Re: [PATCH] CPU Jitter RNG: inclusion into kernel crypto API and/dev/random

From: Pavel Machek
Date: Sun Nov 03 2013 - 18:32:21 EST


> 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

> >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())

(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.
(cesky, pictures)
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