new option to change inputs/outputs of loaded unsigned tx, for coinjoin

[gits7r]

I am trying to achieve coinjoin functionality in electrum in order to simultaneously fund a contract with a counterparty. However, it does not work and this is unfortunate, it's a very useful feature and will become much more requested with lightning channels. I have received requests for such an use case from a journalistic investigation org that uses electrum with tails:

Currently Electrum provides no way to load an unsigned transaction from file and only add inputs / outputs to it and partially sign own added inputs, while leaving the transaction not final and non-broadcastable. I know there is no standard for unsigned transactions, but at least transactions created with Electrum and loaded in Electrum should work.

I have tried a work-around for this, and added one of my addresses from a standard electrum wallet as watch only in an imported wallet, created the transaction and saved it to a file. After that, I tried to load it in the initial wallet that holds the private key of that address but the "Sign" button is disabled, I cannot sign / broadcast it.

Can we improve our transaction format so that such an use case can work? For example:

a) Alice has a standard wallet and creates a transaction in Electrum but does not sign it. She hands it over to Bob, so Bob can also fund a contract with her at the same time.

b) Bob uses Electrum but has a multisig wallet 2-of-2 with Carol. Bob loads the unsigned transaction sent by Alice, adds his own inputs (increasing one output created by Alice, and , if change required, adds more outputs as well) and signs the transaction.

b.1) Bob sends to his cosigner Carol so that his inputs become spendable. Carol signs, and sends back to Bob.

c) Bob sends the half-signed transaction back to Alice, which appends her signature for her initial inputs, making the transaction final and broadcastable.

Step b.1 may not be required din all cases, if Bob would be using a standard wallet too, but it should be supported.

[SomberNight]

I have tried a work-around for this, and added one of my addresses from a standard electrum wallet as watch only in an imported wallet, created the transaction and saved it to a file. After that, I tried to load it in the initial wallet that holds the private key of that address but the "Sign" button is disabled, I cannot sign / broadcast it.

This is known not to work in past releases (#2897) but should work on master. What version did you use to test?

adds his own inputs (increasing one output created by Alice, and , if change required, adds more outputs as well)

indeed this is currently not supported

[gits7r]

I have used the latest version 3.1.3 and it does not recognize own private keys for both standard wallets as well as multisig wallets. For example, I import a transaction that spends from a P2SH multisig 2 out of 2 address. I import it in a wallet that is the first cosigner of the 2 required, and it does not recognize that it can at least partially sign. Standard non multisig wallet will not work and behave the same, the sign button cannot be pressed even the private key is in the wallet.

[SomberNight]

Yes, as I said this only works on master. There will be a 3.2.0 release soon that has it.

[gits7r]

Oh, okay git master -- thought you were referring to latest release tag.

What about adding a feature to add inputs / outputs to created but unsigned / partially signed transaction (that either belongs to our wallet or not). Something like "extend transaction". Like I said, I do know that there is no standard for this yet, but it could be a long time until there will be one - maybe there will never be. Until then, we should at least support it from electrum to electrum, and when / if a standard will show up we will adapt it. We can maintain even backwards compatibility by encoding a 'extend transaction protocol number' so that electrum knows how to interpret it. This will ensure Electrum's leading position in the ecosystem. Simultaneous funding by different people is needed for services that provide liquidity for lightning channels and much more other things.

[gits7r]

As soon as I learn about #4514 I will draft a proposal to implement this -- there is high demand for this feature.