Re: [PATCH 0/5] Feed entropy pool via high-resolution clocksources

Matt Mackall wrote:

On Wed, 2011-06-15 at 10:49 -0400, Jarod Wilson wrote:

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Matt Mackall wrote:

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On Tue, 2011-06-14 at 18:51 -0400, Jarod Wilson wrote:

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Matt Mackall wrote:

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But that's not even the point. Entropy accounting here is about
providing a theoretical level of security above "cryptographically
strong". As the source says:

"Even if it is possible to  analyze SHA in some clever way, as long as
the amount of data returned from the generator is less than the inherent
entropy in the pool, the output data is totally unpredictable."

This is the goal of the code as it exists. And that goal depends on
consistent _underestimates_ and accurate accounting.

Okay, so as you noted, I was only crediting one bit of entropy per byte
mixed in. Would there be some higher mixed-to-credited ratio that might
be sufficient to meet the goal?

As I've mentioned elsewhere, I think something around .08 bits per
timestamp is probably a good target. That's the entropy content of a
coin-flip that is biased to flip heads 99 times out of 100. But even
that isn't good enough in the face of a 100Hz clock source.

And obviously the current system doesn't handle fractional bits at all.

What if only one bit every n samples were credited? So 1/n bits per
timestamp, effectively, and for an n of 100, that would yield .01 bits
per timestamp. Something like this:

Something like that would "work", sure. But it's a hack/abuse -relative
to the current framework-. I'm reluctant to just pile on the hacks on
the current system, as that just means getting it coherent is that much
further away.

Something I should have probably made clearer was that we were looking 
for a semi-quick improvement for a particular use case that involves a 
certain 2.6.32-based kernel... For the short term, we're looking at some 
userspace alternatives, such as a Linux implementation of an entropy 
seed approach BSI approves of. Reference doc here, but in German:

https://www.bsi.bund.de/ContentBSI/Publikationen/TechnischeRichtlinien/tr02102/index_htm.html

Longer-term, we're having some discussions related to the revamped 
framework you've laid out. I'll table this clocksource-based entropy 
contribution code for now.

Also, thank you for putting up with me. ;)

-- 
Jarod Wilson
jarod@redhat.com