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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