[Cryptech Tech] trng ready for play

[Joachim Strömbergson]

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Aloha!

Bernd Paysan wrote:
> The way the hashing and the CSPRNG work also influence this design:
> As the current design uses SHA-2 and ChaCha, the reseeding procedure
> replaces the previous seed, and therefore each seed has to be really
> good.
> 
> I would have used Keccak as sponge function (and so does Bill Cox)
> today, which has different constraints for the mixer. The sponge
> function allows partial reseeds whenever there is data from some
> entropy provider, and if you just fill up a buffer and do a sponge
> cycle whenever the buffer is full,

And this is what I can do too. The difference being that it is the mixer
that do the partial updates and then sometimes reinitializes the output
function (the CSPRNG). This allows me to really mixing without
interfering with RND generation whatsoever. With only Keccak you need to
find those free cycles now and again to do absorption operations.


> Feeding in a full block of known stuff into SHA-2 as key for ChaCha
> however does reduce the entropy (we only have the counter, which
> isn't reset at reseeding).

I don't know what "we" refers to. The current Cryptech TRNG certainly
resets the counter to a new start value.

- -- 
Med vänlig hälsning, Yours

Joachim Strömbergson - Alltid i harmonisk svängning.
========================================================================
 Joachim Strömbergson          Secworks AB          joachim at secworks.se
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