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Re: How can e-cash, even on-line cleared, protect payee identity?

Jiri Baum <[email protected]> writes:
>Hello Hal <[email protected]>
>H wrote:
>> There could be an issue of fraud, though,
>> where Bob insists that Alice's coin was no good even though it actually
>> was.

>Cut'n'choose between Alice and Bob? Ie Alice asks Bob for half the blinds
>to check that the proto-coins are true?

This would work to protect Alice from certain kinds of fraud by Bob, but
it increases the amount of data considerably, and it still does not
resolve the main issue that Bob claims that his coin didn't unlind to
clean data.  Who is at fault in that case?  How can this be resolved?

>Apart from no-good proto-coins, is there any other way the coin
>could be no good?

Alice could give Bob bogus data, Bob could give Alice bogus data, Bob
could claim that Alice gave him bogus data (even though it was good).

>As for no-good proto-coins, it's Bob's fault, isn't it? Alice has 
>a record of what Bob sent, and what she sent back. Anybody can check
>that the latter is a bank-signed version of the former.

If what she got from Bob was signed by him, she can prove that she gave him
back a bank-signed version of that.  (He has to sign it, otherwise she
could just exhibit two bogus numbers, one the cube of the other.)  Given
that, your idea seems good.  Alice can prove that she did her part OK, so
if she is able to show such a proof then Bob must be at fault.

>Given this,
>there's no need (from this) for Alice to know that the proto-coins are
>good (if they aren't, Bob's an idiot, but there's not much Alice
>can do about it - I guess given all the blinding factors the bank
>could replace the coin, seeing that it signed a worthless one).

Yes, I think so, so there is no need for the cut and choose.

>An interesting question is whether Bob and Nick can now collude to
>expose Alice. Therefore Alice would at least want to verify that the
>proto-coins are true? Would that suffice? Or is that not necessary?

I don't think they can.  All Bob sees is his own blinded coin, and the
signed version of that.  The bank sees a separately blinded number which
it signed.  Alice's blinding factor can be anything, so there is no
linkage between them.

However, the timing is a problem.  Bob knows _when_ Alice communicated
with the bank.  So he can collude with the bank afterwards to identify
those withdrawals which took place at that time, one of which must have
been Alice.  This could be a problem.

In regular ecash, the timing issue is potentially less serious because
the payee can in principle have a totally anonymous relationship to the
bank, and exchange his received coins for fresh ones.  But in this
system doing that is more difficult.  Alice must withdraw funds rather
than deposit them.  To do so totally anonymously she would have to
present coins to the bank at withdrawal time equal in value to the
amount she wanted to pay Bob.  The bank would replace these coins with
fresh ones that it signs, which are the doubly-blinded ones which Bob
has provided to Alice.  So this is a somewhat more roundabout
approach.  However, if you do this, and Alice communicates with the
bank anonymously, then both sides seem to be pretty well protected
against collusion.