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Date: Sun, 21 Jun 2015 16:39:24 +0200
Message-ID: <CAPswA9z5fo5Vw_cSoUu1qtgU4kKf+W3FxREPJ5UpvvJUSi+doA@mail.gmail.com>
From: Kalle Rosenbaum <kalle@rosenbaum.se>
To: Greg Maxwell <gmaxwell@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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--001a1142ea98a60f780519081ff6
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Hi Greg!

After a lot of constructive discussion, feedback and updating, I'm
requesting that you please assign these proposals BIP numbers. It's both
the "Proof of Payment" proposal and the "Proof of Payment URI scheme"
proposal that I'm referring to.

The wikimedia source is available here:
https://github.com/kallerosenbaum/poppoc/wiki/Proof-of-Payment-BIP and
https://github.com/kallerosenbaum/poppoc/wiki/btcpop-scheme-BIP.

Is this what you need in order to proceed or is there something else you
need from me?

Best regards,
/Kalle

2015-06-17 11:51 GMT+02:00 Kalle Rosenbaum <kalle@rosenbaum.se>:

> 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
> >> >> >
>

--001a1142ea98a60f780519081ff6
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

<div dir=3D"ltr"><div>Hi Greg!<br><br>After a lot of constructive discussio=
n, feedback and updating, I&#39;m requesting that you please assign these p=
roposals BIP numbers. It&#39;s both the &quot;Proof of Payment&quot; propos=
al and the &quot;Proof of Payment URI scheme&quot; proposal that I&#39;m re=
ferring to.<br><br>The wikimedia source is available here: <a href=3D"https=
://github.com/kallerosenbaum/poppoc/wiki/Proof-of-Payment-BIP">https://gith=
ub.com/kallerosenbaum/poppoc/wiki/Proof-of-Payment-BIP</a> and <a href=3D"h=
ttps://github.com/kallerosenbaum/poppoc/wiki/btcpop-scheme-BIP">https://git=
hub.com/kallerosenbaum/poppoc/wiki/btcpop-scheme-BIP</a>.<br><br></div>Is t=
his what you need in order to proceed or is there something else you need f=
rom me?<br><div><br>Best regards,<br>/Kalle<br></div></div><div class=3D"gm=
ail_extra"><br><div class=3D"gmail_quote">2015-06-17 11:51 GMT+02:00 Kalle =
Rosenbaum <span dir=3D"ltr">&lt;<a href=3D"mailto:kalle@rosenbaum.se" targe=
t=3D"_blank">kalle@rosenbaum.se</a>&gt;</span>:<br><blockquote class=3D"gma=
il_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-lef=
t:1ex"><span class=3D"">2015-06-16 21:48 GMT+02:00 Pieter Wuille &lt;<a hre=
f=3D"mailto:pieter.wuille@gmail.com">pieter.wuille@gmail.com</a>&gt;:<br>
&gt; I don&#39;t see why existing software could create a 40-byte OP_RETURN=
 but not<br>
&gt; larger? The limitation comes from a relay policy in full nodes, not a<=
br>
&gt; limitation is wallet software... and PoPs are not relayed on the netwo=
rk.<br>
<br>
</span>You are probably right here. The thing is that I don&#39;t know how =
*all*<br>
wallet signing and validating software is written, so I figure it&#39;s<br>
better to stick to a &quot;valid&quot; output. Since I don&#39;t *need* mor=
e data<br>
than 40 bytes, why bother. There&#39;s another constraint to this as well:<=
br>
The other BIP proposal, &quot;Proof of Payment URI scheme&quot;, includes a=
<br>
nonce parameter in the URI. If the nonce is very long, the QR code<br>
will be unnecessarily big. The server should try to detect a brute<br>
force of the 48 bit nonce, or at least delay the pop requests by some<br>
100 ms or so.<br>
<br>
Do you think this is an actual problem, and why? Is your suggestion to<br>
use a bigger nonce, given the above?<br>
<span class=3D""><br>
&gt;<br>
&gt; Regarding sharing, I think you&#39;re talking about a different use ca=
se. Say<br>
&gt; you want to pay for 1-week valid entrance to some venue. I thought the=
