From: JR@ns.cnb.uam.es
Date: Tue, 30 Jan 1996 08:36:10 +0800
To: IM"cypherpunks@toad.com"@ns.cnb.uam.es
Subject: Re: Time codes for PCs (fromn German Banking)
Message-ID: <960129234228.20402217@ROCK.CNB.UAM.ES>
MIME-Version: 1.0
Content-Type: text/plain


From:	SMTP%"jimbell@pacifier.com" 27-JAN-1996 03:43:05.83

>A peripheral I've long wanted to see, commonly available:  ACCURATE  time, 
>broadcast to the millisecond/microsecond/nanosecond, available from sources 
>as varied as TV VIR's, FM subcarriers, and other sources, available as an 
>easy input (via a peripheral card) to a computer.
>
	Yup! Do you think it is really possible? If I remember well
speed of light is 300.000 Km/s. That means that light takes around
1 ms. to cover 300 Km. If you use a satellite, antenna, whatever
to broadcast a timing signal, the accuracy will depend on when
do you receive it, and that in turn on your distance from the
source.

	By the time the signal reaches you it may be several milliseconds
old (and thousands of microsenconds and millions of nanoseconds). We are
so used to think of TV, whatever as an instantaneous broadcast medium
that we forget that there are speed limits in the Universe. And you
can't exceed them by just paying a ticket.

	Note that this was a best case scenario: using the speed of
light to transfer the information and using the shortest path. In
reality most waves won't travel as fast in the air, and will depend
on atmospheric circumstances.

>I have a 12-year-old Heathkit "Most Accurate Clock" that I assembled myself, 
>and had the foresight to install it with its computer  interface option. 
>(receives 5, 10, or 15 MHz signals broadcast from Boulder, Colorado, 
>containing "exact" time.) 
>
	Just remember that the best you can get would be microseconds if
you're in a 300 meter radius, or milliseconds on a 300 Km. And possibly
nanoseconds at 0.3 m.

	Even then you need to know exactly (i.e. with an accuracy of
between centimeters to a few Km depending on the timing you want) your
position. And depending on the media you use, possibly the atmospheric
conditions in between the emisor and your receptor.

	Then remains the cypherpunk part on all this: how can you
trust the *signal* your receptor receives? How do you know no one is
interferring it or sending an inaccurate or false one?

	So you need a GPS... And a timing source that can be trusted.
You'd wantthe signal not to be tamperable or at least to be able to
detect when it has been tampered.

	And that on a broadcast system. A system owned by someone who
you may not trust (say a private TV channel, radio or satellite). So
you may want to have several sources, and to be able to verify that
the signals you receive all come from their respective sources.

	Yum! a nice problem to think about. One factor is that you
wouldn't expect changes in public sources used by sensible systems
since those could not pass unnoticed and might raise big protests.
But you still have the MITM attack to consider...

	Oh well, it's too late now. See ya...

				jr