Ideally, the token is issued by a "bank" where both sending MTA and
receiving MTA have accounts.
You probably don't want to do that, since the performance of book
entry transactions is worse than for bearer ones. If you're doing
book entry, you need at least a journalling system to log the entries
to be added to each account, if not locks on each account to maintain
the balance.
Here's the least awful way I know to do small value online payments:
Sender goes to his bank and buys a bunch of bearer stamps. There are
two things one can do with a bearer stamp, cancel/reissue, and redeem
it.
Cancel/reissue simply creates a new stamp worth the same as the old
stamp (or maybe a little less if the bank keeps its cut) but promises
that the new stamp is good. If the old stamp has already been
cancelled, this fails. This transaction is supposed to be fast, since
it needs only check the one bit to see if it's been used and generate
the new stamp or maybe take one from a precomputed list of them.
Redeeming is a batch process, collect a whole bunch of stamps, send
them in to the bank, and in a while you get a credit for all of them,
maybe in an account in that bank, maybe sent to your real bank.
The point of bearer stamps is that the bank doesn't try to track who's
got them, they're like cash and whoever presents a stamp first wins.
So when you get those reissued stamps for incoming postage, you have
to keep those private, since if someone else finds out what they are
and sends them to the bank first, he wins and you lose.
None of this is new, this was all worked out a decade ago. But it's
all still way too slow to keep up with an interesting amount of e-mail.
R's,
John
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