<br>
&gt; purpose of the PoP was to be sure that only the person who paid for it=
, and<br>
&gt; not anyone else can use it during that week.<br>
&gt;<br>
<br>
</span>That&#39;s right. That&#39;s one use case. You pay for the 1-week en=
trance and<br>
then you use your wallet to sign PoPs when you enter the venue.<br>
<span class=3D""><br>
&gt; My argument against that is that the original payer can also hand the<=
br>
&gt; private keys in his wallet to someone else, who would then become able=
 to<br>
&gt; create PoPs for the service. He does not lose anything by this, assumi=
ng the<br>
&gt; address is not reused.<br>
&gt;<br>
<br>
</span>Yes, that is possible. It&#39;s about the same as giving out a<br>
username/password for a service that you have paid for. In the case of<br>
a concert ticket, it&#39;s simple. Just allow one entrance per payment.<br>
But in the example you gave, it&#39;s a bit more complicated. You could<br>
for example give all guests a bracelet upon first entry or upon first<br>
exit. Or you can put a stamp on people leaving the venue, and demand<br>
that all re-entries show the stamp, possibly along with a new PoP.<br>
Pretty much as is done already. Different use cases will need<br>
different protection. In this example, the value added by PoP is that<br>
the venue does not have to distribute tickets in advance. This in turn<br>
allows for better privacy for the customer, who don&#39;t have to give out<=
br>
personal information such as an email-address.<br>
<span class=3D""><br>
&gt; So, using a token does not change anything, except it can be provided =
to the<br>
&gt; payer - instead of relying on creating an implicit identity based on w=
ho<br>
&gt; seems to have held particular private keys in the past.<br>
&gt;<br>
<br>
</span>Yes, that&#39;s a difference, but it comes at the cost of security. =
The<br>
stolen token can be used over and over. In the case of PoP it&#39;s only<br=
>
usable once, and it&#39;s only created when it&#39;s actually needed,<br>
minimizing the window of opportunity for the thief.<br>
<div class=3D"HOEnZb"><div class=3D"h5"><br>
Regards,<br>
Kalle<br>
<br>
&gt; On Jun 16, 2015 9:41 PM, &quot;Kalle Rosenbaum&quot; &lt;<a href=3D"ma=
ilto:kalle@rosenbaum.se">kalle@rosenbaum.se</a>&gt; wrote:<br>
&gt;&gt;<br>
&gt;&gt; 2015-06-16 21:25 GMT+02:00 Pieter Wuille &lt;<a href=3D"mailto:pie=
ter.wuille@gmail.com">pieter.wuille@gmail.com</a>&gt;:<br>
&gt;&gt; &gt; You can&#39;t avoid sharing the token, and you can&#39;t avoi=
d sharing the<br>
&gt;&gt; &gt; private<br>
&gt;&gt; &gt; keys used for signing either. If they are single use, you don=
&#39;t lose<br>
&gt;&gt; &gt; anything by sharing them.<br>
&gt;&gt;<br>
&gt;&gt; Forwarding the PoP request would be a way to avoid sharing keys, a=
s<br>
&gt;&gt; suggested above.<br>
&gt;&gt;<br>
&gt;&gt; &gt;<br>
&gt;&gt; &gt; Also you are not creating a real transaction. Why does the OP=
_RETURN<br>
&gt;&gt; &gt; limitation matter?<br>
&gt;&gt;<br>
&gt;&gt; This was discussed in the beginning of this thread: &quot;The idea=
 is to<br>
&gt;&gt; simplify implementation. Existing software can be used as is to si=
gn<br>
&gt;&gt; and validate PoPs&quot;<br>
&gt;&gt;<br>
&gt;&gt; Regards,<br>
&gt;&gt; Kalle<br>
&gt;&gt;<br>
&gt;&gt; &gt;<br>
&gt;&gt; &gt; On Jun 16, 2015 9:22 PM, &quot;Kalle Rosenbaum&quot; &lt;<a h=
ref=3D"mailto:kalle@rosenbaum.se">kalle@rosenbaum.se</a>&gt; wrote:<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; Thank you for your comments Pieter! Please find my answer=
s below.<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; 2015-06-16 16:31 GMT+02:00 Pieter Wuille &lt;<a href=3D"m=
