[USING] fix PayJoin

[MaxHillebrand]

This ready for review branch adds some minor fixes to the PayJoin chapter, mainly that it works over clearnet too.
also adds a link to bip79 with a link to the btcpay spec for how our implementation differs.

[yahiheb]

Does Wasabi support BIP79?

[MaxHillebrand]

mostly, yes.
There are some differences, mainly using PSBT instead of raw transactions.
The couple deviations are in the btcpay server spec.

[yahiheb]

But Wasabi doesn't support receiver side does it?

[MaxHillebrand]

yes. But this doesn't mean that it is strictly compliant with BIP79.
For example, another thing with BIP79 is that the URI has a bp=xxx extension for bustapay.
Wasabi doesn't recognize this, I think, but only the BTCPay spec of pj=xxx for PayJoin.

[yahiheb]

Then maybe we shouldn't add this to the supported BIPs?

[MaxHillebrand]

For example, we also have BIP158 on there, but we don't 100% support it, cause we have segwit v0 single pubkey filters, not the basic filters.

I think, that if we support the general idea / most of the details of a BIP, then we should reference it here.

BTCPay might open a new BIP with the updated specifics. If they do, then we should of course link to this one.

FYI, the changes really are minimal...

- The standard way of letting a sender know where to send a bustapay transaction is done via a bip21 encoded address. The key value "bpu" (short for "BustaPayUrl") should be used. An example of such address would be bitcoin:2NABbUr9yeRCp1oUCtVmgJF8HGRCo3ifpTT?bpu=https://bp.bustabit.com/submit It is highly encouraged that urls are kept short. 
+ The standard way of letting a sender know where to send a bustapay transaction is done via a bip21 encoded address. The key value "pj" (short for "payjoin URL") should be used. An example of such address would be bitcoin:2NABbUr9yeRCp1oUCtVmgJF8HGRCo3ifpTT?pj=https://example.com/submit It is highly encouraged that urls are kept short.

- Step 1. Sender creates a bitcoin transaction paying the receiver
+ Step 1. Sender creates a bitcoin transaction OR PSBT paying the receiver
+ While we also support the use of raw transaction instead of PSBT, we strongly advise against it.

- This transaction must use segwit for all inputs, and be fully valid and signed. The transaction must be eligible for propagation on the network (but not done so at this stage) 
+ If using PSBT, the PSBT must be finalized with witness UTXO information.
+ If using transactions, the transaction must use segwit for all inputs, and be fully valid and signed. The transaction must be eligible for propagation on the network (but not done so at this stage

- Step 2. Sender gives the "template transaction" to the receiver
+ Step 2. If using PSBT, sender gives "original PSBT" to the receiver, in base64 or hex format. 
+ If using transactions, sender gives "original transaction" to the receiver, in hex format.
- This is done via an HTTP POST request, sent to a "[bustapay] url" 
+ This is done via an HTTP POST request, sent to a "[payjoin] url" 

Step 3. Receiver processes the transaction and returns a partially signed coinjoin

- The receiver validates the transaction, and pays himself. The receiver then adds one or more of his own inputs (known as the contributed inputs) and (optionally) increases the output that pays himself (generally by the sum of the contributed inputs). Doing so creates a partial transaction, which the receiver returns to the sender. It is called such as it requires the sender to re-sign his own inputs. 
+ The receiver extracts the transaction, optionally verifying it is valid. (by calling testmempoolaccept on the extracted transaction)
+ The receiver extracts the feerate, adds one of his inputs and decreases the change output of the payer by the increased fee difference his input added such that the feerate is the same as the "orginal PSBT" or "original transaction".

[yahiheb]

Thank you for the explanation. I am just trying to make sure that we add what we really support.
I remember @nopara73 removing some bips before, so that is why I thought we shouldn't add it.

[yahiheb]

WalletWasabi/WalletWasabi#3639 (comment)

Does this change anything?

[MaxHillebrand]

nope, I don't think so...