Privacy-enhancing uses for TCPA

3 Aug 2002

      Here are some alternative applications for TCPA/Palladium technology which
could actually promote privacy and freedom.  A few caveats, though: they
do depend on a somewhat idealized view of the architecture.  It may be
that real hardware/software implementations are not sufficiently secure
for some of these purposes, but as systems become better integrated
and more technologically sound, this objection may go away.  And these
applications do assume that the architecture is implemented without secret
backdoors or other intentional flaws, which might be guaranteed through
an open design process and manufacturing inspections.  Despite these
limitations, hopefully these ideas will show that TCPA and Palladium
actually have many more uses than the heavy-handed and control-oriented
ones which have been discussed so far.

To recap, there are basically two technologies involved.  One is "secure
attestation".  This allows machines to securely receive a hash of the
software which is running remotely.  It is used in these examples to
know that a trusted client program is running on the remote machine.
The other is "secure storage".  This allows programs to encrypt data
in such a way that no other program can decrypt it.

In addition, we assume that programs are able to run "unmolested";
that is, that other software and even the user cannot peek into the
program's memory and manipulate it or learn its secrets.  Palladium has
a feature called "trusted space" which is supposed to be some special
memory that is immune from being compromised.  We also assume that
all data sent between computers is encrypted using something like SSL,
with the secret keys being held securely by the client software (hence
unavailable to anyone else, including the users).

The effect of these technologies is that a number of computers across
the net, all running the same client software, can form their own
closed virtual world.  They can exchange and store data of any form,
and no one can get access to it unless the client software permits it.
That means that the user, eavesdroppers, and authorities are unable to
learn the secrets protected by software which uses these TCPA features.
(Note, in the sequel I will just write TCPA when I mean TCPA/Palladium.)

Now for a simple example of what can be done: a distributed poker game.
Of course there are a number of crypto protocols for playing poker on the
net, but they are quite complicated.  Even though they've been around
for almost 20 years, I've never seen game software which uses them.
With TCPA we can do it trivially.

Each person runs the same client software, which fact can be tested
using secure attestation.  The dealer's software randomizes a deck and
passes out the cards to each player.  The cards are just strings like
"ace of spades", or perhaps simple numerical equivalents - nothing fancy.
Of course, the dealer's software learns in this way what cards every
player has.  But the dealer himself (i.e. the human player) doesn't
see any of that, he only sees his own hand.  The software keeps the
information secret from the user.  As each person makes his play, his
software sends simple messages telling what cards he is exposing or
discarding, etc.  At the end each person sends messages showing what
his hand is, according to the rules of poker.

This is a trivial program.  You could do it in one or two pages of code.
And yet, given the TCPA assumptions, it is just as secure as a complex
cryptographically protected version would be that takes ten times as
much code.

Of course, without TCPA such a program would never work.  Someone would
write a cheating client which would tell them what everyone else's cards
were when they were the dealer.  There would be no way that people could
trust each other not to do this.  But TCPA lets people prove to each
other that they are running the legitimate client.

So this is a simple example of how the secure attestation features of
TCPA/Palladium can allow a kind of software which would never work today,
software where people trust each other.  Let's look at another example,
a P2P system with anonymity.

Again, there are many cryptographic systems in the literature for
anonymous communication.  But they tend to be complicated and inefficient.
With TCPA we only need to set up a simple flooding broadcast network.
Let each peer connect to a few other peers.  To prevent traffic
analysis, keep each node-to-node link at a constant traffic level using
dummy padding.  (Recall that each link is encrypted using SSL.)

When someone sends data, it gets sent everywhere via a simple routing
strategy.  The software then makes the received message available to the
local user, if he is the recipient.  Possibly the source of the message
is carried along with it, to help with routing; but this information is
never leaked outside the secure communications part of the software,
and never shown to any users.

That's all there is to it.  Just send messages with flood broadcasts,
but keep the source locked inside the secure part.  Messages can be
sent and received, and neither participants nor outsiders can tell
what the source of any message is.

As with the earlier example, such a system would never work without TCPA.
Rogue software would easily determine which direction messages were coming
from, and the anonymity provided would be extremely limited at best.
But by eliminating rogues using secure attestation, and keeping the
sensitive data safe from molestation, we are able to achieve using a
very simple system what otherwise takes tremendous complexity.

Here's one more example, which I think is quite amazing: untraceable
digital cash with full anonymity, without blinding or even any
cryptography at all! (Excepting of course the standard TCPA pieces like
SSL and secure storage and attestation.)

The idea is, again, trivial.  Making a withdrawal, the client sends the
user's password and account ID to the bank (this information is kept in
secure storage).  The bank approves, and the client increments the local
"wallet" by that amount (also kept in secure storage).  To make a payment,
use the anonymous network for transport, and just send a message telling
how much is being paid!  The recipient increments his wallet by that
amount and the sender decrements his.  Deposit works analogously to
withdrawal.

Again, that's all there is to it.  Nothing could be simpler.  Yet it
provides for secure (assuming TCPA is secure), anonymous, untraceable
payments.  The secure attestation is crucial, of course, to make sure
that people are running legitimate clients, otherwise cheating would
be rampant.  And the secure storage is equally crucial, otherwise any
software could increment the sum stored in the wallet and everyone would
accept and believe those payments.

I understand, of course, that this specific example is not very practical
unless we have an extremely secure version of TCPA.  If anyone who
can break the security can give themselves unlimited money, it means
that the security has to be essentially perfect.  So this is more of a
proof of concept than a realistic proposal.  But eventually, with TCPA
technology integrated into a tamper-proof, nanotech CPU with molecular
sensors and built-in self-destructs, possibly this might be good enough.

Or you could augment this solution with some crypto, similar with the
"wallets with observers" proposals from Chaum and from Brands.  Note that
we can make the client open-source, allowing anyone to verify that it
has no back doors or cheating potentials, which allows all users to
trust that it is not going to hurt them (a problem that takes great
complexity to solve with the observer protocols).  But still the bare
simplicity of the system should make clear how powerful something like
TCPA can be for this kind of application.

I could go on and on, but the basic idea is always the same, and hopefully
once people see the pattern they will come up with their own ideas.
Being able to write software that trusts other computers allows for an
entirely new approach to security software design.  TCPA can enhance
freedom and privacy by closing off possibilities for surveillance and
interference.  The same technology that protects Sony's music content
in a DRM application can protect the data exchanged by a P2P system.
As Seth Schoen of the EFF paraphrases Microsoft, "So the protection of
privacy was the same technical problem as the protection of copyright,
because in each case bits owned by one party were being entrusted to
another party and there was an attempt to enforce a policy."
(http://vitanuova.loyalty.org/2002-07-05.html, 3rd bullet point)

In fact, TCPA and Palladium have tremendous potential for enhancing
and protecting privacy, if people will just look at them with an
open mind.

AARG! Anonymous

Jay Sulzberger

Seth David Schoen

Jay Sulzberger

James A. Donald

Mike Rosing

Peter N. Biddle

Adam Shostack

Bill Frantz

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