Introduce `FundingTransactionReadyForSignatures` event #3889

dunxen · 2025-06-24T12:12:50Z

Cherry-picked from #3735 as it is relevant to splicing and will unblock testing after #3736 lands.

The FundingTransactionReadyForSignatures event requests witnesses from the client for their contributed inputs to an interactively constructed transaction.

The client calls ChannelManager::funding_transaction_signed to provide the witnesses to LDK.

ldk-reviews-bot · 2025-06-24T12:12:52Z

👋 Thanks for assigning @jkczyz as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

lightning/src/events/mod.rs

lightning/src/ln/channelmanager.rs

jkczyz · 2025-06-24T16:31:27Z

lightning/src/ln/channelmanager.rs

+		let witnesses: Vec<_> = transaction
+			.input
+			.into_iter()
+			.filter_map(|input| if input.witness.is_empty() { None } else { Some(input.witness) })
+			.collect();


Don't have a strong opinion here, but seems we can avoid this by passing the Transaction through and only collecting witnesses when we are ready to construct TxSignatures.

Cool, yeah will look at changing this after fixups.

lightning/src/ln/interactivetxs.rs

codecov · 2025-06-25T11:08:53Z

Codecov Report

Attention: Patch coverage is 9.70874% with 93 lines in your changes missing coverage. Please review.

Project coverage is 88.82%. Comparing base (6771d84) to head (ff1489d).

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	1.85%	52 Missing and 1 partial ⚠️
lightning/src/ln/channel.rs	11.11%	32 Missing ⚠️
lightning/src/ln/interactivetxs.rs	45.45%	6 Missing ⚠️
lightning/src/events/mod.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3889      +/-   ##
==========================================
- Coverage   88.88%   88.82%   -0.07%     
==========================================
  Files         165      165              
  Lines      118886   118971      +85     
  Branches   118886   118971      +85     
==========================================
  Hits       105676   105676              
- Misses      10892    10976      +84     
- Partials     2318     2319       +1

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ldk-reviews-bot · 2025-06-26T12:13:27Z

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ldk-reviews-bot · 2025-06-26T12:13:29Z

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optout21

Looks good!

lightning/src/ln/channel.rs

ldk-reviews-bot · 2025-06-28T12:14:21Z

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ldk-reviews-bot · 2025-06-30T12:15:10Z

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jkczyz

Not sure if there was a problem rebasing, but some comments that had been marked resolved weren't fixed.

lightning/src/events/mod.rs

lightning/src/ln/channelmanager.rs

lightning/src/events/mod.rs

dunxen · 2025-07-01T08:08:04Z

Not sure if there was a problem rebasing, but some comments that had been marked resolved weren't fixed.

Yeah, they got lost on a rebase and somehow lost the commit. Rebased to get the one CI fix in. Fixing.

ldk-reviews-bot · 2025-07-02T12:15:51Z

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ldk-reviews-bot · 2025-07-07T00:01:47Z

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lightning/src/events/mod.rs

wpaulino · 2025-07-07T19:08:26Z

lightning/src/ln/channel.rs

+	fn verify_interactive_tx_signatures(&mut self, _witnesses: &Vec<Witness>) {
+		if let Some(ref mut _signing_session) = self.interactive_tx_signing_session {
+			// Check that sighash_all was used:
+			// TODO(dual_funding): Check sig for sighash


Any reason this isn't done yet?

Oh yeah we're actually unable to do this reliably, especially if inputs were spend P2WSH. The public comments warn the client to ensure it is SIGHASH_ALL (or SIGHASH_DEFAULT/SIGHASH_ALL) if spending a P2TR UTXO.

These TODOs need to be removed.

We should be able to use ScriptBuf::instructions to identify all pushes that match a signature length, parse those that match, and make sure they come with SIGHASH_ALL.

I'm worried we assume something is a sig just based on length when it's not. I mean it's probably a sig, but we'd then actually need to check if there is a corresponding checksig in the script.

If it wasn't a sig then we'd always treat that as invalid if it happens to have the same length and something that looks like a sighash_all byte (or the case no explicit sighash as in p2tr sighash_default).

It's probably unlikely but I haven't considered how "probably".

For ECDSA sigs we can be a bit more sure since they have to parse as DER-encoded. P2WSH and P2TR-script-spend always reveal the witness script so we can certainly check for corresponding OP_CHECKSIGs there. P2WPKH and P2TR-key-spend have a specific index within the witness that corresponds to the signature.

If we don't check this for holder witnesses, then we're susceptible to malleability attacks.

