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From: Omar Shibli <omarshib@gmail.com>
Date: Fri, 29 Sep 2017 07:21:09 +0300
Message-ID: <CAE3EOfi6wz=5oduOambZgBT51=OsYGUG2Ps_gJHis=fTCq80Dg@mail.gmail.com>
To: Peter Todd <pete@petertodd.org>, 
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] Why the BIP-72 Payment Protocol URI Standard is
 Insecure Against MITM Attacks
Precedence: list

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Thank you for sharing, this is indefinitely valuable.

I think that risk could be mitigated if instead of ignoring the bitcoin
address/amount/..., the wallet use this address for integrity checks.
Furthermore, I think this BIP could be improved by actually applying the
homomorphic property and deriving the bitcoin address from merchant pub key
and the hash itself. that would allow both the customer and merchant to be
able generate address independently.

On Fri, Sep 29, 2017 at 5:55 AM, Peter Todd via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> On Thu, Sep 28, 2017 at 03:43:05PM +0300, Sjors Provoost via bitcoin-dev
> wrote:
> > Andreas Schildbach wrote:
> > > This feels redundant to me; the payment protocol already has an
> > > expiration time.
> >
> > The BIP-70 payment protocol has significant overhead and most
> importantly requires back and forth. Emailing a bitcoin address or printing
> it on an invoice is much easier, so I would expect people to keep doing
> that.
>
> The BIP-70 payment protocol used via BIP-72 URI's is insecure, as payment
> qr
> codes don't cryptographically commit to the identity of the merchant, which
> means a MITM attacker can redirect the payment if they can obtain a SSL
> cert
> that the wallet accepts.
>
> For example, if I have a wallet on my phone and go to pay a
> merchant, a BIP-72 URI will look like the following(1):
>
>     bitcoin:mq7se9wy2egettFxPbmn99cK8v5AFq55Lx?amount=0.11&r=https://
> merchant.com/pay.php?h%3D2a8628fc2fbe
>
> A wallet following the BIP-72 standard will "ignore the bitcoin
> address/amount/label/message in the URI and instead fetch a PaymentRequest
> message and then follow the payment protocol, as described in BIP 70."
>
> So my phone will make a second connection - likely on a second network
> with a
> totally different set of MITM attackers - to https://merchant.com
>
> In short, while my browser may have gotten the correct URL with the correct
> Bitcoin address, by using the payment protocol my wallet is discarding that
> information and giving MITM attackers a second chance at redirecting my
> payment
> to them. That wallet is also likely using an off-the-shelf SSL library,
> with
> nothing other than an infrequently updated set of root certificates to use
> to
> verify the certificate; your browser has access to a whole host of better
> technologies, such as HSTS pinning, certificate transparency, and
> frequently
> updated root certificate lists with proper revocation (see Symantec).
>
> As an ad-hoc, unstandardized, extension Android Wallet for Bitcoin at least
> supports a h= parameter with a hash commitment to what the payment request
> should be, and will reject the MITM attacker if that hash doesn't match.
> But
> that's not actually in the standard itself, and as far as I can tell has
> never
> been made into a BIP.
>
> As-is BIP-72 is very dangerous and should be depreciated, with a new BIP
> made
> to replace it.
>
> 1) As an aside, it's absolutely hilarious that this URL taken straight from
>    BIP-72 has the merchant using PHP, given its truly terrible track
> record for
>    security.
>
> --
> https://petertodd.org 'peter'[:-1]@petertodd.org
>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
>

