[Cryptography] OTP USB TLA

[Sampo Syreeni]

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 at iki.fi, http://decoy.iki.fi/front 
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