Messages in this thread |  | | Date | Mon, 4 Nov 2013 00:32:07 +0100 | From | Pavel Machek <> | Subject | Re: [PATCH] CPU Jitter RNG: inclusion into kernel crypto API and /dev/random |
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Hi!
> Another friend of mine mentioned that he assumes the rise and fall times > of transistors varies very slightly and could be the main reason for the > jitter. I do not think that this is really the case, because our gates > that form the CPU instructions comprise of many transistors. The > combined raise/fall jitter should cancel each other out.
Plus, there's clock that should make sure that this jitter does not matter.
> >There should be way to extract entropy more directly from various > >oscillator effects, no? > > I am working a different way of measuring such oscillator effects by > using the high-resolution timer of the CPU and measure it with a > Jiffies-based snapshotting window. So, here I would combine two timers > that are differently generated. If their frequencies would be relative > prime to each other, we would measure a direct oscillator effect.
I guess main problem is machines that do not have high-resolution timer on the CPU (rdtsc). They do not get enough entropy during boot, and the hell breaks loose.
But they usually _do_ have RTC or other clock, not driven by CPU oscilator. Good.
What about just
while (!enough_entropy) { cur_time = read_rtc(); simulated_tsc = 0; while (cur_time == read_rtc()) simulated_tsc++; gain_entropy_from(simulated_tsc) }
(Where enough_entropy should be something like 128 bits).
This should work, we know why it works (drift between rtc and cpu clock) and it does _not_ need rdtsc-style fast source.
Disadvantage is that it burns cpu, but, well, you only need 128 bits. Asuming the rtc used has 100Hz resolution, enough entropy should be collected in under 2 seconds. That's acceptable adition to time it takes generating ssh keys on slow cpu. Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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