[Cryptography] Entropy of a diode

[Bill Cox]

On Sat, Jul 23, 2016 at 2:48 PM, David Johnston <dj at deadhat.com> wrote:

> The entropy in a diode is a very open question indeed. In practice the
> entropy in the diode is vast relative to anyone's cryptographic needs.
>
> The problems are
> A) Getting it out and turning it to useful random bits.
> B) Knowing how much entropy is intrinsic to the diode and how much is
> externally sourced that you don't want to count.
> C) Knowing what kind of extractor to use
> D) Knowing how to conduct online health tests, which means knowing the
> failure modes.


I agree.  Good TRNGs can be built using diode noise, and my favorite of
these is the OneRNG.  They build these devices with lots of shielding and
redundancy (mixing with an RF source).  They characterize the reverse
breakdown of emitter-base junctions they use as noise sources themselves,
rather than relying on any data sheet or physics model.  You can overcome
the problems of a diode noise source, but for future designs, I hope
designers will use better circuits.

A new diode-based TRNG on the way is ChaosKey:
http://altusmetrum.org/ChaosKey.  I am glad to see more open-source TRNGs,
so kudos to the developer for that.  The source code and schematics are
both available.  However, I wish we were moving away from such noise
sources.

Bill
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