With a $10 dongle, you can go from tracking airplanes, to reading your wireless electric meter, to listening to local amateur radio, and then to an additional entropy source.
I double I'll ever get tired of playing with this little radio.
Ive used the RTL as a blended entropy source for my kubernetes pi cluster project, but blended it with signal data from the open source FST-01 from the flying stone project. this in turn gets randomly audited by dieharder. http://webhome.phy.duke.edu/~rgb/General/dieharder.php
My RTL entropy generator changes frequencies every 6 hours to a frequency determined by a script referencing /dev/urandom.
> If you're serious about the cryptographic security of your entropy source, you should probably short, or put a 75 Ohm load on the antenna port, and put the whole assembly in a shielded box. Then you're getting entropy from the thermal noise of the amplifiers which is much harder to interfere with than atmospheric radio.
Or you could randomize it, the RTL has a pretty wide receive range.
Here is one for $99, I'm sure you could DIY one for less with reduced output power: https://www.tekbox.net/test-equipment/comb-generators/tbcg1-...
I still think it's a great effort but as others have pointed out here it shouldn't be your only source of entropy and I'd feel a lot better about it if it were properly reviewed/hammered on.
Still, it's really cool to randomly see the project front page on HN years later!
There are all kinds of conceivable failure modes that software cannot detect, let alone handle.
Statistic tests are good, but only of limited use.
Otherwise I’d be happier using a similarly priced USB tool that is at least designed for the job such as the ChaosKey which is also open hardware and firmware and has a Linux driver in tree
Edit: Just curious: are people downvoting because they disagree with my proposed spelling, or because they don't like the correction? Please let me know so I can integrate this knowledge into future comments.
Hardware fails. Can you reliably detect that? The board you linked to cannot at all. Most HWRNGs can, but only partially.
It's probably a fun project to try, but I'd be wary of using it for cryptography if only because we have other better methods.
I collected a bunch of links a while ago. Here they are, I don't know if these are still working or interesting, but they might be useful: https://news.ycombinator.com/item?id=6060636
An old project of mine:
It was good enough to use for a financial application when whitened / XORed with system sources.
I've seen enough of these discussions to be able to parrot back to you some of the usual responses. Basically, say that you point your webcam at a perfectly black sheet (0x000000 in color). Will you get any actual entropy from this? No.
Now say you point your webcam at the night sky. On a clear night, I will more or less know what your camera is looking at, and will be able to to narrow down the huge field of possible values presented by sha256 to a much smaller set of possibilities. Knowing when you generated your private key and your method of getting entropy, I will be able to crack your key much easier (doesn't mean easy, just many orders of magnitude easier).
Now, say you point your webcam at a lava lamp, or the display showing characteristics of some quantum process, or something else provably random. Here I can't really tell wtf your camera is looking at. I can of course go and slide that black sheet in front of your camera for a bit. Or stand in front of it. But if you physically secure your camera, then yeah you are doing something right (though of course not everything, as it's quite possible that something in your algorithm is in fact reducing your possible output to only some subset of possible values produced by sha256).
It's the same thing with radio. If you tune it to a signal that's producing a sine wave with a stable frequency... I know exactly what your input is. If you tune it to something like the BBC, I know exactly what your input is, even with some noise. If you tune it to something really random, can I just put a very strong transmitter near your receiver and overwhelm the spectrum such that you are listening to only my signal? And the best part is that you won't know that this is happening even more than with a webcam.
Personally, I wouldn't really want to trust these devices. They are fun. but they aren't practical for this purpose. Where you need real entropy is a data center. And a data center won't be receiving too much radio.
Again, parroting every other thread about this, a reverse biased transistor will give you true quantum randomness which is very cheap, very easy to produce, and is difficult to temper with at a distance without disrupting the rest of your equipment too.
Again, you don't need as much entropy as you think. Your kernel will seed a CSPRNG with a tiny bit of entropy, which is enough to keep you safe for eons after that. You can't "run out" of entropy with that scheme. The biggest issue is to initialize the random number generator with true entropy before you start using it, which is only difficult if you are running virtual machines and the host doesn't provide entropy to it. I believe at least in some schemes the seed used is based on time or some such predictable factor, which doesn't help.
And if I can affect the image in a way that makes it even more predictable (black sheet over the camera, bright white light into the camera), it gets even easier.
I think it is valid to question how you could reduce the search space enough to be more beneficial than brute forcing SHA256.
Umm... Why not?
The first frame would give you some entropy, but you would terribly weaken it by re-using identical "random" data over and over again.
Note, however, you should not use this for any real purpose. I saw a video on Youtube a while back of the author presenting this (at a LUG meeting or some such) and even he warns against using it for any real purpose.
So play around with it, hack on it, etc., but do not, by any means, incorporate this into any process that requires actual entropy for cryptographic purposes.
A digital TV signal is a lot more repeatable than an analog one. A digital signal doesn't have the "TV static" effect of an analog signal that may introduce different entropy between two devices pulling the same signal in different locations
That said, although the DVB-T dongles were intended for digital TV use, the typical demodulators (such as http://www.amateur-radio-wiki.net/index.php?title=RTL2832) have an extremely wide frequency range and can be repurposed all over that range.
Absolutely use extreme caution when adopting a new source of entropy.
It has a few issues like the usb packaging does some retiming, plus the rtl chip documentation is under nda and else.