CPRNGs are still an issue.

Charles Jackson clj at jacksons.net
Tue Dec 16 16:22:08 EST 2008

-Michael Heyman

Before we give up on using drive timings [as an entropy source], does anyone
have evidence to
verify this assertion [that SSD drives will have much less variation in
read/write timing]?  The reviews I have seen using tools like HD
Tune and HD Tach seem to show timing noise reading and writing SSDs. I
don't know where the noise comes from - it is probably not turbulence
<grin/> - but it may be random enough that a long series of tests, say
for a second or so (don't forget, these drives are fast), could
provide a nice pool of unguessable bits.

I probably should not be commenting, not being a real device guy.  But,
variations in temperature and time could be expected to change SSD timing.
Temperature changes will probably change the power supply voltages and shift
some of the thresholds in the devices.  Oscillators will drift with changes
in temperature and voltage.  Battery voltages tend to go down over time and
up with temperature.  In addition, in some systems the clock frequency is
purposely swept over something like a 0.1% range in order to smooth out the
RF emissions from the device.  (This can give a 20 or 30 dB reduction in
peak emissions at a given frequency.  There is, of course, no change in
total emissions.)

Combine all of these factors, and one can envision the SSD cycles taking
varying numbers of system clock ticks and consequently the low order bits of
a counter driven by a system clock would be "random."  However, one would
have to test this kind of entropy source carefully and would have to keep
track of any changes in the manufacturing processes for both the SSD and the
processor chip.     

Is there anyone out there who knows about device timing that can say more?  

Chuck Jackson 

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