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Date: Wed, 17 Jun 2015 11:51:54 +0200
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From: Kalle Rosenbaum <kalle@rosenbaum.se>
To: Pieter Wuille <pieter.wuille@gmail.com>
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Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] BIP for Proof of Payment
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2015-06-16 21:48 GMT+02:00 Pieter Wuille <pieter.wuille@gmail.com>:
> I don't see why existing software could create a 40-byte OP_RETURN but not
> larger? The limitation comes from a relay policy in full nodes, not a
> limitation is wallet software... and PoPs are not relayed on the network.

You are probably right here. The thing is that I don't know how *all*
wallet signing and validating software is written, so I figure it's
better to stick to a "valid" output. Since I don't *need* more data
than 40 bytes, why bother. There's another constraint to this as well:
The other BIP proposal, "Proof of Payment URI scheme", includes a
nonce parameter in the URI. If the nonce is very long, the QR code
will be unnecessarily big. The server should try to detect a brute
force of the 48 bit nonce, or at least delay the pop requests by some
100 ms or so.

Do you think this is an actual problem, and why? Is your suggestion to
use a bigger nonce, given the above?

>
> Regarding sharing, I think you're talking about a different use case. Say
> you want to pay for 1-week valid entrance to some venue. I thought the
> purpose of the PoP was to be sure that only the person who paid for it, and
> not anyone else can use it during that week.
>

That's right. That's one use case. You pay for the 1-week entrance and
then you use your wallet to sign PoPs when you enter the venue.

> My argument against that is that the original payer can also hand the
> private keys in his wallet to someone else, who would then become able to
> create PoPs for the service. He does not lose anything by this, assuming the
> address is not reused.
>

Yes, that is possible. It's about the same as giving out a
username/password for a service that you have paid for. In the case of
a concert ticket, it's simple. Just allow one entrance per payment.
But in the example you gave, it's a bit more complicated. You could
for example give all guests a bracelet upon first entry or upon first
exit. Or you can put a stamp on people leaving the venue, and demand
that all re-entries show the stamp, possibly along with a new PoP.
Pretty much as is done already. Different use cases will need
different protection. In this example, the value added by PoP is that
the venue does not have to distribute tickets in advance. This in turn
allows for better privacy for the customer, who don't have to give out
personal information such as an email-address.

> So, using a token does not change anything, except it can be provided to the
> payer - instead of relying on creating an implicit identity based on who
> seems to have held particular private keys in the past.
>

Yes, that's a difference, but it comes at the cost of security. The
stolen token can be used over and over. In the case of PoP it's only
usable once, and it's only created when it's actually needed,
minimizing the window of opportunity for the thief.

Regards,
Kalle

> On Jun 16, 2015 9:41 PM, "Kalle Rosenbaum" <kalle@rosenbaum.se> wrote:
>>
>> 2015-06-16 21:25 GMT+02:00 Pieter Wuille <pieter.wuille@gmail.com>:
>> > 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.
>>
>> Forwarding the PoP request would be a way to avoid sharing keys, as
>> suggested above.
>>
>> >
>> > Also you are not creating a real transaction. Why does the OP_RETURN
>> > limitation matter?
>>
>> This was discussed in the beginning of this thread: "The idea is to
>> simplify implementation. Existing software can be used as is to sign
>> and validate PoPs"
>>
>> Regards,
>> Kalle
>>
>> >
>> > On Jun 16, 2015 9:22 PM, "Kalle Rosenbaum" <kalle@rosenbaum.se> wrote:
>> >>
>> >> 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
>> >> >