[PATCH v3 1/3] crypto: hw_random - Add new Exynos RNG driver
krzk at kernel.org
Tue Mar 28 09:48:24 PDT 2017
On Mon, Mar 27, 2017 at 03:53:03PM +0200, Stephan Müller wrote:
> Am Montag, 27. März 2017, 06:23:11 CEST schrieb PrasannaKumar Muralidharan:
> Hi PrasannaKumar,
> > > Oh my, if you are right with your first guess, this is a bad DRNG design.
> > >
> > > Just out of curiousity: what happens if a caller invokes the seed function
> > > twice or more times (each time with the sufficient amount of bits)? What
> > > is
> > > your guess here?
> > Should the second seed use the random data generated by the device?
> A DRNG should be capable of processing an arbitrary amount of seed data. It
> may be the case that the seed data must be processed in chunks though.
As I said, I do not know the implementation details about hardware. They
are just not disclossed.
> That said, it may be the case that after injecting one chunk of seed the
> currently discussed RNG simply needs to generate a random number to process
> the input data before another seed can be added. But that is pure speculation.
> But I guess that can be easily tested: inject a known seed into the DRNG,
> generate a random number, inject the same seed again and again generate a
> random number. If both are identical (which I do not hope), then the internal
> state is simply overwritten (strange DRNG design).
> A similar test can be made to see whether a larger set of seed simply
> overwrites the state or is really processed.
> 1. seed
> 2. generate random data
> 3. reset
> 4. seed with anther seed
> 5. generate random data
> 6. reset
> 7. seed with same data from 1
> 8. seed with same data from 2
> 9. generate random data
> If data from 9 is identical to 2, then additional seed data is discarded ->
> bad design. If data from 9 is identical to 5, then the additional data
> overwrites the initial data -> bad DRNG design. If data from 9 neither matches
> 2 or 5, then all seed is taken -> good design.
I tested a little bit and:
1. Seeding with some value
2. generating random,
3. kcapi_rng_destroy+kcrng_init, (I cannot do a hardware reset except
reboot of entire system)
4. seeding with the same value as in (1) - different random numbers.
Doing a system reboot and repeating above - different random numbers
(all are different: step (2) and in (4)).
Your test case also produces different random values every time.
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