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<div dir=3D"ltr"><div>Thank you for sharing, this is indefinitely valuable.=
</div><div><br></div><div>I think that risk could be mitigated if instead o=
f ignoring the bitcoin address/amount/..., the wallet use this address for =
integrity checks.</div><div>Furthermore, I think this BIP could be improved=
 by actually applying the homomorphic property and deriving the bitcoin add=
ress from merchant pub key and the hash itself. that would allow both the c=
ustomer and merchant to be able=C2=A0generate address independently.</div><=
/div><div class=3D"gmail_extra"><br><div class=3D"gmail_quote">On Fri, Sep =
29, 2017 at 5:55 AM, Peter Todd via bitcoin-dev <span dir=3D"ltr">&lt;<a hr=
ef=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank">bitco=
in-dev@lists.linuxfoundation.org</a>&gt;</span> wrote:<br><blockquote class=
=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padd=
ing-left:1ex">On Thu, Sep 28, 2017 at 03:43:05PM +0300, Sjors Provoost via =
bitcoin-dev wrote:<br>
&gt; Andreas Schildbach wrote:<br>
&gt; &gt; This feels redundant to me; the payment protocol already has an<b=
r>
&gt; &gt; expiration time.<br>
&gt;<br>
&gt; The BIP-70 payment protocol has significant overhead and most importan=
tly requires back and forth. Emailing a bitcoin address or printing it on a=
n invoice is much easier, so I would expect people to keep doing that.<br>
<br>
The BIP-70 payment protocol used via BIP-72 URI&#39;s is insecure, as payme=
nt qr<br>
codes don&#39;t cryptographically commit to the identity of the merchant, w=
hich<br>
means a MITM attacker can redirect the payment if they can obtain a SSL cer=
t<br>
that the wallet accepts.<br>
<br>
For example, if I have a wallet on my phone and go to pay a<br>
merchant, a BIP-72 URI will look like the following(1):<br>
<br>
=C2=A0 =C2=A0 bitcoin:<wbr>mq7se9wy2egettFxPbmn99cK8v5AFq<wbr>55Lx?amount=
=3D0.11&amp;r=3D<a href=3D"https://merchant.com/pay.php?h%3D2a8628fc2fbe" r=
el=3D"noreferrer" target=3D"_blank">https://<wbr>merchant.com/pay.php?h%<wb=
r>3D2a8628fc2fbe</a><br>
<br>
A wallet following the BIP-72 standard will &quot;ignore the bitcoin<br>
address/amount/label/message in the URI and instead fetch a PaymentRequest<=
br>
message and then follow the payment protocol, as described in BIP 70.&quot;=
<br>
<br>
So my phone will make a second connection - likely on a second network with=
 a<br>
totally different set of MITM attackers - to <a href=3D"https://merchant.co=
m" rel=3D"noreferrer" target=3D"_blank">https://merchant.com</a><br>
<br>
In short, while my browser may have gotten the correct URL with the correct=
<br>
Bitcoin address, by using the payment protocol my wallet is discarding that=
<br>
information and giving MITM attackers a second chance at redirecting my pay=
ment<br>
to them. That wallet is also likely using an off-the-shelf SSL library, wit=
h<br>
nothing other than an infrequently updated set of root certificates to use =
to<br>
verify the certificate; your browser has access to a whole host of better<b=
r>
technologies, such as HSTS pinning, certificate transparency, and frequentl=
y<br>
updated root certificate lists with proper revocation (see Symantec).<br>
<br>
As an ad-hoc, unstandardized, extension Android Wallet for Bitcoin at least=
<br>
supports a h=3D parameter with a hash commitment to what the payment reques=
t<br>
should be, and will reject the MITM attacker if that hash doesn&#39;t match=
. But<br>
that&#39;s not actually in the standard itself, and as far as I can tell ha=
s never<br>
been made into a BIP.<br>
<br>
As-is BIP-72 is very dangerous and should be depreciated, with a new BIP ma=
de<br>
to replace it.<br>
<br>
1) As an aside, it&#39;s absolutely hilarious that this URL taken straight =
from<br>
=C2=A0 =C2=A0BIP-72 has the merchant using PHP, given its truly terrible tr=
ack record for<br>
=C2=A0 =C2=A0security.<br>
<span class=3D"HOEnZb"><font color=3D"#888888"><br>
--<br>
<a href=3D"https://petertodd.org" rel=3D"noreferrer" target=3D"_blank">http=
s://petertodd.org</a> &#39;peter&#39;[:-1]@<a href=3D"http://petertodd.org"=
 rel=3D"noreferrer" target=3D"_blank">petertodd.org</a><br>
</font></span><br>______________________________<wbr>_________________<br>
bitcoin-dev mailing list<br>
<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org">bitcoin-dev@lists.=
<wbr>linuxfoundation.org</a><br>
<a href=3D"https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev" =
rel=3D"noreferrer" target=3D"_blank">https://lists.linuxfoundation.<wbr>org=
/mailman/listinfo/bitcoin-<wbr>dev</a><br>
<br></blockquote></div><br></div>

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