John Larkin wrote:

Relay pullin time and bounce would make a nice randomizer, too.

I am not sure how many random bits could be extracted from one click of
a relay. Once I tested reed relay for life expectancy; the bounce
pattern looked very repeatable. BTW, the life term appeared to be not so
great: somewhat 500K cycles or so.

Unclocked ring oscillators are not deterministic. They will drift all
over the place with time, temperature, tiny Vcc variations, thermal
noise, and EMI from other sources. CMOS oscillators have ghastly phase
noise.

Agreed. Typical RMS jitter ~ 1% of the period, and a big part of it is
due to the unpredictable noise of the components.

The external RC thing can be made truly random, too.

Random, in this context, means that future states can't be predicted,
even given total knowledge of the hardware and the current state.
Which means that bad guys can never decode the pattern.

One should be very careful with the assesment of the amount of the true
random information. It is easy to make a RNG which could seem very solid
from first glance, but it would fail to the basic analysis.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com