[Cryptech Tech] Suggested changes to TRNG

[Joachim Strömbergson]

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Aloha!

Basil Dolmatov wrote:
>> It cannot reflect changes (including ageing as well as tweaking and
>> tampering), so it cannot be used as the reason to set "OUTPUT_GOOD"
>> signal.

Exactly. And I've never suggested anything to that effect. At least not
intentionally.

The output good signal will be based on startup tests. And during
production tests by online tests that constantly monitors the entropy
providers as well as the output of the csprng.

The _only_ reason for the warmup delay is to avoid possibly really bad
entropy that appears directly after configuration and system startup. To
give the physical processes time to become less predictable.

It is my guess that this is esp true for the rosc based entropy provider
since the oscillators will always be in a defined state directly after
FPGA configuration. Letting the oscillators free run for a while before
starting to collect entropy from them seems prudent to me.

This is the only purpose of the functionality I've added.

One could of course spend a fair bit of time measuring the behaviour of
the entropy providers at time zero and then a number of times up to some
limit (seconds, minutes) and try to come up with a number where initial
conditions seems to have disappeared. Something for a later time. Right
now I just kill a bunch of ms which means that the rosc oscillators runs
for several millions of cycles before using them.

- -- 
Med vänlig hälsning, Yours

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