ecash moneychangers (Was: Kid Gloves or Megaphones)

[iang@cs.berkeley.edu (Ian Goldberg)]

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In article <199605051818.LAA13740@dns2.noc.best.net>,
 <jamesd@echeque.com> wrote:
>
>>>It is true that the issuer is unable to discover that double blinding is
>>>being used. The real problem with the protocol is that it requires
>>>payor/payee collusion, which may make it difficult to execute.
>
>At 07:58 PM 5/4/96 EDT, E. ALLEN SMITH wrote:
>>=09Can the payee discover that the payor isn't colluding before the bank
>>can figure out who the payee is?
>
>If the payor is not colluding, then the payee will immediately discover
>he has not been paid, because the checksums are wrong, and his software
>says "bad payment"
>
>If the payor is colluding, then no matter what he reveals to the bank,
>the bank cannot discover the payee.  Note that with payee anonymity,
>the payee does not have to promptly check in his money, so the bank
>has no hope of narrowing the search by coincidence in time.
>
>But if the payee is colluding, then the payor can be detected by=20
>coincidence in time.

Ah, but if we have the capability to do the fully-anon protocol, we can
suddenly do change-making stations.

The change problem is similar to the problem described above:  what if the
payor wants to buy something, but doesn't have the right change?  Going
to the bank to get change will give away who he is.  The solution:
go to your local moneychanger.

A moneychanger accepts, say, a coin for $0.02 and two blinded half-coins
for $0.01 each.  He deposits the $0.02, and if it clears, has the bank sign
the half-coins, which he returns to the payor (he'll probably blind and
unblind those half-coins, too).  The payor now has the right change, and
all the bank can see is that the moneychanger deposited a $0.02 coin and
withdrew 2 $0.01 coins.  Of course, the moneychanger may charge the payor
an extra bit for the privilege.

In the case of the fully-anon protocol, the payee gives a blinded half-coin
to the payor.  The payor then, as above, sends it (and a service fee)
to the moneychanger, who sends it to the bank (or maybe another moneychanger...
echos of remailers...), yadda yadda.

A moneychanger is a very useful construct for protecting _payor_ privacy
when exact change isn't handy.

Note also that with a system like this, there's no real reason for the payor
to even _have_ an account with the bank...

If (when) the ecash library is released, this will all become pretty
straightforward to implement.

   - Ian "who thinks he understands the ecash protocol, right down to the wire"

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