Re: [PATCH] CPU Jitter RNG: inclusion into kernel crypto API and /dev/random

[Sandy Harris]

Stephan Mueller <smueller@xxxxxxxxxx> wrote:

> [quoting me]

>> ...your code is basically, with 64-bit x:
>>
>>   for( i=0, x = 0 ; i < 64; i++, x =rotl(x) )
>>        x |= bit()
>
> Why not declare some 64-bit constant C with a significant
>>number of bits set and do this:
>>
>>   for( i=0, x = 0 ; i < 64; i++, x =rotl(x) ) // same loop control
>>      if( bit() ) x ^= C ;
>>
>>This makes every output bit depend on many input bits
>>and costs almost nothing extra.

> Ok, let me play a bit with that. Maybe I can add another flag to the
> allocation function so that the caller can decide whether to use that.
> If the user is another RNG, you skip that mixing function, otherwise you
> should take it.

I'd say just do it. It is cheap enough and using it does no harm
even where it is not strictly needed. Adding a flag just gives the
calling code a chance to get it wrong. Better not to take that risk
if you don't have to.
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