tapping undersea fibers?

John Denker jsd at research.att.com
Mon Jun 4 12:09:52 EDT 2001

I wrote:

>>AfricaONE has a backbone that circles the continent offshore, plus 
>>separate drops for each country, when it would have been vastly cheaper 
>>to go by land....[Offshore is] less likely to be tapped by hostile powers.

At 12:38 AM 6/4/01 -0400, Lenny Foner wrote:

>My understanding of this, which could very well be wrong, was not
>concern about tapping, but concern over damage.

Well, of course there is concern about all sorts of threats:
  *) Tapping
  *) Damage
     -- inadvertent
     -- otherwise

But let's ask what are the costs and benefits of various options:
  1) One obvious option is to build a _ring_ of fiber on dry land.
    1a) Sure, there will be an inadvertent cut now and then (due to some 
klutz with a backhoe) but such cuts can be repaired.  The cost of repairs 
is infinitesimal compared to the cost of running the cable 
offshore.  Because it is a ring, customers won't notice cuts if they happen 
one at a time (and are repaired promptly);  it would take _two_ cuts to 
cause a partition.
    1b) There will be wars now and then.  Each combatant _will_ want to cut 
the adversary's cable.  (Ever hear of a foreign minister named 
Zimmerman?)  There's not a lot that the cable operator can do to maintain 
service in the war zone.  But remember it takes two cuts, including one 
_not_ in the war zone, before anybody outside the war zone is left in the dark.
    1c) It is not impossible to have damage (inadvertent or otherwise) to 
an offshore cable, as Herr Zimmerman found out.
    1d) A dry-land configuration would provide !much! more functionality -- 
more places for paying customers to connect.
  2) You can do even better if build something more like a mesh than a 
simple ring.  Such a structure would be tremendously robust against damage, 
and would provide even more places for customers to connect.

So it seems to me that the principal rationale for putting the cable 
offshore is the expectation that the two-bit warlord next door would have a 
hard time tapping the backbone.

>Is there technology in existence that can usefully encrypt the entire
>contents of such cables' data stream, end-to-end?

It's certainly doable.  You might think it would get easier year by year, 
as electronics gets cheaper and faster -- but fibers are getting faster, 
too.  The crucial factor is that the crypto market has grown to the point 
where people are actually making stuff for this market, such as 3DES in 
hardware at (nearly) OC-48 speeds:

Each wavelength on the AfricaONE fiber is OC-192 (10 Gb/sec).  You would 
have to pick the incoming OC-192 data apart into four or five OC-48 
streams, encrypt it, re-assemble it, and send it to the laser.  That would 
work OK for a point-to-point link, but more generally you would need to add 
"outer headers" à la IPsec.  And you might want to worry about key 
management :-).
  -- A full solution would be a nifty piece of engineering.  You can't buy 
it at K-mart.
  -- But it would be a lot cheaper than putting the cable offshore 

People say that CBC prevents you from parallelizing the encipherment, but 
in practice it's straightforward to make it work (for modest degrees of 
parallelism).  Or you can use counter mode.

>It seems to avoid a whole class of threat models---such as discouraging 
>random entities from destroying the cable due to a bungled tap, ......

That's a good argument, in peacetime anyway.  OTOH in wartime, a link that 
can't be tapped is _more_ likely to be destroyed.

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