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Re: Magic Money Digicash System

From: [email protected] (Francis Barrett)
>  > Magic Money is a digital cash system designed for use over
>  > electronic mail.
> This is the neatest thing I have read in a long time.  Where can I get
> one?

FTP to csn.org, cd to /mpj, read the file README.MPJ which will tell you
a directory to switch to, do that, cd to pgp-tools (or pgp_tools, or
pgptools, I forget which), and get magicmny.zip.  Then unzip and build it.

> A few questions.  Since the client which generates the proto-coins is
> under the control of the consumer, the bank has no way of making sure
> that he is not running his own code, or that the RNG he is using is
> cryptographically strong, or even that he is not distributing modified
> client programs to other users.

None of these things should cause major problems.  At worst useless coins
would be generated.  Initially, users might send their coins in right away
to confirm that they are OK until they get some confidence in the program.

> How does the bank deal with collisions in the 16 byte values of coins?

This will practially never happen if they are chosen randomly.  Bad
randomness could produce coins which match ones which have already been
spent (if somehow your RNG got into exactly the same state as someone
else's), so they would be valueless.  I think the program makes you
initialize a random file before using it, so just make sure you put
something random there!

> What if the user picks the numeric values for the server to sign in a
> way which leaks information about the banks private key?  RSA is much
> more secure when signing random-esque data, like a message digest,
> than it is when signing numbers provided to it by some outside party.

I don't think there are any values you can sign which would give away a
private key.  Even signing "1" or "2" should be safe, I think, since the
secret key is the size of the modulus.

I ftp'd a paper recently mentioned on imp-interest (on "anonymous
credit cards") which claimed that new cash could be generated from sets
of old cash in Chaum's scheme.  I don't believe this, and the ref was
to a paper "in preparation" by the authors.  I'll try sending them
email to ask about this.

> Similarly, how can the consumer trust the bank's representation that
> money has already been spent?  Surely the bank should be required to
> publish a list of cancelled coins and timestamps with a running MD5
> hash periodically for inspection by the unwashed masses.

Here is how this problem would arise.  Alice has some cash, which she
sends to Bob to buy something.  Bob sends it to the bank to be verified
and turned into fresh cash before he will send the goods to Alice.  But
the bank says the cash has been spent before, and Bob reports this to
Alice.  Alice insists that she has never spent this cash before.

Now, this is like a mystery story.  Who is telling the truth?  Maybe Alice
is lying.  Maybe the bank is lying.  Maybe they are both telling the truth
and someone broke in and stole Alice's cash while she was sleeping, copying
it from her computer and spending it before she could.

Ignoring that last possibility for a minute, it is basically Alice's word
against the bank's.  In general, in situations like this, we often go by
the reputation of the parties involved.  If the bank really is cheating,
there will be lots of other people like Alice, people with good reputations,
who are making similar charges.  This will make people stop trusting the
bank.  On the other hand, if Alice is cheating, this is probably not the first
time.  In time she will get a reputation for being untrustworthy.

The idea of publishing lists of used coins is interesting but I'm not sure
it helps.  Double-spending could easily occur close together in time, between
publication of lists.  A cheating bank could claim a coin had been spent
just before the actual coin came in.

> What do you do about lost messages from the server to the client.
> Once coins have been recorded as spent, they cannot be redeemed again.
> Yet the mail message containing the new coins may have been lost in
> transit.

The server should re-transmit the message if it does not arive.  We
discussed this a while back and it appears safe for everyone in these
protocols to re-transmit messages freely if the other person claims
never to have gotten them.  Even if they are lying, what is the harm -
you are just sending them information they already have.

Good questions.