Subject: Re: [Bitcoin-development] Bitcoin at POS using BIP70, NFC and offline payments - implementer feedback

From: Jan Vornberger <[email-masked]>
Date: Mon, 23 Feb 2015 16:09:37 +0100

Hey!

On Sun, Feb 22, 2015 at 05:37:16PM -0500, Andy Schroder wrote:
> It's maybe not a bad idea for the wallet to try all payment_url
> mechanisms in parallel. Should we add this as a recommendation to
> wallets in TBIP75?

It doesn't need to be a recommendation I think, but maybe it would be
good to mention that a wallet may do that, if it wants.

> I actually also happen to be using nfcpy. I am having some
> reliability issues as well with it. What exactly are your problems?

Aw, interesting. Sometimes transfers seem to start and then not complete
in some way and occasionally the NFC dongle is then totally 'stuck' in some
way afterwards, that even after restarting the Python script or
reloading the driver nothing works anymore. I have to actually unplug
the dongle and plug it in again. Obviously not exactly production ready.
I had the same problems with the command line tools based on libnfc, so
it might be a problem lower down the stack. I'm not sure I have the
expertise to troubleshoot that.

> I have seen your video before. I guess I'm wondering how your
> prototype works with bitpay and bluetooth. Doesn't bitpay sign the
> payment request for you with an https based payment_url? If so, how
> do you add the bluetooth payment_url while keeping their signature
> valid?

Good point, I'm currently simply removing the signature, so that I can
modify the payment request. I haven't spoken with BitPay yet, but I hope
that they will extend their API at some point to set additional
payment_urls or provide a Bluetooth MAC and then I can do it properly
with signed requests.

> In your video it looks like the phone still has cellular and
> wifi reception (it is not offline).

You are right, I forgot to actually disable wifi and cellular data when
recording the video. But as you know it would work the same way offline.

> Regarding the NFC data formats. I would like to clarify that the
> wallets are having those events dispatched by the android OS. The
> "URI" and "mime type" events are sent to the application in the same
> way as from other sources such as a web browser, e-mail, stand alone
> QR code scanner app, etc.. So, I don't think the wallet actually
> knows it is receiving the event from NFC. That is one reason why so
> many existing wallets happen to support BIP21 payment request via
> NFC. Andreas can correct me if I am wrong on these statements. I'm a
> little weary sending the "mime type" based format over NFC because
> of backwards compatibility and because of the long certificate chain
> that needs to be transferred. You want that tap to be as robust and
> fast as possible. A bluetooth connection can have a retry without
> any user interaction.

There is a specific NFC intent that you have to list in your Android
manifest, but you are right that if you already support BIP21 URIs then
it is often fairly easy and quick to also support them via NFC.

Whereas the mime type approach means that you necessarily need to be
able to actually understand BIP70, which a lot of wallet don't yet. But
personally that wouldn't hold me back using the mime type if I feel it's
the better experience. Those wallets simply have to fall back on
scanning the QR code in the meantime and then get up to speed on their
NFC and BIP70 support.

I'm still concerned that the fact, that Bluetooth is often disabled, is a
problem for the UX. And it's not just a one-time thing as with NFC,
which is - in my experience - also often disabled, but then people turn
it on and leave it on. But with Bluetooth the Android system is geared
much more towards turning it off after use and people have this general
idea of 'it uses energy, so I should disable it' and sometimes also
'Bluetooth is insecure and if I leave it on I will get hacked'. So
chances are, Bluetooth will be off most of the time, which means
everytime you pay the dialog 'Turn on Bluetooth?' will pop up, which
isn't exactly streamlined.

So the advantage of transmitting the whole BIP70 payment request via NFC
I see is, that you don't need Bluetooth to get the payment request and
for sending the transaction back the wallet can then make an intelligent
decision and first try via HTTP and only after that fails, say something
like: "You are currently offline, turn on and transmit via Bluetooth
instead?". Much less confusing to the user, in my opinion.

Another idea could be to request the permission BLUETOOTH_ADMIN which,
as far as I know, allows you to programmatically turn on Bluetooth
without user interaction. The wallet could then have a setting somewhere
that says 'automatically turn on Bluetooth during payments' which would
enable and then disable (if it was off before) Bluetooth during the
payment process. That should also be a decent compromise, at the cost of
another permission.

> There is also the "ack" memo that I mentioned in reference [2]. I
> think we can improve upon this really. Can we make a new status
> field or different bluetooth message header? I know Andreas didn't
> want to change it because that is how his app already works, but I
> don't think the way it is is ideal.

I'm fine with doing changes here - I don't think there is all that much
stuff out there yet which would break from it. At the moment I'm also
modifying BitPay's memo field to contain 'ack', as Andreas' wallet
otherwise reports a failure if I transmit the original via Bluetooth. :-)
But I was assuming that was temporary anyway (?).

Jan