Re: [PATCH v2 02/03]: hwrng: create filler thread

[H. Peter Anvin]

There are a number of things wrong with this post, but I'll respond in detail when I get to a keyboard.

On March 26, 2014 6:11:53 PM PDT, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>[cc: Greg Price, might be working on this stuff]
>
>On Wed, Mar 26, 2014 at 6:03 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> I'm wondering more about the default.  We default to 50% for
>arch_get_random_seed, and this is supposed to be the default for in
>effect unverified hwrngs...
>
>TBH I'm highly skeptical of this kind of entropy estimation.
>/dev/random is IMO just silly, since you need to have very
>conservative entropy estimates for the concept to really work, and
>that ends up being hideously slow.  Also, in the /dev/random sense,
>most hardware RNGs have no entropy at all, since they're likely to be
>FIPS-approved DRBGs that don't have a real non-deterministic source.
>
>For the kernel's RNG to be secure, I think it should have the property
>that it still works if you rescale all the entropy estimates by any
>constant that's decently close to 1.
>
>If entropy estimates are systematically too low, then a naive
>implementation results in an excessively long window during early
>bootup in which /dev/urandom is completely insecure.
>
>If entropy estimates are systematically too high, then a naive
>implementation fails to do a catastrophic reseed, and the RNG can be
>brute-forced.
>
>So I think that the core code should do something along the lines of
>using progressively larger reseeds.  Since I think that /dev/random is
>silly, this means that we only really care about the extent to which
>"entropy" measures entropy conditioned on whatever an attacker can
>actually compute.  Since this could vary widely between devices (e.g.
>if your TPM is malicious), I think that the best we can do is to
>collect ~256 bits from everything available, shove it all in to the
>core together, and repeat.  For all I know, the core code already does
>this.
>
>The upshot is that the actual rescaling factor should barely matter.
>50% is probably fine.  So is 100% and 25%.  10% is probably asking for
>trouble during early boot if all you have is a TPM.
>
>--Andy

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