[Cryptography] Entropy of a diode

[Bill Cox]

Please stop using reverse-breakdown of emitter-base diodes as noise sources
for crypto.

These circuits have several limitations, such as sensitivity to power
supply noise, changing noise patterns as they age, and have no reliable
model for predicting the rate or quality of random bits, making it
difficult to monitor the health of the noise source.  A better circuit
is a modular
entropy multiplier <https://github.com/waywardgeek/infnoise>, which costs
around $1 in parts, is simple to breadboard
<http://www.openrandom.org/blog/breadboard-infinite-noise-trng>, and has
open-source implementations including what parts to buy, and where to get
them.  Alternatively, if you happen to have an FPGA on your board, you can
build a reliable TRNG using phase noise from multiple ring oscillators.  If
you happen to have access to a 24-bit A/D, the lowest bits will be reliably
random if you sample audio from a mic, or even an unconnected mic jack.  If
all you want is to collect 256 true random bits for crypto to seed a CPRNG,
you can easily use the mouse and keyboard to accumulate your bits from a
highly random source: humans.

Bill
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