does it mean?" -- Whitney Houston, "Run To You"
> > > An even better idea: disconnect the antenna. Most of the noise comes > > from the front end amplifier, not the galactic and cosmic background, > > at least in your average consumer grade receiver. And this is a quantum > > process that someone else definitely can't predict or copy. > This is a bad idea, the computer it self will generate clocking noise which will appear in the noise and destroy the randomness. The standard, and even most high-end, recievers don't have the shielding to prevent this sort of intrussion. Heck, that digital clock on your desk (and possibly your wrist if close enough) will cause problems as well. > And if that doesn't work, crawl up the spectrum a bit. The higher in > frequency you go, the more thermal noise you'll see. > Only up to a point. Past a certain point and the processes will start to roll off their energy production.
>This is a bad idea, the computer it self will generate clocking noise which >will appear in the noise and destroy the randomness. The standard, and even >most high-end, recievers don't have the shielding to prevent this sort of >intrussion. Heck, that digital clock on your desk (and possibly your wrist if >close enough) will cause problems as well. In my ham experience, almost all of the computer noise you might pick up in such a receiver comes through the antenna. If you disconnect the antenna, what remains is almost entirely thermal noise from the receiver front end. And even if there were some computer noise mixed in, MD5 hashing the output to concentrate the entropy should satisfy even the most paranoid. It's advisable to replace the antenna with a dummy load (matched resistive terminator) to ensure that the front end remains stable; if the receiver is sensitive enough you also pick up the thermal noise generated by the terminator itself. (BTW, you can easily demonstrate thermal noise with a *good* low-noise preamp and a linear, e.g., SSB or AM -- not FM, receiver this way. Turn off the receiver AGC and dunk the terminator in liquid nitrogen. The noise level will decrease dramatically. Take the terminator out of the N2 and let it warm up; the noise level will return to normal. Be prepared to sacrifice the coax you dunk into the N2; I've cracked a few rubber connector boots this way. >> And if that doesn't work, crawl up the spectrum a bit. The higher in >> frequency you go, the more thermal noise you'll see. >> >Only up to a point. Past a certain point and the processes will start to >roll off their energy production. It's a little more complicated than that. In general, as you go higher in frequency the natural background noise power (lightning, sun noise, galactic synchrotron radiation, 3K cosmic background, etc) decreases rapidly, reaching a minimum in the 1-10 GHz range. Above that, atmospheric components such as water vapor and oxygen again start to contribute quite a bit of thermal noise. (This low-noise window is why the Search for Extraterrestrial Intelligence projects concentrate on the 1-10Ghz range). On the other hand, the noise contributed by state-of-the-art receivers tends to increase with frequency, though again the state of the art has gotten very good. For cryptographic quality random numbers you want *only* local receiver noise; if you rely on external sources like the galactic background, your attacker also has access to them. Phil
Phil Karn says: > An even better idea: disconnect the antenna. Most of the noise comes > from the front end amplifier, not the galactic and cosmic background, > at least in your average consumer grade receiver. And this is a quantum > process that someone else definitely can't predict or copy. This is also cheap. If you have audio input on your computer, just put an El-Cheapo radio with its antenna off tuned to dead air into your workstation, and distill what comes out with MD5 or SHA or something similar. Assume that things are much less random than they seem and distill every N bytes down to 1 byte with the hashing algorithm -- N depends on your paranoia. Perry