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

[Eric Voskuil <eric@voskuil.org>]

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On 02/22/2015 02:37 PM, Andy Schroder wrote:
> I'd like to see some discussion too about securing the bluetooth
> connection. Right now it is possible for an eavesdropper to monitor the
> data transferred. 

Yes, this should be a prerequisite issue to all others.

> I'd personally like to see if wrapping the current
> connection with SSL works or if we can run https over a bluetooth
> socket. 

There is no reason to add this significant complexity. The purpose of
SSL/TLS is to establish privacy over a *public* channel. But to do so
requires verification by the user of the merchant's public certificate.
Once we rely on the channel being *private*, the entire SSL process is
unnecessary.

Presumably we would not want to require PKI for privacy, since that's a
bit of a contradiction. But if one wants to do this NFC is not required,
since the private session can be established over the public (Bluetooth)
network.

> There was some criticism of this, but I don't think it has been
> tested to know if it is really a problem or not. If we just run https
> over bluetooth, then a lot of my concerns about the message header
> inconsistencies will go away and the connection will also be secure. We
> don't have to reinvent anything.
> 
> 
> 
> Andy Schroder
> 
> On 02/22/2015 02:08 PM, Jan Vornberger wrote:
>> 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
>>
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> 
> 
> 
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