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Date: Tue, 16 Jun 2015 21:22:54 +0200
Message-ID: <CAPswA9zzKvh6G+NqzTmhrNXMSWOPtZ2DGW2t_2Jrisy8x9m2qw@mail.gmail.com>
From: Kalle Rosenbaum <kalle@rosenbaum.se>
To: Pieter Wuille <pieter.wuille@gmail.com>
Content-Type: text/plain; charset=UTF-8
Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] BIP for Proof of Payment
Precedence: list

Thank you for your comments Pieter! Please find my answers below.

2015-06-16 16:31 GMT+02:00 Pieter Wuille <pieter.wuille@gmail.com>:
> On Mon, Jun 15, 2015 at 1:59 PM, Kalle Rosenbaum <kalle@rosenbaum.se> wrote:
>>
>> 2015-06-15 12:00 GMT+02:00 Pieter Wuille <pieter.wuille@gmail.com>:
>> 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
>