Re: [PATCH 0/6] crypto: DRBG - improve 'nopr' reseeding

From: Stephan Müller
Date: Wed Oct 27 2021 - 14:43:53 EST


Am Mittwoch, 27. Oktober 2021, 10:40:12 CEST schrieb Nicolai Stange:

Hi Nicolai,

> Hi Stephan,
>
> first of all, many thanks for your prompt review!
>
> Stephan Müller <smueller@xxxxxxxxxx> writes:
> > Am Montag, 25. Oktober 2021, 11:25:19 CEST schrieb Nicolai Stange:
> >> - Replace the asynchronous random_ready_callback based DRBG reseeding
> >>
> >> logic with a synchronous solution leveraging rng_is_initialized().
> >
> > Could you please help me why replacing an async method with a sync method
> > is helpful? Which problems do you see with the async method that are
> > alleviated with the swtich to the sync method? In general, an async
> > method is more powerful, though it requires a bit more code.
>
> There is no problem with the async method (*), I just don't see any
> advantage over the less complex approach of doing all reseeding
> work synchronously from drbg_generate().
>
> Before the change, there had been two sites taking care of reseeding:
> the drbg_async_seed() work handler scheduled from the
> random_ready_callback and drbg_generate().
>
> After the change, all reseeding is handled at a single place only, namely
> drbg_generate(), which, in my opinion, makes it easier to reason about.
> In particular, in preparation for patch 6/6 from this series introducing
> yet another condition for triggering a reseed...

That makes sense. Thanks for clarifying.

Ciao
Stephan
>
> Thanks,
>
> Nicolai
>
> (*) Except for that a wait_for_random_bytes() issued by DRBG users won't
> give any guarantees with respect to a subsequent drbg_generate()
> operation, c.f. my other mail in reply to your review on 3/6 I'm
> about to write in a second. As of now, there aren't any DRBG users
> invoking wait_for_random_bytes(), but one might perhaps consider
> changing that in the future.
>
> >> This
> >> move simplifies the code IMO and, as a side-effect, would enable DRBG
> >> users to rely on wait_for_random_bytes() to sync properly with
> >> drbg_generate(), if desired. Implemented by patches 1-5/6.
> >>
> >> - Make the 'nopr' DRBGs to reseed themselves every 5min from
> >>
> >> get_random_bytes(). This achieves at least kind of a partial prediction
> >> resistance over the time domain at almost no extra cost. Implemented
> >> by patch 6/6, the preceding patches in this series are a prerequisite
> >> for this.


Ciao
Stephan