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Message-ID: <54EA5CB4.5030302@voskuil.org>
Date: Sun, 22 Feb 2015 14:48:20 -0800
From: Eric Voskuil <eric@voskuil.org>
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Subject: Re: [Bitcoin-development] Bitcoin at POS using BIP70,
 NFC and offline payments - implementer feedback
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One correction inline below.

e

On 02/22/2015 02:39 PM, Eric Voskuil wrote:
> Hi Jan,
>=20
> This is really nice work.
>=20
> WRT the Schroder and Schildbach proposal, the generalization of the "r"=

> and "payment_url" parameters makes sense, with only the potential
> backward compat issue on payment_url.
>=20
>> 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.
>> ...
>> 3) Are there other comments regarding 'h' parameter as per TBIP75?
>=20
> Yes, this design is problematic from a privacy standpoint. Anyone withi=
n
> the rather significant range of the Bluetooth terminal is able to
> capture payment requests and correlate them to people. In other words i=
t
> can be used to automate tainting.
>=20
> The problem is easily resolved by recognizing that, in the envisioned
> face-to-face trade, proximity is the source of trust. Even in the above=

> proposal the "h" parameter is trusted because it was obtained by
> proximity to the NFC terminal. The presumption is that this proximity
> produces a private channel.
>=20
> As such the "tap" should transfer a session key used for symmetric bloc=
k
> cipher over the Bluetooth channel. This also resolves the issue of
> needing to formulate the payment request before the NFC.
>=20
> As an aside, in other scenarios, such as an automated dispenser, this
> presumption does not hold. The merchant is not present to guard against=

> device tampering. Those scenarios can be secured using BIP70, but canno=
t
> guarantee privacy.
>=20
> The other differences I have with the proposal pertain to efficiency,
> not privacy or integrity of the transaction:
>=20
> The proposed resource name is redundant with any unique identifier for
> the session. For example, the "h" parameter is sufficient. But with the=

> establishment of a session key both as I propose above, the parties can=

> derive a sufficiently unique public resource name from a hash of the
> key. An additional advantage is that the resource name can be
> fixed-length, simplifying the encoding/decoding.
>=20
> The MAC address (and resource name) should be encoded using base58. Thi=
s

The MAC address (and session key) should be encoded using base58. This

> is shorter than base16, is often shorter than base64, better
> standardized and does not require URI encoding, and is generally
> available to implementers.
>=20
> There is no need for the establishment of two Bluetooth services.
>=20
> I would change the payment_url recommendation so that the list order
> represents a recommended ordering provided by the terminal for the wall=
et.
>=20
> I wrote up my thoughts on these considerations last year and recently
> revised it by adding a section at the end to incorporate the "r" and
> "payment_url" generalizations from Andreas and Andy.
>=20
> https://github.com/evoskuil/bips/tree/master/docs
>=20
> e
>=20
>=20
> On 02/22/2015 11:08 AM, 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 walle=
t
>> supports it, the sender can be completely offline. Only the terminal n=
eeds 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=3DP7vKHMoapr8
>>
>> The above demo uses a release version of Schildbach's Bitcoin Wallet, =
so it
>> works as shown today. However, some parts - especially the Bluetooth s=
tuff - are
>> custom extensions of Schildbach's wallet which are not yet standard.
>>
>> I'm writing this post to document my experience implementing NFC and o=
ffline
>> 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 de=
cided to
>> go with the combination of NFC and Bluetooth:
>>
>> Doing everything via NFC is an interesting option to keep things simpl=
e, but the
>> issue is, that one usually can't maintain the connection while the use=
r confirms
>> the transaction (as they take the device back to press a button or may=
be 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 somethi=
ng like
>> this.)
>>
>> 2. Confirm beforehand: User confirms, then taps and everything can hap=
pen in one
>> go. The disadvantage is, that you confirm the transaction before you h=
ave seen
>> the details. (I believe Google Wallet can also work this way.)
>>
>> 3. Tap the phone, then establish a Bluetooth connection which allows y=
ou 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 o=
n right
>> now, but there are pros and cons to all options. One disadvantage of o=
ption 3 in
>> practice is, that many users - in my experience - have Bluetooth turne=
d 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 fol=
lowing the
>> work that Airbitz has done around that, but personally I prefer the NF=
C
>> interaction of "I touch what I want to pay" rather than "a payment req=
uest comes
>> to me through the air and I figure out whether it is meant for me/is l=
egitimate".
>>
>>
>> ## 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) a=
s far as I
>> am aware (with CoinBlesk being an exception, which uses host-based car=
d
>> emulation, if I understand it correctly). NDEF defines a number of rec=
ord 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-paymen=
trequest'
>> 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 sc=
enarios to
>> consider below. Not every possible combination is listed, but it shoul=
d cover a
>> bit of everything.
>>
>> Scenarios:
>>
>> 1) Scan QR code, transmit transaction via Bitcoin network
>>    Example QR code: bitcoin:1asdf...?amount=3D42
>>
>> 2) Touch NFC pad, transmit transaction via Bitcoin network
>>    Example NFC URI: bitcoin:1asdf...?amount=3D42
>>
>> 3) Scan QR code, fetch BIP70 details via HTTP, post transaction via HT=
TP
>>    Example QR code: bitcoin:1asdf...?amount=3D42&r=3Dhttps://example.o=
rg/bip70paymentrequest
>>
>> 4) Touch NFC pad, fetch BIP70 details via HTTP, post transaction via H=
TTP
>>    Example NFC URI: bitcoin:1asdf...?amount=3D42&r=3Dhttps://example.o=
rg/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 v=
ia Bluetooth
>>    Example QR code: bitcoin:1asdf...?amount=3D42&bt=3D1234567890AB
>>    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=3D42&bt=3D1234567890AB
>>    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 n=
ow, 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 re=
quest 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 transfe=
r 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 amou=
nt 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 usi=
ng, so it
>> might just be my NFC setup that is causing these problems. I will prob=
ably 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' pa=
rameter is
>> the non-standard extension of Andreas' wallet that I was mentioning. T=
BIP75
>> proposes to change 'bt' into 'r1' as part of a more generic approach o=
f
>> numbering different sources for the BIP70 payment request. I think tha=
t 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=3D42&r=3Dhttps://example.org/bip70&r1=3Dbt:1=
234567890AB/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 =3D 6;
>>         optional bytes merchant_data =3D 7;
>>         repeated string additional_payment_urls =3D 8;
>>           // ^-- new; to hold things like 'bt:1234567890AB'
>>     }
>>
>> TBIP75 proposes to just change 'optional string payment_url' into 'rep=
eated
>> 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 r=
equest via
>> both HTTP and Bluetooth) and go with whatever completes first. But tha=
t is of
>> course up to each wallet to decide how to handle.
>>
>> TBIP75 furthermore proposes to include an additional 'h' parameter whi=
ch would
>> be a hash of the BIP70 payment request, preventing a MITM attack on th=
e
>> 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 rais=
ed
>> concerns about this approach. One being, that one needs to finalize th=
e 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_pay=
ment_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.sourcefor=
ge.net/msg06354.html
>> [3] https://github.com/AndySchroder/bips/blob/master/tbip-0074.mediawi=
ki
>> [4] https://github.com/AndySchroder/bips/blob/master/tbip-0075.mediawi=
ki
>>
>> ----------------------------------------------------------------------=
--------
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>> Bitcoin-development@lists.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/bitcoin-development
>>
>=20


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