We checked that all prev outputs are "standard" during tx construction, so if a witness is not one of the recognised witnesses shall we just ignore and not error? (considering it'll fail anyway). I'd say we should only care about checking for sighash types for witnesses corresponding to "standard" scriptpubkeys.

We shouldn't ever get to this point if that were the case, we should return an error when the inputs are provided.

The function name should probably include "sighash_type" to make it clear that's all we check.

We could also consider using the validation feature on the bitcoin crate to actually verify witnesses.

We shouldn't ever get to this point if that were the case, we should return an error when the inputs are provided.

Yip, exactly.

We could also consider using the validation feature on the bitcoin crate to actually verify witnesses.

Yeah we could and since we’re only verifying the ones provided by the client, we don’t add much to the “DoS budget” from an “untrusted” peer.

We could also consider using the validation feature on the bitcoin crate to actually verify witnesses.

I'm not super excited to start depending on this in non-test. Its been removed from Bitcoin Core so is slowly getting more and more outdated and I don't think anyone else uses it aside from us anyway. If we checked the list of scripts is in one of several standard formats, we can (and should, IMO) just use that knowledge to check things.

For P2TR script path spend we’d be uncertain if it was a signature or just random 64 bytes (65 with explicit sighash).

The solution to also look for corresponding op_checksigs becomes tricky since we’d also need to consider that for op_checksigadd where there is a threshold.

It could be done but you’d at least need to do some very basic script validation to see if the supposed signatures are consumed by checksig opcodes and then only verify the sighash type of those. Conditional branching in script further complicates that (even though people should probably use a different leaf script in that case).

It feels like we’d run into the problem of writing our own validation just to check this.

I can’t imagine we’d get many false positives if we assumed 64/65 byte things are signatures. It just feels wrong.

Discussed this in the meeting. Will push another update tomorrow re-adding scriptpubkeys to NegotiatedTxInput and fully verifying key-spends, but not P2WSH or P2TR script-path (we'll make it very clear the client is on their own as they should know what they're doing and we'll describe the consequences). We will check all signatures in witnesses use sighash_all as well.

lightning/src/ln/channel.rs

The `FundingTransactionReadyForSignatures` event requests witnesses from the client for their contributed inputs to an interactively constructed transaction. The client calls `ChannelManager::funding_transaction_signed` to provide the witnesses to LDK.

…igned exchange

We directly get the holder `TxSignatures` when necessary.

dunxen added Dual-funding Splicing labels Jun 24, 2025

dunxen requested review from wpaulino, optout21 and jkczyz June 24, 2025 12:13

jkczyz reviewed Jun 24, 2025

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dunxen mentioned this pull request Jun 25, 2025

Contribute funding inputs on accepting dual-funded channel #3735

Draft

dunxen force-pushed the 2025-06-readyforsigningevent branch from 165d59d to 8ca6d79 Compare June 25, 2025 10:59

optout21 reviewed Jun 26, 2025

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dunxen force-pushed the 2025-06-readyforsigningevent branch from 890633d to a1de384 Compare June 27, 2025 07:20

jkczyz reviewed Jun 27, 2025

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lightning/src/ln/channel.rs Outdated Show resolved Hide resolved

dunxen force-pushed the 2025-06-readyforsigningevent branch 2 times, most recently from 7df5779 to c8f981c Compare June 30, 2025 10:29

jkczyz reviewed Jun 30, 2025

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dunxen force-pushed the 2025-06-readyforsigningevent branch 2 times, most recently from c15f426 to ff1489d Compare July 1, 2025 09:18

dunxen force-pushed the 2025-06-readyforsigningevent branch from ff1489d to 0a586e6 Compare July 4, 2025 07:05

wpaulino reviewed Jul 7, 2025

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dunxen force-pushed the 2025-06-readyforsigningevent branch 2 times, most recently from a9e1a3a to 83e78d6 Compare July 14, 2025 09:34

dunxen added 5 commits July 16, 2025 11:59

f only emit FundingTransactionReadyForSigning on initial commitment_s…

60fb2f4

…igned exchange

wip: Check that holder witnesses sighash types are SIGHASH_(ALL/DEFAULT)

2262f33

f add splicing check in funding_transaction_signed

8a5d821

Use bool for monitor_pending_tx_signatures over Option<TxSignatures>

1ab3286

We directly get the holder `TxSignatures` when necessary.

dunxen force-pushed the 2025-06-readyforsigningevent branch from 83e78d6 to 1ab3286 Compare July 16, 2025 11:38

Introduce FundingTransactionReadyForSignatures event #3889

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Introduce FundingTransactionReadyForSignatures event #3889

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Introduce `FundingTransactionReadyForSignatures` event #3889

Introduce `FundingTransactionReadyForSignatures` event #3889

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