Expose the payload of an Address

[ConorOkus]

When we close a channel in LDK, we want the funds to go directly to a BDK-based on-chain wallet.

We do this by creating a custom KeysInterface where we override the get_destination_script & get_shutdown_scriptpubkey methods and return addresses from BDK. See here for an example in LDK node.

We'll need to expose the payload of an address in order to extract the witness programme and eventually create the ShutdownScript in LDK. This is less error-prone and cumbersome than getting an address from BDK and creating a new script from that, hashing it, and finally creating LDK‘s ShutdownScript

[notmandatory]

I'm having a little trouble getting my head around this, is the idea to get the script template used to create receive addresses by BDK? If so LDK would still need to contact BDK to get which index is being used for the xkeys so that it doesn't use one that the BDK wallet has already used for receiving on-chain. I found the below function, which given a (non-taproot) Descriptor gives you the corresponding Script, is that the sort of function you're looking for?

https://docs.rs/miniscript/latest/miniscript/descriptor/struct.Wsh.html#method.ecdsa_sighash_script_code

Once we can figure out what the functions are you need in the rust lib we should be able to expose them for the language bindings.

[tnull]

I'm having a little trouble getting my head around this, is the idea to get the script template used to create receive addresses by BDK?

Not exactly, the idea is really only to be able to access the payload field given an bitcoin::Address in order to extract and reuse the WitnessProgram in LDK's ShutdownScript::new_witness_program.

Essentially, we'd want to enable this in bindings:

fn get_shutdown_scriptpubkey(&self) -> ShutdownScript {
		let address = self.wallet.get_new_address().expect("Failed to retrieve new address from wallet.");
		match address.payload {
			bitcoin::util::address::Payload::WitnessProgram { version, program } => {
				return ShutdownScript::new_witness_program(version, &program)
					.expect("Invalid shutdown script.");
			}
			_ => panic!("Tried to use a non-witness address. This must not ever happen."),
		}
	}

An alternate path would maybe be to expose access to address.script_pubkey().wscript_hash() or similar, but this would probably take some more unnecessary conversions on our end.

[notmandatory]

Ah thanks, I'll get a PR started.