Re: [PATCH 0/2] Add support for Qualcomm's PRNG

[H. Peter Anvin]

On 10/10/2013 03:41 AM, Paul Mackerras wrote:
> On Wed, Oct 09, 2013 at 08:07:35AM -0700, H. Peter Anvin wrote:
> 
>> consider the PowerPC random number generator[1]) and
> 
> [snip]
> 
>> [1] which has a known first-order bias which they "correct" for by
>> XORing two datums together in a very simple data reduction step.
> 
> 65 actually, not two.
> 
>> However, if their random source has bias it is extremely likely it also
>> has nonzero correlations, which require stronger reductions.  It would
> 
> The correlations are essentially zero, by design, and experiment
> confirms it.  Did you see my mail on the kvm list where I explained
> how it works?
> 

No, sorry... I got a bit of detached discussion as part of benh talking
about KVM and randomness (for the record, I'm all for better randomness
on all platforms.)

Either way, XORing samples is a pretty inefficient (both in terms of
anticorrelation and in terms of entropy efficiency) form of data
reduction/conditioning.  It would still be better to feed the output
into the pool with a 65x derating.

	-hpa


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