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Date: Mon, 27 Jan 2014 20:34:36 +0000
From: Roy Badami <roy@gnomon.org.uk>
To: Jeremy Spilman <jeremy@taplink.co>
Message-ID: <20140127203435.GU38964@giles.gnomon.org.uk>
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Cc: bitcoin-development@lists.sourceforge.net,
	Andreas Schildbach <andreas@schildbach.de>
Subject: Re: [Bitcoin-development] Payment Protocol for Face-to-face Payments
Precedence: list

On Mon, Jan 27, 2014 at 09:11:08AM -0800, Jeremy Spilman wrote:
> On Mon, 27 Jan 2014 03:59:25 -0800, Andreas Schildbach  
> <andreas@schildbach.de> wrote:
> 
> > SCAN TO PAY
> > For scan-to-pay, the current landscape looks different. I assume at
> > least 50% of Bitcoin transactions are initiated by a BIP21 URL encoded
> > into a QR-code. Nevertheless, I tried to encode a payment request into
> > the bitcoin URL. I used my existing work on encoding transactions into
> > QR-codes. Steps to encode:
> 
> Really interesting work. When using scan-to-pay, after the payer scans the  
> QR code with the protobuf PaymentRequest (not a URL to download the  
> PaymentRequest) are they using their own connectivity to submit the  
> Payment response?
> 
> If we assume connectivity on the phone, might as well just get a URL from  
> the QR code and re-use existing infrastructure for serving that?

My first thought was likewise.  In the case where the phone needs
Internet connectivity anyway, why not include an HTTPS URL in a BIP 72 URL?

I'm assuming that every client will have to support this is any case,
since it's effectively mandated by the BIP, so why add another mode of
operation?

However, PaymentRequest-over-QR-code does seem to me to have one
rather attractive advantage: the authentication model is orders of
magnitude simpler and more intuitive for a face-to-face transaction
than anything else.  You're saying "pay the coins to that thing over
there displaying that QR code".  Which, most of the time, is exactly
what you want.

In the web case, it's fine to ignore the case where the URL domain has
been subverted (and an cert obtained by the attacker) because in that
case you've lost before you even get to payments (MitM attacker shows
you a modified web page with different payment details).  But the
face-to-face case isn't intrinsically dependent on SSL security, and
it's nice not to introduce that attack vector...

roy