ailto:pieter.wuille@gmail.com">pieter.wuille@gmail.com</a>&gt;:<br>
&gt;&gt; &gt;&gt; &gt; On Mon, Jun 15, 2015 at 1:59 PM, Kalle Rosenbaum &lt=
;<a href=3D"mailto:kalle@rosenbaum.se">kalle@rosenbaum.se</a>&gt;<br>
&gt;&gt; &gt;&gt; &gt; wrote:<br>
&gt;&gt; &gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; &gt;&gt; 2015-06-15 12:00 GMT+02:00 Pieter Wuille &lt;<a =
href=3D"mailto:pieter.wuille@gmail.com">pieter.wuille@gmail.com</a>&gt;:<br=
>
&gt;&gt; &gt;&gt; &gt;&gt; I&#39;m not sure if we will be able to support P=
oP with CoinJoin. Maybe<br>
&gt;&gt; &gt;&gt; &gt;&gt; someone with more insight into CoinJoin have som=
e input?<br>
&gt;&gt; &gt;&gt; &gt;<br>
&gt;&gt; &gt;&gt; &gt;<br>
&gt;&gt; &gt;&gt; &gt; Not really. The problem is that you assume a transac=
tion corresponds<br>
&gt;&gt; &gt;&gt; &gt; to<br>
&gt;&gt; &gt;&gt; &gt; a<br>
&gt;&gt; &gt;&gt; &gt; single payment. This is true for simple wallet use c=
ases, but not<br>
&gt;&gt; &gt;&gt; &gt; compatible<br>
&gt;&gt; &gt;&gt; &gt; with CoinJoin, or with systems that for example woul=
d want to combine<br>
&gt;&gt; &gt;&gt; &gt; multiple payments in a single transaction.<br>
&gt;&gt; &gt;&gt; &gt;<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; Yes, you are right. It&#39;s not compatible with CoinJoin=
 and the likes.<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; &gt;<br>
&gt;&gt; &gt;&gt; &gt; 48 bits seems low to me, but it does indeed solve th=
e problem. Why<br>
&gt;&gt; &gt;&gt; &gt; not<br>
&gt;&gt; &gt;&gt; &gt; 128<br>
&gt;&gt; &gt;&gt; &gt; or 256 bits?<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; The nonce is limited because of the OP_RETURN output bein=
g limited to<br>
&gt;&gt; &gt;&gt; 40 bytes of data: 2 bytes version, 32 bytes txid, 6 bytes=
 nonce.<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; &gt;<br>
&gt;&gt; &gt;&gt; &gt;&gt; &gt; Why does anyone care who paid? This is like=
 walking into a<br>
&gt;&gt; &gt;&gt; &gt;&gt; &gt; coffeshop,<br>
&gt;&gt; &gt;&gt; &gt;&gt; &gt; noticing I don&#39;t have money with me, le=
t me friend pay for me, and<br>
&gt;&gt; &gt;&gt; &gt;&gt; &gt; then<br>
&gt;&gt; &gt;&gt; &gt;&gt; &gt; have<br>
&gt;&gt; &gt;&gt; &gt;&gt; &gt; the shop insist that I can&#39;t drink it b=
ecause I&#39;m not the buyer.<br>
&gt;&gt; &gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; &gt;&gt; If you pay as you use the service (ie pay for co=
ffee upfront),<br>
&gt;&gt; &gt;&gt; &gt;&gt; there&#39;s<br>
&gt;&gt; &gt;&gt; &gt;&gt; no need for PoP. Please see the Motivation secti=
on. But you are<br>
&gt;&gt; &gt;&gt; &gt;&gt; right<br>
&gt;&gt; &gt;&gt; &gt;&gt; that you must have the wallet(s) that paid at ha=
nd when you issue a<br>
&gt;&gt; &gt;&gt; &gt;&gt; PoP.<br>
&gt;&gt; &gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; &gt;&gt; &gt;<br>
&gt;&gt; &gt;&gt; &gt;&gt; &gt; Track payments, don&#39;t try to assign ide=
ntities to payers.<br>
&gt;&gt; &gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; &gt;&gt; Please elaborate, I don&#39;t understand what yo=
u mean here.<br>
&gt;&gt; &gt;&gt; &gt;<br>
&gt;&gt; &gt;&gt; &gt;<br>
&gt;&gt; &gt;&gt; &gt; I think that is a mistake. You should not assume tha=
t the wallet who<br>
&gt;&gt; &gt;&gt; &gt; held<br>
&gt;&gt; &gt;&gt; &gt; the coins is the payer/buyer. That&#39;s what I said=
 earlier; you&#39;re<br>
&gt;&gt; &gt;&gt; &gt; implicitly<br>
&gt;&gt; &gt;&gt; &gt; creating an identity (the one who holds these keys) =
based on the<br>
&gt;&gt; &gt;&gt; &gt; transaction. This seems fundamentally wrong to me, a=
nd not necessary.<br>
&gt;&gt; &gt;&gt; &gt; The<br>
&gt;&gt; &gt;&gt; &gt; receiver should not care who paid or how, he should =
care what was<br>
