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

From: Jan Vornberger <[email-masked]>
Date: Sun, 22 Feb 2015 20:08:39 +0100

Hi everyone,

I am working on a Bitcoin point of sale terminal based on a Raspberry Pi, which
displays QR codes, but also provides payment requests via NFC. It can optionally
receive the sender's transaction via Bluetooth, so if the sender wallet
supports it, the sender can be completely offline. Only the terminal needs an
internet connection.

Typical scenario envisioned: Customer taps their smartphone (or maybe smartwatch
in the future) on the NFC pad, confirms the transaction on their phone
(or smartwatch) and the transaction completes via Bluetooth and/or the phone's
internet connection.

You can see a prototype in action here:

  https://www.youtube.com/watch?v=P7vKHMoapr8

The above demo uses a release version of Schildbach's Bitcoin Wallet, so it
works as shown today. However, some parts - especially the Bluetooth stuff - are
custom extensions of Schildbach's wallet which are not yet standard.

I'm writing this post to document my experience implementing NFC and offline
payments and hope to move the discussion forward around standardizing some of
this stuff. Andy Schroder's work around his Bitcoin Fluid Dispenser [1,2]
follows along the same lines, so his proposed TBIP74 [3] and TBIP75 [4] are
relevant here as well.

## NFC vs Bluetooth vs NFC+Bluetooth ##

Before I get into the implementation details, a few words for why I decided to
go with the combination of NFC and Bluetooth:

Doing everything via NFC is an interesting option to keep things simple, but the
issue is, that one usually can't maintain the connection while the user confirms
the transaction (as they take the device back to press a button or maybe enter a
PIN). So there are three options:

1. Do a "double tap": User taps, takes the device back, confirms, then taps
again to transmit the transaction. (I think Google Wallet does something like
this.)

2. Confirm beforehand: User confirms, then taps and everything can happen in one
go. The disadvantage is, that you confirm the transaction before you have seen
the details. (I believe Google Wallet can also work this way.)

3. Tap the phone, then establish a Bluetooth connection which allows you to do
all necessary communication even if the user takes the device back.

I feel that option 3 is the nicest UX, so that is what I am focusing on right
now, but there are pros and cons to all options. One disadvantage of option 3 in
practice is, that many users - in my experience - have Bluetooth turned off, so
it can result in additional UI dialogs popping up, asking the user to turn on
Bluetooth.

Regarding doing everything via Bluetooth or maybe BLE: I have been following the
work that Airbitz has done around that, but personally I prefer the NFC
interaction of "I touch what I want to pay" rather than "a payment request comes
to me through the air and I figure out whether it is meant for me/is legitimate".

## NFC data formats ##

A bit of background for those who are not that familiar with NFC: Most Bitcoin
wallets with NFC support make use of NDEF (NFC Data Exchange Format) as far as I
am aware (with CoinBlesk being an exception, which uses host-based card
emulation, if I understand it correctly). NDEF defines a number of record types,
among them 'URI' and 'Mime Type'.

A common way of using NFC with Bitcoin is to create a URI record that contains a
Bitcoin URI. Beyond that Schildbach's wallet (and maybe others?) also support
the mime type record, which is then set to 'application/bitcoin-paymentrequest'
and the rest of the NFC data is a complete BIP70 payment request.

## Implementation ##

To structure the discussion a little bit, I have listed a number of scenarios to
consider below. Not every possible combination is listed, but it should cover a
bit of everything.

Scenarios:

1) Scan QR code, transmit transaction via Bitcoin network
   Example QR code: bitcoin:1asdf...?amount=42

2) Touch NFC pad, transmit transaction via Bitcoin network
   Example NFC URI: bitcoin:1asdf...?amount=42

3) Scan QR code, fetch BIP70 details via HTTP, post transaction via HTTP
   Example QR code: bitcoin:1asdf...?amount=42&r=https://example.org/bip70paymentrequest

4) Touch NFC pad, fetch BIP70 details via HTTP, post transaction via HTTP
   Example NFC URI: bitcoin:1asdf...?amount=42&r=https://example.org/bip70paymentrequest

