[Cryptography] Measuring propagation times

[John Gilmore]

On 22 Sep 2025, at 17:54, Sampo Syreeni wrote:
> 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?

Steven M. Bellovin <smb at cs.columbia.edu> wrote:
> I think that there are real-world protocols that do that. Or see [sec]5 of
> https://www.cs.columbia.edu/~smb/papers/netmeas.pdf from 1992.

Ultrawideband communication devices use very carefully measured
propagation delays of infrequent wideband pulses, for locating peer
communicators in relative 3d space.  See:

  https://en.wikipedia.org/wiki/Ultra-wideband

This tech is starting to be built into mobile phones, watches, and item
tags.  I don't know if anyone is using it for security yet -- it seems
mostly used for locating things.  And, of course, it tells you about where
a peer is, but not whether they are trustworthy.

	John