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Re-word the CORS header issue and add some fixes

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docs/research/user-experience/xmr-btc-user-interface-definition.md
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# XMR<>BTC Atomic Swap - User Interface Prototypes
# XMR<>BTC Atomic Swap - User Interface Prototype Definition
This document specifies assumptions concerning possible user interfaces on top of the [Monero-Bitcoin swap as described by Lucas on the COMIT-blog](https://comit.network/blog/2020/10/06/monero-bitcoin/).
@ -9,13 +9,16 @@ This document will be used as a basis for defining and evaluating low-fidelity U
This section sums up assumptions around the current setup, protocol and existing software.
### Technical Limitations
### Security Assumptions
Swap-solutions that run completely in the browser (webpage) can be problematic because blockchain nodes, notably [bitcoind](https://github.com/bitcoin/bitcoin/issues/11833), do not specifying [CORS headers](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) in their response.
Swap-solutions that run completely in the browser (webpage) and rely on connecting to a locally running blockchain node are currently not possible because specifying [CORS headers](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) is a security risk.
This means the browser will reject a response from a locally running Bitcoin node.
Discussions have been ongoing, notably for [bitcoind](https://github.com/bitcoin/bitcoin/issues/11833), to ad features that would allow webpages to communicate with locally running blockchain nodes directly.
However, it is hard to verify the code of a webpage when accessing the webpage in a browser and thus not recommended to add features that would allow to do this.
Theoretically this could be overcome by creating a node infrastructure behind a proper REST API, or with a locally running proxy. However, it is questionable if these are favourable solution, as they adds additional software on top of the blockchain nodes.
We will focus on solutions that favour the use of locally running blockchain nodes.
We will focus on solutions that favour the use of locally running blockchain nodes and explore solutions that do not access locally running blockchain nodes from a webpage directly.
### Current Setup
@ -31,10 +34,15 @@ Given the current setup this results in the following software necessary to achi
* **Interface to connect the trading partners**: The two parties have to find each other and start the `Swap Execution Daemon` with the correct parameters. The interface can be realised in multiple different ways, from single-maker, P2P trading platform to P2P marketplace.
For Bitcoin the current setup relies on a synced bitcoind node and bitcoind's wallet.
For Monero the current setup relies on a synced monerod node and monero's wallet RPC.
For funding Monero an existing wallet is unlocked and used for the swap.
For funding Bitcoin an existing wallet is unlocked and used for the swap.
The swap execution daemon has control over the wallets during the swap execution to allow automated broadcasting of the transactions.
For Monero the current setup relies on a synced monerod node and monero's wallet RPC. For funding Monero an existing wallet is unlocked and used for the swap. The current setup reliese on generating a *new* Monero wallet for `redeeming` or `refunding`.
This is not optimal for the user experience, as it means that a user cannot redeem or refund to an existing Monero wallet in the current setup. We decided **not** to model this restriction of the current setup in the prototypes, but to model the prototypes using an existin Monero wallet.
We believe that it should be possible to use an existing wallet with investing more implementation time into the setup and protocol.
The current setup relies on generating a *new* Monero wallet for `redeeming` or `refunding`.
One trivial solution would be to add an additional transaction in the end, that transfers the Monero to the user's wallet.
More optimized solutions are possible as well, but require more research.
Since there are solutions to overcome the need of a second wallet we decided **not** to mock this part of the current setup in the UI prototypes.
### Current Protocol
@ -60,8 +68,8 @@ Hardware wallet support can be added at a later point if requested as feature by
## Possible Solutions
This section outlines possible solutions based on the technical limitations and assumptions stated above.
This section does not focus on the UX of the swap and the use-case specifics, but on the flow of the application considering restrictions to the browser and applications running next to it.
This section outlines possible solutions based on the assumptions stated above.
This section does not focus on the UX of the swap and the use-case specifics, but on the interaction between browser and other applications needed to achieve the swap.
We have explored the following options:
@ -82,15 +90,15 @@ For the time being the way forward seems to be a swap tool that plugs into exist
### Webpage
A complete webpage based solution is unfortunately not easily possible because of the restrictions of CORS headers and blockchain nodes not being able to supply CORS headers.
The swap protocol implementation could be done in javascript, or run as web-assembly, but since the current setup requires extensive communication with a (local) wallet
Allowing webpages access to a locally running
A complete webpage based solution is not easily possible because of the CORS header restriction of browsers.
We could enable communication between blockchain nodes and the browser through e.g. a proxy, but such a solution would not have an advantage over running a desktop application, because either way one has to run additional software next to the blockchain nodes to enable the swap.
### Browser Extension
In the previous section we discussed that webpage based solutions are currently not possible, due to blockchain nodes inability to specify CORS headers.
Browser extensions, however, are not as restrictive as webpages as can be seen in the [Google Chrome's documentsion](https://developer.chrome.com/extensions/xhr).
A browser extension would have the advantage of being able to offer a well-defined API, that allows multiple different web-sites to offer services within the extension.
Browser extensions, are not as restrictive as webpages as can be seen in the [Google Chrome's documentsion](https://developer.chrome.com/extensions/xhr).
The code of a browser extension can be signed and is easily verifiable.
A browser extension would be able to offer a well-defined API, that allows multiple different web-sites to offer services within the extension.
This could range from discovery, negotiation to execution.
Implementing a browser extension that handles the communication between maker and taker, and is able to communicate with locally running wallets and blockchain nodes is a valid solution.
@ -103,14 +111,15 @@ Mozilla's [web-ext tool](https://github.com/mozilla/web-ext) - besides [other to
### Desktop Application
A desktop application is a valid solution that is not subject to restrictions by the browser.
For previous MVPs we used Electron to implement cross-platform desktop applications.
The problem of finding a trading partner can be outsourced to a webpage (e.g. single-maker only).
Custom URL schemes can help to provide a better user experience, as they allow us to open the swap desktop application with parameters passed to the application through the customer URL.
It can, however, be be quite a pain to achieve the proper registration of custom URL schemas for different kinds of operating systems.
A stand-alone desktop applications is possible, but can be seen as a hurdle, because every user first has to download the binary to see any form of user interface.
For completx applications that involve a decentralized orderbook a desktop application might be advisable for the moment.
For complex applications that involve a decentralized orderbook a desktop application might be advisable for the moment.
For previous MVPs we used [Electron](https://www.electronjs.org/) to implement cross-platform desktop applications.
## Use-case specific prototypes