To: bitcoin-dev@lists.linuxfoundation.org
From: Andreas Schildbach <andreas@schildbach.de>
Date: Sat, 30 Sep 2017 17:33:01 +0200
Message-ID: <oqodf6$30b$1@blaine.gmane.org>
References: <20170927160654.GA12492@savin.petertodd.org>
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	<B5DE4E92-C5B3-4C01-A148-E3C46C897323@sprovoost.nl>
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In-Reply-To: <20170929025538.GC12303@savin.petertodd.org>
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Subject: Re: [bitcoin-dev] Why the BIP-72 Payment Protocol URI Standard is
 Insecure Against MITM Attacks
Precedence: list

Generally agreed. This is why I nack'ed BIP72 years ago when we
discussed about standardization.

However, there are many ways to use BIP70 without BIP72. BIP72 is just a
kludge to biggy-pack the payment protocol onto BIP21. And also, as you
note, BIP72 can be easily fixed using a hash parameter.


On 09/29/2017 04:55 AM, Peter Todd via bitcoin-dev 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.
> 
> 
> 
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