5) Touch NFC pad, receive BIP70 details directly, post transaction via HTTP
   Example NFC MIME record: application/bitcoin-paymentrequest + BIP70 payment request

6) Scan QR code, fetch BIP70 details via Bluetooth, post transaction via Bluetooth
   Example QR code: bitcoin:1asdf...?amount=42&bt=1234567890AB
   Payment request has 'payment_url' set to 'bt:1234567890AB'

7) Touch NFC pad, fetch BIP70 details via Bluetooth, post transaction via Bluetooth
   Example NFC URI: bitcoin:1asdf...?amount=42&bt=1234567890AB
   Payment request has 'payment_url' set to 'bt:1234567890AB'

Scenarios 1 and 2 are basically the 'legacy'/pre-BIP70 approach and I am just
listing them here for comparison. Scenario 3 is what is often in use now, for
example when using a checkout screen by BitPay or Coinbase.

I played around with both scenarios 4 and 5, trying to decide whether I should
use an NFC URI record or already provide the complete BIP70 payment request via
NFC.

My experience here has been, that the latter was fairly fragile in my setup
(Raspberry Pi, NFC dongle from a company called Sensor ID, using nfcpy). I tried
with signed payment requests that were around 4k to 5k and the transfer would
often not complete if I didn't hold the phone perfectly in place. So I quickly
switched to using the NFC URI record instead and have the phone fetch the BIP70
payment request via Bluetooth afterwards. Using this approach the amount of data
is small enough that it's usually 'all or nothing' and that seems more robust to
me.

That said, I continue to have problems with the NFC stack that I'm using, so it
might just be my NFC setup that is causing these problems. I will probably give
the NXP NFC library a try next (which I believe is also the stack that is used
by Android). Maybe I have more luck with that approach and could then switch to
scenario 5.

Scenarios 6 and 7 is what the terminal is doing right now. The 'bt' parameter is
the non-standard extension of Andreas' wallet that I was mentioning. TBIP75
proposes to change 'bt' into 'r1' as part of a more generic approach of
numbering different sources for the BIP70 payment request. I think that is a
good idea and would express my vote for this proposal. So the QR code or NFC URI
would then look something like this:

  bitcoin:1asdf...?amount=42&r=https://example.org/bip70&r1=bt:1234567890AB/resource

In addition the payment request would need to list additional 'payment_url's. My
proposal would be to do something like this:

    message PaymentDetails {
        ...
        optional string payment_url = 6;
        optional bytes merchant_data = 7;
        repeated string additional_payment_urls = 8;
          // ^-- new; to hold things like 'bt:1234567890AB'
    }

TBIP75 proposes to just change 'optional string payment_url' into 'repeated
string payment_url'. If this isn't causing any problems (and hopefully not too
much confusion?) I guess that would be fine too.

In my opinion a wallet should then actually attempt all or multiple of the
provided mechanisms in parallel (e.g. try to fetch the BIP70 payment request via
both HTTP and Bluetooth) and go with whatever completes first. But that is of
course up to each wallet to decide how to handle.

TBIP75 furthermore proposes to include an additional 'h' parameter which would
be a hash of the BIP70 payment request, preventing a MITM attack on the
Bluetooth channel even if the BIP70 payment request isn't signed. This would
have also been my suggestion, although I know that Mike Hearn has raised
concerns about this approach. One being, that one needs to finalize the BIP70
payment request at the time the QR code and NFC URI is generated.

## Questions ##

My questions to the list:

1) Do you prefer changing 'optional string payment_url' into 'repeated string
payment_url' or would you rather introduce a new field 'additional_payment_urls'?

2) @Andreas: Is the r, r1, r2 mechanism already implemented in Bitcoin Wallet?

3) Are there other comments regarding 'h' parameter as per TBIP75?

4) General comments, advice, feedback?

I appreciate your input! :-)

Cheers,
Jan

[1] http://andyschroder.com/BitcoinFluidDispenser/
[2] https://www.mail-archive.com/bitcoin-development%40lists.sourceforge.net/msg06354.html
[3] https://github.com/AndySchroder/bips/blob/master/tbip-0074.mediawiki
[4] https://github.com/AndySchroder/bips/blob/master/tbip-0075.mediawiki