You can't avoid sharing the token, and you can't avoid sharing the private
keys used for signing either. If they are single use, you don't lose
anything by sharing them.

Also you are not creating a real transaction. Why does the OP_RETURN
limitation matter?
On Jun 16, 2015 9:22 PM, "Kalle Rosenbaum" <> wrote:

> Thank you for your comments Pieter! Please find my answers below.
> 2015-06-16 16:31 GMT+02:00 Pieter Wuille <>:
> > On Mon, Jun 15, 2015 at 1:59 PM, Kalle Rosenbaum <>
> wrote:
> >>
> >> 2015-06-15 12:00 GMT+02:00 Pieter Wuille <>:
> >> I'm not sure if we will be able to support PoP with CoinJoin. Maybe
> >> someone with more insight into CoinJoin have some input?
> >
> >
> > Not really. The problem is that you assume a transaction corresponds to a
> > single payment. This is true for simple wallet use cases, but not
> compatible
> > with CoinJoin, or with systems that for example would want to combine
> > multiple payments in a single transaction.
> >
> Yes, you are right. It's not compatible with CoinJoin and the likes.
> >
> > 48 bits seems low to me, but it does indeed solve the problem. Why not
> 128
> > or 256 bits?
> The nonce is limited because of the OP_RETURN output being limited to
> 40 bytes of data: 2 bytes version, 32 bytes txid, 6 bytes nonce.
> >
> >> > Why does anyone care who paid? This is like walking into a coffeshop,
> >> > noticing I don't have money with me, let me friend pay for me, and
> then
> >> > have
> >> > the shop insist that I can't drink it because I'm not the buyer.
> >>
> >> If you pay as you use the service (ie pay for coffee upfront), there's
> >> no need for PoP. Please see the Motivation section. But you are right
> >> that you must have the wallet(s) that paid at hand when you issue a
> >> PoP.
> >>
> >> >
> >> > Track payments, don't try to assign identities to payers.
> >>
> >> Please elaborate, I don't understand what you mean here.
> >
> >
> > I think that is a mistake. You should not assume that the wallet who held
> > the coins is the payer/buyer. That's what I said earlier; you're
> implicitly
> > creating an identity (the one who holds these keys) based on the
> > transaction. This seems fundamentally wrong to me, and not necessary. The
> > receiver should not care who paid or how, he should care what was payed
> for.
> You are saying that it's a problem that the wallet used to pay, must
> also be used to issue the PoP? That may very well be a problem in some
> cases. People using PoP should of course be aware of it's limitations
> and act accordingly, i.e. don't pay for concert tickets for a friend
> and expect your friend to be able to enter the arena with her wallet.
> As Tom Harding noted, it is possible to transfer keys to your friend's
> wallet, but that might not be desirable if those keys are also used
> for other payments. Also that would weaken the security of an HD
> wallet, since a chain code along with a private key would reveal all
> keys in that tree. Another solution is that your friend forwards the
> PoP request to your wallet, through twitter or SMS, and you send the
> PoP for her. Maybe that forwarding mechanism can be built into wallets
> and automated so that the wallet automatically suggests to sign the
> PoP for your friend. This is probably something to investigate
> further, but not within the scope of this BIP.
> Of course the simplest solution would be to send money to your friend
> first so that she can pay for the ticket from her own wallet, but
> that's not always feasible.
> >
> > The easiest solution to this IMHO would be an extension to the payment
> > protocol that gives you (or your wallet) a token in return for paying,
> and
> > that knowledge of that token is used to gain access to the services you
> > provide.
> >
> That token would then be reusable. Someone stealing it would be able
> to use it as much as she wants. That is what I want to avoid with PoP.
> The BIP proposal briefly mentions something like this in the
> rationale. I also had a discussion about this with Mike Hearn on this
> list on Mars 13 that I think covers most pros and cons of the
> different approaches.
> While your suggestion does indeed separate the transaction from the
> proof of payment, it also assumes that the token is held in the wallet
> that pays. Otherwise you would need to keep it in another safe place,
> remember it's reusable. Where would that be? How would you transfer
> that token to your friend?
> Thank you again for your comments. I appreciate it.
> Best regards,
> Kalle
> > --
> > Pieter
> >
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