entropy

[Roberto Ragusa]

Tony Nelson wrote:
> On 10-01-07 12:40:02, Roberto Ragusa wrote:
>> Luca wrote:
>>> Hi all,
>>>  if I simply write to /dev/random, will that increase the entropy 
>>> of my  system? (I'm assuming that the data I'm writing are random 
>>> and that somehow I got them).
>> Wikipedia says so.
>>
>> My tests say no.
>>
>> In particular this brutal approach does not increase the entropy
>>   cat /dev/urandom >/dev/random
>> (it is stupid to do that, I know, but it's just a test)
>  ...
> 
> `man 4 random` says that the current entropy can be read and written 
> from /dev/urandom, not /dev/random.  This is used to preserver entropy 
> across reboots.

That's true.
But as far as I can see neither writing to random nor to urandom will
increase the entropy availability.

After checking the sources of rngd, I found it uses a specific ioctl:
  ioctl(random_fd, RNDADDENTROPY, &entropy);

So I think Luca can inject entropy by using the same ioctl in his
own application, or by using rngd in some way (you can tell it where
to take entropy from).

-- 
   Roberto Ragusa    mail at robertoragusa.it