[Cryptech Tech] Seeking comments on a proposal for changes to the Cryptech RNG design.

Joachim Strömbergson joachim.strombergson at assured.se
Thu Mar 29 08:59:59 UTC 2018


Pavel Shatov wrote:
>> Hi, Joachim, sorry for a late reply.

NP, thanks for thoughts.

>> I was thinking, in what place of SHA-512 exactly do timing checks fail?
>> I bet it's in the carry chains of 64-bit adders.

yes, or more correctly the chained 64-bit ALU ops. In SHA-512 there are
a chain of three 64-bit adders in the worst path (and the carry will be
the longest through it all). See op-chain in the attached image.

I have considered pipelining between these ops before. This will fix the
clock speed issue. The downside is of course that performance will be
halved. This might still be ok for the RNG though.

I had dismissed this solution since it doesn't also reduce the resources
consumes. (It actually increases slightly, of course. But a 64-bit reg
and an extra FSM state is is really insignificant.)

But your post and Robs comment makes me think that we should treat this
as a multi stage development of the RNG. Fix clock speed now by
pipelining and then work on resource reduction at a later step.

In fact, I see more pressing things to improve in the RNG that we should
probably focus on before resource reduction. Implementing online tests
for the entropy sources (based on AIS31) for example.

Decision: Pipeline the SHA-512 to solve clock speed.

Thanks to all who has given feedback. I'm pretty certain we will replace
the SHA-512 in a future update. And do other changes to the RNG. I'll
post ideas about such ideas in separate mails.

Med vänlig hälsning, Yours

Joachim Strömbergson - Assured AB

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