&gt;&gt; &gt;&gt; &gt; payed<br>
&gt;&gt; &gt;&gt; &gt; for.<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; You are saying that it&#39;s a problem that the wallet us=
ed to pay, must<br>
&gt;&gt; &gt;&gt; also be used to issue the PoP? That may very well be a pr=
oblem in some<br>
&gt;&gt; &gt;&gt; cases. People using PoP should of course be aware of it&#=
39;s limitations<br>
&gt;&gt; &gt;&gt; and act accordingly, i.e. don&#39;t pay for concert ticke=
ts for a friend<br>
&gt;&gt; &gt;&gt; and expect your friend to be able to enter the arena with=
 her wallet.<br>
&gt;&gt; &gt;&gt; As Tom Harding noted, it is possible to transfer keys to =
your friend&#39;s<br>
&gt;&gt; &gt;&gt; wallet, but that might not be desirable if those keys are=
 also used<br>
&gt;&gt; &gt;&gt; for other payments. Also that would weaken the security o=
f an HD<br>
&gt;&gt; &gt;&gt; wallet, since a chain code along with a private key would=
 reveal all<br>
&gt;&gt; &gt;&gt; keys in that tree. Another solution is that your friend f=
orwards the<br>
&gt;&gt; &gt;&gt; PoP request to your wallet, through twitter or SMS, and y=
ou send the<br>
&gt;&gt; &gt;&gt; PoP for her. Maybe that forwarding mechanism can be built=
 into wallets<br>
&gt;&gt; &gt;&gt; and automated so that the wallet automatically suggests t=
o sign the<br>
&gt;&gt; &gt;&gt; PoP for your friend. This is probably something to invest=
igate<br>
&gt;&gt; &gt;&gt; further, but not within the scope of this BIP.<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; Of course the simplest solution would be to send money to=
 your friend<br>
&gt;&gt; &gt;&gt; first so that she can pay for the ticket from her own wal=
let, but<br>
&gt;&gt; &gt;&gt; that&#39;s not always feasible.<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; &gt;<br>
&gt;&gt; &gt;&gt; &gt; The easiest solution to this IMHO would be an extens=
ion to the<br>
&gt;&gt; &gt;&gt; &gt; payment<br>
&gt;&gt; &gt;&gt; &gt; protocol that gives you (or your wallet) a token in =
return for<br>
&gt;&gt; &gt;&gt; &gt; paying,<br>
&gt;&gt; &gt;&gt; &gt; and<br>
&gt;&gt; &gt;&gt; &gt; that knowledge of that token is used to gain access =
to the services<br>
&gt;&gt; &gt;&gt; &gt; you<br>
&gt;&gt; &gt;&gt; &gt; provide.<br>
&gt;&gt; &gt;&gt; &gt;<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; That token would then be reusable. Someone stealing it wo=
uld be able<br>
&gt;&gt; &gt;&gt; to use it as much as she wants. That is what I want to av=
oid with PoP.<br>
&gt;&gt; &gt;&gt; The BIP proposal briefly mentions something like this in =
the<br>
&gt;&gt; &gt;&gt; rationale. I also had a discussion about this with Mike H=
earn on this<br>
&gt;&gt; &gt;&gt; list on Mars 13 that I think covers most pros and cons of=
 the<br>
&gt;&gt; &gt;&gt; different approaches.<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; While your suggestion does indeed separate the transactio=
n from the<br>
&gt;&gt; &gt;&gt; proof of payment, it also assumes that the token is held =
in the wallet<br>
&gt;&gt; &gt;&gt; that pays. Otherwise you would need to keep it in another=
 safe place,<br>
&gt;&gt; &gt;&gt; remember it&#39;s reusable. Where would that be? How woul=
d you transfer<br>
&gt;&gt; &gt;&gt; that token to your friend?<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; Thank you again for your comments. I appreciate it.<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; Best regards,<br>
&gt;&gt; &gt;&gt; Kalle<br>
&gt;&gt; &gt;&gt;<br>
&gt;&gt; &gt;&gt; &gt; --<br>
&gt;&gt; &gt;&gt; &gt; Pieter<br>
&gt;&gt; &gt;&gt; &gt;<br>
</div></div></blockquote></div><br></div>

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