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I had an idea many years ago about using those cheap tv tuner cards that were briefly somewhat popular in PCs as an entropy source. If it could reliably tune to

by User23 5y ago

I had an idea many years ago about using those cheap tv tuner cards that were briefly somewhat popular in PCs as an entropy source. If it could reliably tune to a dead channel I imagine it would be a good source of quality entropy.

denton-scratch 5y ago
Using a radio receiver as a source of entropy opens you to a rather obvious attack channel.
- adrian_b 5y ago
  If you replace the radio signal source at the radio receiver input with a shielded resistive terminator, you no longer have any attack channel (because now the radio receiver has only an output, but no input). For this purpose, radio receivers are just very sensitive amplifiers. Any very sensitive amplifier will output noise when its input is terminated on a resistor, both amplified resistor noise and additional noise from sources internal to the amplifier. When you use much less sensitive amplifiers than radio receivers, e.g. audio amplifiers, a resistor at the input may be not enough and you must put there a source of greater noise, e.g. an avalanche breakdown diode (a.k.a. Zener diode).
adrian_b 5y ago
Many years ago I have indeed used this method, which has the advantage of a very high rate for the random bits. There is no need to tune to a dead channel. Instead of connecting a TV cable or antenna, it is enough to connect a coaxial resistive terminator on the TV input connector. Then all the channels are dead. Unfortunately this is a dead technology, so I have abandoned it after I retired my last motherboard with PCI connectors. You can do something similar with the audio microphone input, but the TV tuners had a much higher bandwidth and a much higher sensitivity. For good results with the microphone input you need to make an analog noise generator, instead of using just a resistor, as it was enough for a TV tuner.
- astrange 5y ago
  You could still do it with a USB capture card or an SDR. Analog capture certainly isn't dead though it might be a little expensive.
  - adrian_b 5y ago
    Yes, the only problem is the price. Analog TV tuners were very cheap. Now it no longer makes sense to use an expensive analog capture board for a RNG. There are much cheaper solutions. For no more than $10, you can make a RNG using the ADC of either a microcontroller board with USB interface or of an audio USB dongle or of the microphone connector of the motherboard audio.
- dTal 5y ago
  >For good results with the microphone input you need to make an analog noise generator, instead of using just a resistor, as it was enough for a TV tuner. Is this true? If you crank the gain on the cheap mic amplifier that comes with basically any laptop or motherboard, you'll get a ton of noise. Yes, the high bits will be correlated between samples, but if you XOR all the bits in a sample - or multiple samples - together, that ought to be a pretty solid random bit, no? You just need to make your XOR-ing "window" span at least 1 bit's worth of entropy in the stream. It's not megabytes per second, but it's a lot better than having the user wiggle the mouse, surely?

by User23 5y ago

denton-scratch 5y ago

Using a radio receiver as a source of entropy opens you to a rather obvious attack channel.

adrian_b 5y ago
If you replace the radio signal source at the radio receiver input with a shielded resistive terminator, you no longer have any attack channel (because now the radio receiver has only an output, but no input). For this purpose, radio receivers are just very sensitive amplifiers. Any very sensitive amplifier will output noise when its input is terminated on a resistor, both amplified resistor noise and additional noise from sources internal to the amplifier. When you use much less sensitive amplifiers than radio receivers, e.g. audio amplifiers, a resistor at the input may be not enough and you must put there a source of greater noise, e.g. an avalanche breakdown diode (a.k.a. Zener diode).

adrian_b 5y ago

Many years ago I have indeed used this method, which has the advantage of a very high rate for the random bits. There is no need to tune to a dead channel. Instead of connecting a TV cable or antenna, it is enough to connect a coaxial resistive terminator on the TV input connector. Then all the channels are dead. Unfortunately this is a dead technology, so I have abandoned it after I retired my last motherboard with PCI connectors. You can do something similar with the audio microphone input, but the TV tuners had a much higher bandwidth and a much higher sensitivity. For good results with the microphone input you need to make an analog noise generator, instead of using just a resistor, as it was enough for a TV tuner.

astrange 5y ago
You could still do it with a USB capture card or an SDR. Analog capture certainly isn't dead though it might be a little expensive.
- adrian_b 5y ago
  Yes, the only problem is the price. Analog TV tuners were very cheap. Now it no longer makes sense to use an expensive analog capture board for a RNG. There are much cheaper solutions. For no more than $10, you can make a RNG using the ADC of either a microcontroller board with USB interface or of an audio USB dongle or of the microphone connector of the motherboard audio.
dTal 5y ago
>For good results with the microphone input you need to make an analog noise generator, instead of using just a resistor, as it was enough for a TV tuner. Is this true? If you crank the gain on the cheap mic amplifier that comes with basically any laptop or motherboard, you'll get a ton of noise. Yes, the high bits will be correlated between samples, but if you XOR all the bits in a sample - or multiple samples - together, that ought to be a pretty solid random bit, no? You just need to make your XOR-ing "window" span at least 1 bit's worth of entropy in the stream. It's not megabytes per second, but it's a lot better than having the user wiggle the mouse, surely?