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Why not get randomness from sensors?

Why not get randomness from sensors?

Posted May 11, 2006 5:48 UTC (Thu) by Ross (guest, #4065)
In reply to: On the safety of Linux random numbers by Thalience
Parent article: On the safety of Linux random numbers

There are some untapped sources of entropy: the hardware sensors on most motherboards. The temperature, voltage, fan speed, etc. sensors could be manipulated by someone with physical access, but only to a limited precision. Doing some kind of differential sampling and not counting zero inputs as having any entropy should help.

Is there a reason the drivers for those devices don't contribute to the entropy pool?


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Why not get randomness from sensors?

Posted May 11, 2006 6:08 UTC (Thu) by jwb (guest, #15467) [Link]

It is fantastically expensive to communicate with the most common sensor hardware. Often the system has to bang bits over a two-wire port. It is reasonable to access these devices once every second or so, but I doubt that you'd be able to sample them often enough to generate substantial entropy.

It would be nice if hardware entropy devices were more common. There are plenty of random processes out there in the electrical world, like shot noise.

Why not get randomness from sensors?

Posted May 11, 2006 14:44 UTC (Thu) by pjones (subscriber, #31722) [Link]

I think you're wrong about what "substantial" means here. It doesn't need to be enough entropy to use as the system's only source. It needs to be enough to pervert the data from all the other sources in a way that masks their (potential) weeknesses. That requires surprisingly little data, if it is truly unavailable to attackers.

To that end, the bigger worry here is that it's just the sort of data you might want to stick in SNMP for your monitoring infrastructure to check on.

Why not get randomness from sensors?

Posted May 11, 2006 21:54 UTC (Thu) by giraffedata (subscriber, #1954) [Link]

It doesn't need to be enough entropy to use as the system's only source. It needs to be enough to pervert the data from all the other sources in a way that masks their (potential) weeknesses.

You seem to be describing a means of creating entropy out of nothing. If all the other sources provide 1000 bits per second of entropy and the hardware sensor gives you another 10 bits per second, you've got at most 1010 bits per second of entropy no matter what you do with those 10 new bits.

So I think "substantial" is the same amount no matter how you look at it.

Actually, I think the sensors mentioned have negligible entropy to contribute. You read them all digitally, and given one reading, you can predict very well what the reading will be a second later, to the full precision of the sensor.


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