[149147] in cryptography@c2.net mail archive
Re: [Cryptography] cheap sources of entropy
daemon@ATHENA.MIT.EDU (Jerry Leichter)
Tue Jan 21 02:38:50 2014
X-Original-To: cryptography@metzdowd.com
From: Jerry Leichter <leichter@lrw.com>
In-Reply-To: <52DD7223.5050806@av8n.com>
Date: Mon, 20 Jan 2014 15:55:43 -0500
To: John Denker <jsd@av8n.com>
Cc: cryptography@metzdowd.com
Errors-To: cryptography-bounces+crypto.discuss=bloom-picayune.mit.edu@metzdowd.com
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On Jan 20, 2014, at 1:59 PM, John Denker <jsd@av8n.com> wrote:
>> Getting quality random bits when you have (a) almost any kind of
>> high-rate real-world sensor and (b) a human being willing to help is
>> an easy problem. Any modern cellphone can provide tons of randomness
>> if a person moves it around, talks to it, waves his hands around in
>> front of it.
>=20
> That is not the smart way to think about it....
I have no problem with Turbid and with the general notion of getting =
randomness from good physical sources. But you missed the point of my =
message entirely.
We keep coming back to discussions of randomness generators in general, =
and being side-tracked by focusing on generators in "easy" situations. =
Situations where you can add hardware and software to do something like =
Turbid are easy. (It's not that about whether *Turbid* is easy, it's =
about what you can do given that you already have it.) Situations like =
the one I mentioned above are also easy.
Perhaps it would be better to get away from "randomness" and talk about =
"unpredictability". Yes, I consider the inputs from a bunch of sensors =
in a cell phone being swung around by a human being to be unpredictable. =
Even to someone who has no access to the cellphone but does have =
several high-quality sight and sound recordings of the event. All the =
processes in play are noisy and have large chaotic components. A sound =
field in a complex natural environment, if looked at to high precision, =
is extremely variable from place to place and from moment to moment. =
Sure, you can get the general field pretty precisely - but knowing =
whether the bottom bit of output from a sensitive A/D converter is set =
at any particular sampling time (which you don't know exactly either) - =
no. Human motion is governed by the firing of multiple nerve cells =
triggering multiple muscle cells, subject to multiple layers of neural =
control, all interacting in complicated ways with feedback from sensors =
of internal and external state. Even the most practiced of movements =
vary in unpredictable ways when repeated and measured to the kind of =
accuracy that you can easily get from cell phone sensors. And you know =
... some of the variation in neural behavior is due to exactly the kind =
of quantum noise down at the synaptic level that you're basing Turbid =
on. A real neuron isn't the same as the neurons we use for nice CS =
examples - it's much more complicated.
If I had a *choice* between a carefully implemented physical circuit =
based on shot noise or some similar well-understood source of "core =
randomness" or something fairly ad hoc based on sensors and human =
interaction, *of course* I would choose the former. But that may not be =
available. Still, the latter isn't bad, even if the actual randomness =
available can't be as easily quantified.
Very few things in the real world are subject to proof in any =
mathematical sense. No one can compute the strength of steel beams from =
first principles; we measure the strengths of materials in particular =
quantities, configurations, at particular temperatures and other real =
word conditions, make tables, and go from there. Even then, exact =
computation of the dynamic forces - often even the static forces - =
present on the beams in a building are way beyond our capabilities. We =
make approximations, and we add a safety factor just in case. Still - =
buildings and bridges and such stay up and behave as predicted, for the =
most part.
I contend that I can build a system of the sort I described and have =
sufficient confidence in the unpredictability of the values, and of =
their range ("entropy", if you like - I find that word so often misused =
in cryptographic discussions that I try to avoid it), that I'd have no =
qualms using it as a source of "randomness" for pretty much any purpose.
Would I *prefer* to have a Turbid-style generator to mix the values up =
with?Absolutely, but I can live without it if I have to. (Why "mix it =
up"? Because you can prove all you like about the *design*, but you can =
prove absolutely nothing about the *physical artifact* I have sitting on =
the desk in front of me. That thing over there is *supposed* to be just =
a 47K resistor; and that's *supposed* to be a plain USB connector. But =
can you know for sure?)
-- Jerry
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