[Bitcoin-development] Two Proposed BIPs - Bluetooth Communication and bitcoin: URI Scheme Improvements

[Andy Schroder <info@AndySchroder.com>]

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Hello,

I'd like to introduce two proposed BIPs. They are primarily focused on 
implementing the payment protocol using bluetooth connections. I've been 
working on automated point of sale devices and bluetooth communication 
is critical in my mind due to the potential lack of internet access at 
many points of sale, either due to lack of cellular internet coverage, 
lack of payee providing wireless internet, and/or due to financial 
constraints of the payer prohibiting them from maintaining a cellular 
internet service plan. These BIPs are largely modeled after the current 
functionality of Andreas Schildbach's android Bitcoin Wallet's bluetooth 
capability. I've discussed the communication scheme with him in depth 
and believe these proposals to clearly and accurately represent the 
communication scheme.

There is also an additional &h= parameter added to the bitcoin: URI 
scheme which applies to both bluetooth and http payment protocol 
requests which allows for a hash of the payment request to be included. 
This hash was proposed by Andreas as an amendment to BIP72, but others 
preferred not to amend BIP72 since it has already been put into place. 
The current version of Schildbach's bitcoin wallet already supports the 
"h parameter".

I'd appreciate feedback from everyone, particularly wallet developers as 
widespread bluetooth support among wallets is very important to me. I'm 
also very new to this mailing list as well as the BIP writing process, 
so I'd appreciate your understanding if my conventions are not standard. 
I am currently using the naming conventions "TBIP", so that I can 
propose /temporary/ BIP numbers, and cross reference between the two. 
Obviously these will change if the BIPs are formally adopted. You can 
find a copy of these proposed BIPs at the following links:

  * https://github.com/AndySchroder/bips/blob/master/tbip-0074.mediawiki
  * https://github.com/AndySchroder/bips/blob/master/tbip-0075.mediawiki


If you are interested, you can see a demonstration of many of the 
proposed features using Schildbach's wallet and my fuel pump in a video 
I recently created: https://youtu.be/kkVAhA75k1Y . The main thing not 
implemented is multiple URLs for the payment protocol, so, as a hack, 
I'm just presenting https vi QR code and bluetooth via NFC on my fuel 
pump for now.



There are a few known issues that could be improved to this bluetooth 
communication scheme as well as the general payment protocol and myself 
and Andreas would like to receive feedback regarding concerns and 
potential solutions. Some of the known issues are:

  * There may seem to be some inconsistency in the connection header
    messages between the payment request connection and the payment
    connection. This is largely because it is how Andreas originally
    implemented the communication and is hesitant to change it since
    there are many instances of is software already deployed that
    implement this scheme.
  * The current method uses an unauthenticated bluetooth connection for
    bluetooth 2.1 and newer devices (subject to man in the middle
    attacks, but not passive eavesdroppers), and an unsecure and
    unauthenticated connection for older devices. The known concerns
    here are that someone within 100 meters of the payer could track the
    bitcoin addresses used for the transaction and could possibly
    replace the refund address by submitting a forged payment message to
    the payee. Requiring bluetooth 2.1 and authenticating the connection
    out of band unfortunately don't seem to be as straightforward/simple
    of a task with most bluetooth libraries (although I'd love for
    someone to prove me wrong). It's possible this communication scheme
    could be extended to use an https "like" protocol that would not
    care if the underlying bluetooth connection is authenticated or
    encrypted. It's actually possible that http over a bluetooth socket
    (instead of tcp socket) could be implemented, however it is
    presently uncertain whether this would be too slow, too much
    overhead (both on the devices software and communication), or if
    http could easily be run over bluetooth sockets on all platforms.
  * There is no acknowledgement failure message possible in the payment
    protocol, only an acknowledgement message or lack of acknowledgement
    message. This issue seems to be a concern and as a result, the memo
    field is used to send an "ack" or "nack" in Schildbach's wallet. Can
    we add a boolean status field to the payment acknowledgement message?
  * I'd personally like a new optional boolean field added to the
    "PaymentDetails" portion of the "PaymentRequest" to allow for the
    payer's wallet to match the "Output" optional "amount" fields as a
    total amount of all Outputs, rather than requiring the amount for
    each output to be matched exactly. As it currently is, the payee can
    specify multiple receiving addresses in order to require a payer
    split up the payments so that when the payee then goes to spend the
    funds later, they don't necessarily have to give their payees as
    much knowledge of their balances and spending and receiving habits
    and sources. As the payment protocol currently is requiring all
    output amounts to be matched exactly for each output, there is no
    flexibility given to the payer in order to reduce a merging or
    unnecessary diverging of account funds, which can reduce the privacy
    of both the payer and the payee. If the payee were given the option
    to allow the payer the option to divide the amounts amount the
    outputs intelligently, there can be some privacy gained.
  * Amount of data stored in QR codes may be getting large when a
    backwards compatible URL is used (for wallets that don't support the
    payment protocol) and can be difficult to scan with outdoor screens
    that have an extra weather resistant pane when in direct sunlight.
  * The number of offline transactions of a wallet is limited to the
    known unspent outputs when they go offline. Long term, I'd like to
    see wallet devices that can use systems such as Kryptoradio's DVB-T
    based broadcast (but this will need yet another radio!). Another
    project may be to develop a blockchain query protocol of some kind
    where retailers can provide access to blockchain data so that
    customer's wallets can update their known unspent outputs via
    bluetooth. It's possible such a bluetooth system could be used in
    combination of "Kryptoradio" like broadcasts to provide multiple
    blockchain references.
  * The additional payment_url approach is a bit sloppy of a solution in
    the PaymentDetails portion of the PaymentRequest. It would have been
    ideal to just change this from an optional field to a repeated
    field, however, the backwards compatibility in the protocol buffer
    format will provide the last item in the array for a repeated field
    (to a code that expects it to be an optional field), rather than the
    first. Because of this, backwards compatibility with https payment
    requests wouldn't work if the payment_url field is just changed to a
    repeated field.
      o Possible alternatives to what is described in the proposed BIP
          + Change payment_url to a repeated field and then reverse the
            order of the parameter numbers in the payment_url, compared
            to the bitcoin URL "r parameter".
          + Create an additional, new payment_url_multi repeated field
            (or some better name), and then leave the original
            payment_url field in there for backwards compatibility (and
            then maybe phase it out in the future).
      o Reference
          + https://developers.google.com/protocol-buffers/docs/proto#updating
              # "|optional| is compatible with |repeated|. Given
                serialized data of a repeated field as input, clients
                that expect this field to be |optional| will take the
                last input value if it's a primitive type field or merge
                all input elements if it's a message type field."



Your comments and suggestions would be greatly appreciated.

-- 
Andy Schroder


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