Re: [PATCH v2] stdlib: Simplify arc4random_uniform

[Yann Droneaud <ydroneaud@opteya.com>]

Hi,

Le 01/08/2022 à 21:20, Noah Goldstein a écrit :
>> diff --git a/stdlib/arc4random_uniform.c b/stdlib/arc4random_uniform.c
>> index 1326dfa593..5aa98d1c13 100644
>> --- a/stdlib/arc4random_uniform.c
>> +++ b/stdlib/arc4random_uniform.c
>>
>>   uint32_t
>>   __arc4random_uniform (uint32_t n)
>>   {
>> @@ -57,83 +38,33 @@ __arc4random_uniform (uint32_t n)
>> +  while (1)
>>       {
>> +      uint32_t value = __arc4random ();
>> +
>> +      /* Return if the lower power of 2 minus 1 satisfy the condition.  */
>> +      uint32_t r = value & mask;
>> +      if (r < n)
>> +       return r;
>> +
>> +      /* Otherwise check if remaining bits of entropy provides fits in the
>> +        bound.  */
>> +      for (int bits_left = z; bits_left >= bits; bits_left -= bits)
>> +       {
>> +         value >>= bits;
> Can this just be shift by 1 and repeat (32 - z) times or does that
> introduce bias (not seeing exactly why it would)?


That was bothering me too, as I was thinking a rotation would be 
possible instead of shift.

I posted the question 
https://crypto.stackexchange.com/questions/101325/uniform-rejection-sampling-by-shifting-or-rotating-bits-from-csprng-output-safe

The answer: there's indeed a bias.

This explains why my attempt with rotation leads to dieharder 
complaining. It was so obvious ... Damn


Regards.


-- 

Yann Droneaud

OPTEYA