From: Hal <hfinney@shell.portal.com>
Date: Fri, 12 Apr 1996 07:12:39 +0800
To: cypherpunks@toad.com
Subject: Re: No matter where you go, there they are.
Message-ID: <199604111437.HAA05376@jobe.shell.portal.com>
MIME-Version: 1.0
Content-Type: text/plain


I think the various comments are correct that Denning's scheme won't work
across the Internet as it currently exists.  Any network with latencies in
the multiple milliseconds and up will allow the fraud where the remote node
lies about its latency in order to allow it to move some of the received
data "forward in time", which is necessary but would not be possible if
latency were known and fixed.

Note however that Denning did not mention the Internet in her spiel.

I believe her method would be workable across lower latency networks, if such
exist or eventually exist.  Perhaps direct connections or leased lines would
provide low enough latency; I don't know.  In any case networks are likely
to become faster in the future and her method might eventually work.
Actually the issue is not just latency but whether the latency can be
lied about, and for some kinds of networks that would be harder.

The method of using authenticated devices which provide timestamped
data from satellites not visible to the authenticating site does not
need to provide that data in real time.  Even if it is delayed so it
comes in later than the data from the remote site, the verifying site
can still use it to calculate what the remote site should have been
seeing, and so get the benefit of using timings from all the satellites
visible to the remote site (again, assuming the remote site itself has
a low latency connection to the authenticating site).

They do mention that in urban or other obstructed locations a partial
view of the sky may be adequate.  But of course if all the satellites
visible to the remote site are in the south, it can move its apparently
location north by using older data.  So for the system to work there
must be satellites visible in all parts of the sky (no line you can
draw through your location which puts all satellites on one side of
that line).

Hal