One correction inline below. e On 02/22/2015 02:39 PM, Eric Voskuil wrote: > Hi Jan, > > This is really nice work. > > 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. > >> 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? > > Yes, this design is problematic from a privacy standpoint. Anyone within > the rather significant range of the Bluetooth terminal is able to > capture payment requests and correlate them to people. In other words it > can be used to automate tainting. > > 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. > > As such the "tap" should transfer a session key used for symmetric block > cipher over the Bluetooth channel. This also resolves the issue of > needing to formulate the payment request before the NFC. > > 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 cannot > guarantee privacy. > > The other differences I have with the proposal pertain to efficiency, > not privacy or integrity of the transaction: > > 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. > > The MAC address (and resource name) should be encoded using base58. This 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. > > There is no need for the establishment of two Bluetooth services. > > I would change the payment_url recommendation so that the list order > represents a recommended ordering provided by the terminal for the wallet. > > 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. > > https://github.com/evoskuil/bips/tree/master/docs > > e > > > 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 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 >> >> ------------------------------------------------------------------------------ >> Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server >> from Actuate! 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