[systemd-devel] Why we need to read/save random seed?

[cee1]

2015-06-16 0:21 GMT+08:00 Lennart Poettering <lennart at poettering.net>:
> On Mon, 15.06.15 23:33, cee1 (fykcee1 at gmail.com) wrote:
>
>> Hi,
>>
>> I maybe got confused.
>>
>> First, systemd-random-seed.service will save a "seed" from
>> /dev/urandom when shutdown, and load that "seed" to /dev/urandom when
>> next boot up.
>>
>> My questions are:
>> 1. Can we not save a seed, but load a seed that is read from **
>> /dev/random ** to ** /dev/urandom **?
>
> The seed is used for both. Then you'd feed the stuff you got from the
> RNG back into the RNG which is a pointless excercise.

systemd-random-seed.service will load the "seed on disk" to
/dev/urandom, and save a "seed" to disk when shutdown, right?

The article at http://www.2uo.de/myths-about-urandom/ suggests us
saving the seed as soon as there is enough entropy(means read from
/dev/random? if returns, there's enough entropy),

Saving the seed early, make it more tolerant against the case of
system crashes - that means not shutdown properly(which maybe the case
on some mobile device such as STB.



-- 
Regards,

- cee1