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Re: [PATCH v3 1/3] crypto: hw_random - Add new Exynos RNG driver

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  Re: [PATCH v3 1/3] crypto: hw_random - Add new Exynos RNG driver Krzysztof Kozlowski <krzk@kernel.org> - 2017-03-28 19:00 +0200
    Re: [PATCH v3 1/3] crypto: hw_random - Add new Exynos RNG driver Stephan Müller <smueller@chronox.de> - 2017-03-28 19:50 +0200
      Re: [PATCH v3 1/3] crypto: hw_random - Add new Exynos RNG driver Krzysztof Kozlowski <krzk@kernel.org> - 2017-03-28 19:50 +0200

#1611220 — Re: [PATCH v3 1/3] crypto: hw_random - Add new Exynos RNG driver

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.

Best regards,
Krzysztof

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#1611247

Am Dienstag, 28. März 2017, 18:48:24 CEST schrieb Krzysztof Kozlowski:

Hi Krzysztof,

> 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.

Then I would assume that simply adding an outer loop to your for() loop to 
inject seed larger than the minimum required seed size should be fine.
> 
> Best regards,
> Krzysztof



Ciao
Stephan

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#1611249

On Tue, Mar 28, 2017 at 07:41:47PM +0200, Stephan Müller wrote:
> Am Dienstag, 28. März 2017, 18:48:24 CEST schrieb Krzysztof Kozlowski:
> 
> Hi Krzysztof,
> 
> > 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.
> 
> Then I would assume that simply adding an outer loop to your for() loop to 
> inject seed larger than the minimum required seed size should be fine.

Yes, makes sense. I'll send an updated version of patch.

Best regards,
Krzysztof

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