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<body><div style="font-family: sans-serif;"><div class="plaintext" style="white-space: normal;"><p dir="auto">I think that there are real-world protocols that do that. Or see ยง5 of <a href="https://www.cs.columbia.edu/~smb/papers/netmeas.pdf" style="color: #3983C4;">https://www.cs.columbia.edu/~smb/papers/netmeas.pdf</a> from 1992.</p>
<p dir="auto">On 22 Sep 2025, at 17:54, Sampo Syreeni wrote:</p>
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<div style="display:none !important;display:none;visibility:hidden;mso-hide:all;font-size:1px;color:#ffffff;line-height:1px;height:0px;max-height:0px;opacity:0;overflow:hidden;">On 2025-09-09, John Gilmore wrote: > I had an idea a few years ago that with the ubiquity of high bandwidth > USB interfaces and large flash chips, someone could build a small USB > device that would cache paired True random numbers</div>
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<pre style="font-family: sans-serif; font-size: 100%; white-space: pre-wrap; word-wrap: break-word">On 2025-09-09, John Gilmore wrote:
> I had an idea a few years ago that with the ubiquity of high bandwidth
> USB interfaces and large flash chips, someone could build a small USB
> device that would cache paired True random numbers when physically
> plugged into a second such device.
My recent idea is perhaps even funkier: why not time at the nanosecond
level how non-contact chips communicate with each other, and build a
purposely chatty privacy amplification protocol between them, relying on
the upper bound on propagation guaranteed by relativity? So that Charlie
can't *possibly* interlope/intervene from more than say 1ns==30cm away
from Alice and Bob, in a challenge/response-protocol? He'd *surely* be
hashed out by any stochastic tree walking protocol or such, right?
It'd think something like this isn't just an idea. It's implementable
with current hardware and even the attendant code. Plus it's further
developable on any channel between Alice and Bob, where we know the
minimum physical propagation latency between them.
You could probably do something like this over the 4G Finnish mobile
network I'm using right now. Because it already by necessity meaasures
OFDM slot timing, and then at an even lower level necessarily tracks
carrier phase. I'm reasonably sure that if even some of that data goes
through my local node and truly back, I could build a protocol for
shared key formation over that in...decent...time.
Information theoretical analysis gives you lots of bright ideas. However
it's not the most useful kind of analysis, because it's central idea,
entropy/information is a non-dimensional quantity in the end. It doesn't
and cannot really guide you further. It just is and stands there staring
at you, with its value from the start being arbitrary as well.
--
Sampo Syreeni, aka decoy - decoy@iki.fi, <a href="https://urldefense.com/v3/__http://decoy.iki.fi/front__;!!BDUfV1Et5lrpZQ!TJ43iWZBTYE47ZQmxEAlcvqdHMQMj3Jm7E9Rkiz322uXOsDkz5voBf5eqPIFhRTkgvcJlUfmQi9Qy8k$">https://urldefense.com/v3/__http://decoy.iki.fi/front__;!!BDUfV1Et5lrpZQ!TJ43iWZBTYE47ZQmxEAlcvqdHMQMj3Jm7E9Rkiz322uXOsDkz5voBf5eqPIFhRTkgvcJlUfmQi9Qy8k$</a>
+358-40-3648785, 025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2
</pre>
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<br><p dir="auto"> --Steve Bellovin, <a href="https://www.cs.columbia.edu/~smb" style="color: #3983C4;">https://www.cs.columbia.edu/~smb</a></p>
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