Implement BIP174 (PSBT) for serialising partial and unsigned transactions

[SomberNight]

BIP174 is now no longer in "draft" state. It has also been merged into Bitcoin Core (in e.g. bitcoin/bitcoin#13557), and it looks like they will have support for it in 0.17.

We should consider ditching our custom format and implement it as well, as it would be good to be compatible with Bitcoin Core and potentially others.

Related:
#4514
#4614

[doc-hex]

If it helps, the Coldcard wallet plugin already can make a PSBT: see function built_psbt() in the PR #4470.

We do somewhat cheat because we didn't do any merging or combining, and just assume the result is a finalized transaction coming back from the Coldcard. (That's a safe assumption given the role of Coldcard in this case as a hardware keystore and signer.) We didn't need a PSBT parser.

Another limitation is the "Save as PSBT" button on the transaction dialog, appears only if the transaction is associated with a Coldcard based wallet. It should be possible for Electrum to offer PSBT output there for almost any unsigned transaction.

Naturally as PSBT support matures in Electrum, we will use those features as much as possible for this plugin. Always happy to delete code!

PS: You may find this debug tool, psbt_dump, to be useful, and @achow101 has been really helpful as well.

[SomberNight]

note: @asfin has said he is interested in working on this

[roznalex]

Are there any updates?

[SomberNight]

there are WIP changes at https://github.com/asfin/electrum/tree/bip174-2

[SomberNight]

Note: I am now working on this. Trying to reuse code already written as part of the coldcard plugin, and some stuff from #4883

[craigraw]

I think this would be a valuable feature for many. Although I've only tested it with the coldcard, the plugin PSBT support works well, although it does lose partial sigs when loading PSBTs (fix proposed here: #5671).

Support for output descriptors would be very useful, but at the least Electrum needs to support generalised saving/retrieving the master key fingerprint for all participating keystores, particularly hardware keystores which store the derived xpub only. The coldcard plugin saves as custom attribute ckcc_xfp - other hardware wallet plugins will need to implement to comply with BIP174 if they are to take part in PSBTs. This could be a standard attribute in Hardware_KeyStore along with the existing xpub, label and derivation.

[SomberNight]

at the least Electrum needs to support generalised saving/retrieving the master key fingerprint for all participating keystores, particularly hardware keystores which store the derived xpub only. The coldcard plugin saves as custom attribute ckcc_xfp - other hardware wallet plugins will need to implement to comply with BIP174 if they are to take part in PSBTs. This could be a standard attribute in Hardware_KeyStore along with the existing xpub, label and derivation.

Indeed you are right. I am planning on saving derivation path prefix and root node fingerprint for "all" keystores; maybe in the Xpub keystore mixin:

electrum/electrum/keystore.py

Line 278 in d6d6441

class Xpub: