From: nobody@rebma.rebma.mn.org
Date: Sat, 12 Dec 92 14:04:53 PST
To: cypherpunks@toad.com
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Message-ID: <m0n0efX-0006XfC@rebma.rebma.mn.org>
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Update on the digital cash project

I am having some problems with the port to MSDOS, mostly due to
implicitly assuming 32-bit integers in a few places.  Probably I won't
get it working until next weekend.

To recap, the program provides Chaum-style digital cash via two
executables, one for the "players" and one for the "banker".  The
banker creates a public key which has a single modulus n and multiple
exponents, the prime numbers starting with 17.  He sends n to the
players and all is ready.

Players withdraw money by running their programs and specifying the
denominations they want to withdraw.  For example, you could withdraw
a 1, two 5's, a 10 and a 20.  This would create a file with 5 entries
to be sent to the bank.  PGP should be used to encrypt and
ascii-encode this file (for privacy) and it should be mailed to the
banker.

The banker receives this file and runs his program to RSA-sign the
values in each of the withdrawal-request entries.  This is the
"blinded cash" that Chaum describes.  Again, PGP should be used for
mailing this back to the user.

The player then has to "unblind" the file to make it "real" digital
cash.  This also changes it so that the bank won't recognize it when
it is deposited.  He uses his version of the program to do this,
producing an actual digital money file with the five "digital bills"
in it.

To pay another user, he runs another function to extract the desired
bills from this file.  Suppose he wants to extract a 1 and a 5.  This
leaves a 5, a 10 and a 20 in the original file, and creates a new
digital cash file with a 1 and a 5.  He would then use PGP again to
encrypt this for safety and mail it to the person he wants to pay.

That person can run a "check" function on the incoming digital money
to make sure it has a proper bank signature on it and is not a
forgery.  He would then mail it directly to the bank so that it could
get credited to his account.

The banker runs his program which checks the signatures on the
incoming money, looks in a database file to make sure these bills
haven't been used before, and adds these bills to the database.  (The
database stores 16 bytes per bill.)  He should then record the deposit
and perhaps send a confirmation to the depositor (my program doesn't
get involved with that).

I hope this gives a clearer picture of how the electronic money
program works.  It is a simple implementation but I think many systems
would work similarly.

I appreciated the suggestion to use cash as part of the list
management itself.  Rather than paying people who post, I wonder if it
would be better to make people pay to post.  Many people have
complained about the volume.  :)  Unfortunately, I suspect that this
would involve too much overhead for the mailing list maintainer.

Maybe the thing to do is to just get the software out there and let
people decide what they want to do with it (if anything).  I'm
probably going to take a couple more weeks to clean up the user
interface and get these bugs out, then I'll try sending it someone to
be put on the cypherpunks ftp archive.

It's nice to be able to finally sign these messages!

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