Re: Entropy from disks

Chris Friesen
2002-10-29T17:57:03Z

Oliver Xymoron wrote:

I'm not an expert in this field, so bear with me if I make any blunders
obvious to one trained in information theory.

> The current Linux PRNG is playing fast and loose here, adding entropy
> based on the resolution of the TSC, while the physical turbulence
> processes that actually produce entropy are happening at a scale of
> seconds. On a GHz processor, if it takes 4 microseconds to return a
> disk result from on-disk cache, /dev/random will get a 12-bit credit.

In the paper the accuracy of measurement is 1ms. Current hardware has
tsc precision of nanoseconds, or about 6 orders of magnitude more
accuracy. Doesn't this mean that we can pump in many more bits into the
algorithm and get out many more than the 100bits/min that the setup in
the paper acheives?

> My entropy patches had each entropy source (not just disks) allocate
> its own state object, and declare its timing "granularity".

Is it that straightforward? In the paper they go through a number of
stages designed to remove correlated data. From what I remember of the
linux prng disk-related stuff it is not that sophisticated.

> There's a further problem with disk timing samples that make them less
> than useful in typical headless server use (ie where it matters): the
> server does its best to reveal disk latency to clients, easily
> measurable within the auto-correlating domain of disk turbulence.

If this is truly a concern, what about having a separate disk used for
nothing but generating randomness?

Chris

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