2013/10/18 James A. Donald <[1]jamesd@echeque.com>

     You can, however, be sure a microphone input is a reliable source of
     entropy, since fake entropy would interfere with its microphone
     function.

   This is a syntatic non sequitur. Why would fake entropy interfere with
   a microphone's function? How is the microphone guaranteed to have "its
   microphone function"? Is a microphone input just the microphone's jack
   or an actual soundwave-modulated-magnetic-power-factor?
   In either case it's also a semantic non sequitor. If someone plays a
   darn loud sine wave in the serverroom you can be sure the microphone
   will replicate it. It'd be doable to make any microphone always output
   it's maximum value, through a plenty of means. The sad thing is that
   it's sound, so it might even be doable at distance!
   (scenario: people breaking into a running-but-physically-controlled
   server through manipulation of it's random numbers)
   I think an internal radioactive source such as a smoke alarm makes
   great sense. Be wary to isolate it very well to prevent outside
   interference. If it just goes to MAXINT if someone holds his cube of
   madam curie next to the server's case it'd be a shame.
   @Jim Bell: wouldn't such a ring oscillator aggregate be subject to
   patterns? If you have something that can create more out of fewer
   pieces of randomness, isn't there plenty bad-randomness-sources to go
   on?
   @Jon Callas:
   How is this random generator affected by CPU Interrupts? It seems to be
   a feature added for "more randomness", but given interrupts are far
   from guaranteed (especially in problematic systems) you cannot depend
   upon them. Especially because random numbers are harder on smaller
   systems I'm not sure I'd call this solution elegant.
   I believe stir-back is not always possible but a very strong feature.
   If you can always stir-back, can't you always generate something fairly
   random by simulating a x-times-stir-back? If you can't, how can you
   trust your stir-backs to be spaced enough for your -x-times-stir-back
   to not happen anyway?
   Lastly I feel your way of dealing with the pool-distiller model is
   finicky. If you hash a pool your hash will be able to fold onto itself
   very often and bits of entropy in different places can have different
   effects. You're placing an unusual amount of faith in your hash
   function on it being perfect. Diffusion with a partially known
   sourcetext is very, very murky business. And with predictable data
   going into your pool you're essentially creating a probably partially
   known plaintext. That's complex, and that'll do you good, but it is not
   the kind of nice randomness you'd go for.
   A suggestion I'd like to make is a laser & light strength measurement
   unit. Neatly self contained and accurate enough it can measure the
   bending of spacetime itself. I suppose anything even nearly that
   accurate will measure it's direct environment's noise so well it all
   doesn't matter anyway. Something like Delta(cpu_magneticness_mid_cycle)
   would do wonders. Anything dependent on the activity of the computer
   itself gives problems with people manipulating what the computer is
   doing. More rough it could measure minute changes in the movement and
   heat of flowing air.
   wow this became a lot longer than I expected.

References

   1. mailto:jamesd@echeque.com