Transport Mixes

It might be possible to transport goods in a difficult to trace fashion,
by applying the concept of a digital mix to physical transport.
We can't make them cryptographically hard to trace, but perhaps we 
can minimize the clue trail by substituting crypto for normal 
shipment records.

Here's a courier system for shipping small packages untraceably:

+ put your packages in a suitcase for a flight to a major hub airport,
which is also a courier mix site.  Each bag contains an innocuous
looking Newton with a small wireless radio.  These identify themselves
as mix shipments only if the proper one time key is transmitted over
a spread spectrum wireless channel.  You also put a message in the
Newton indicating the shipment route and enclosing digital cash
as postage; these are successively encrypted with each mix's public 
key, just as with remailers

+ couriers spend all day locating mix bags and taking them to
a nearby hotel room, and taking bags from the hotel room when 
they have been delayed, mixed, and queued for the flight they are
and only take a few bags each at a time

+ the particular hotel and room changes every day

+ a separate courier inside the hotel room takes the following steps:
	-- decrypts a message inside the Newton with his private 
	   key to reveal the next destination for the shipment
	-- decrypts the digital cash fee for this mix and clears it
	-- puts the package and Newton in a new suitcase and
	   puts it in a queue for that new destination (a flight
	   at some point in the future, say 10-30 hours later)

+ like any mix, the delay depends on the rate of traffic going through 
the system: we'd like to mix up at least ten or so shipments at each mix

+ we assume that customs checks any bag at at most n-1 out
of n hubs (highly probable unless the bag contains something
that looks suspicious in the x-ray).

+ the contents of the package should contain no clues as to
source and destination, unless they are securely encrypted

+ each mix is a separate organization, composed of only a few
mutually trusted couriers

+ we need protocols for destroying shipments or shipping them
back to the customer, due to suspicous nature of the package
(customs would catch guns, bombs, sniffable drugs, etc.)
unclearable postage, or other exceptions that might occur,
and informing customers and arbitrators of these actions

Something of this sort might even be possible with larger shipments 
using large ocean ports instead of airports, standard size pallets 
instead of suitcases, and warehouses instead of hotel rooms.  
Again, I make no claim that this would be cryptographically
strong; but in some cases we can use cryptographic protocols
to stop info flows related to transport that are otherwise vulverable
to attack, and concentrate on various techniques to minimize
other vulnerabilities.

The mix concept is limited to important shipments where one is
willing to pay a high preium.   The number of mixes will be small,
because increase in transport costs quickly overwhelms the 
increased security of using additional mixes, because there 
are these other vulnearabilities that become more important.

Can one travel personally, untraceably?  Here is a method
analogous to a mix:

+ instead of flying directly to one's destination, fly through two or 
three hubs

+ pay for each ticket with cash; if ID is necessary use unlinkable nom 
de guerres at each airport

+ remove suitcase tags at each airport

+ dress differently at each airport (just enough to foil routine memories 
of stewardesses, etc.)

+ if one is being followed use the various methods to lose them,
choose three new hub airports and start over

Jim Hart
hart@chaos.bsu.edu

Jim Hart wrote:

> It might be possible to transport goods in a difficult to trace fashion,
> by applying the concept of a digital mix to physical transport.
> We can't make them cryptographically hard to trace, but perhaps we 
> can minimize the clue trail by substituting crypto for normal 
> shipment records.

We last had a major thread on this at least a year and a half ago, so
it's worth looking at again certainly.

Before I get to Jim's scheme, bear in mind a couple of extremely
important aspects of physical package shipping vs. crypto:

1. The physical packages are extremely easy to inspect by sniffing
(for certain chemicals), by examination of the package exteriors
(unless "repackaged" each time), by x-raying of the interiors, and by
weighing and similar mass/moments of inertia characterizations.

2. Physical packages are _often_ inspected, if suspicions arise. The
level of security is vastly lower than for shipping encrypted bits
around. Many people who thought they could carefully wrap some hash up
and ship it home found out otherwise.

3. Since people cannot practically do the "envelope within envelopes"
nesting, for packages [they can, but it's trivially detectable], a
major element of mixes is lost. [Practically, any of the remailers
can mark packages, attach bugs, etc. A killer.]

Onward to Jim's scheme:

> Here's a courier system for shipping small packages untraceably:
> 
> + put your packages in a suitcase for a flight to a major hub airport,
> which is also a courier mix site.  Each bag contains an innocuous
> looking Newton with a small wireless radio.  These identify themselves
> as mix shipments only if the proper one time key is transmitted over
> a spread spectrum wireless channel.  You also put a message in the
> Newton indicating the shipment route and enclosing digital cash
> as postage; these are successively encrypted with each mix's public 
> key, just as with remailers
> 
> + couriers spend all day locating mix bags and taking them to
> a nearby hotel room, and taking bags from the hotel room when 
> they have been delayed, mixed, and queued for the flight they are
> and only take a few bags each at a time
> 
> + the particular hotel and room changes every day

If the mix process it itself trusted, then the airport steps can (and
hence should) be skipped. Unlike the case with software remailers,
where additional steps increase the chance that at least one of them
is reliable (and hence mixes the traffic properly), physical remailers
have the property that each additional mix node increase the chance of
compromisinging things, of attaching bugs, of marking the packages,
and so forth.

> + we need protocols for destroying shipments or shipping them
                                               ^^^^^^^^^^^^^^^^
> back to the customer, due to suspicous nature of the package
  ^^^^^^^^^^^^^^^^^^^^
> (customs would catch guns, bombs, sniffable drugs, etc.)
> unclearable postage, or other exceptions that might occur,
> and informing customers and arbitrators of these actions

Huh? Not much of an anonymous remailer if this is possible. (Our
Cypherpunks remailers can do this trace-back only when people don't
encrypt, and most easily only at the first stage. Properly executed
mixes don't allow such trace-backs at all.)

> Can one travel personally, untraceably?  Here is a method
> analogous to a mix:

Jim's schemes elided. This is familiar stuff to any spy thriller fan,
as I am, and it was "losing a tail" that motivated me to think about
my "Labyrinth" scheme in 1987, which I then described to David Chaum
at Crypto '88, only to find he'd already formalized it several years
earlier. 

Both packages and people are easy enough to tag-and-follow that they
are not even in the same league as the cryptograhic security of
digital mixes.

--Tim May


-- 
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