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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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.

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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.

dunxen · 2025-07-17T09:59:25Z

lightning/src/ln/channel.rs

@@ -7605,6 +7586,188 @@ where
 		}
 	}

+	fn verify_interactive_tx_signatures(


Needs some clean up and tests but pushed it for some review.

Is test coverage for this actually feasible yet?

lightning/src/events/mod.rs

lightning/src/ln/channel.rs

wpaulino · 2025-07-17T22:55:35Z

lightning/src/ln/channel.rs

@@ -1840,15 +1840,15 @@ where
 				let res = if has_negotiated_pending_splice && !session_received_commitment_signed {
 					funded_channel
 						.splice_initial_commitment_signed(msg, logger)
-						.map(|monitor_update_opt| (None, monitor_update_opt))
+						.map(|monitor_update_opt| (None, monitor_update_opt, None))


We will also need to return the unsigned transaction here for splices.

Shall we defer this to a splice PR? I'm not 100% sure of the specifics in splice_initial_commitment_signed().

wpaulino · 2025-07-17T23:09:21Z

lightning/src/ln/channelmanager.rs

+					if let Some(unsigned_transaction) = funding_tx_opt {
+						let mut pending_events = self.pending_events.lock().unwrap();
+						pending_events.push_back((
+							Event::FundingTransactionReadyForSigning {


I think we'll also need to generate this on reestablish when we have already received commitment_signed but not tx_signatures.

Working on this still.

lightning/src/ln/interactivetxs.rs

lightning/src/ln/channel.rs

dunxen

Pushed latest fixups. Comments left on remaining ones.

dunxen · 2025-07-21T17:57:55Z

lightning/src/ln/channel.rs

@@ -1840,15 +1840,15 @@ where
 				let res = if has_negotiated_pending_splice && !session_received_commitment_signed {
 					funded_channel
 						.splice_initial_commitment_signed(msg, logger)
-						.map(|monitor_update_opt| (None, monitor_update_opt))
+						.map(|monitor_update_opt| (None, monitor_update_opt, None))


Shall we defer this to a splice PR? I'm not 100% sure of the specifics in splice_initial_commitment_signed().

dunxen · 2025-07-21T17:58:09Z

lightning/src/ln/channelmanager.rs

+					if let Some(unsigned_transaction) = funding_tx_opt {
+						let mut pending_events = self.pending_events.lock().unwrap();
+						pending_events.push_back((
+							Event::FundingTransactionReadyForSigning {


Working on this still.

lightning/src/ln/interactivetxs.rs

wpaulino · 2025-07-23T17:49:18Z

lightning/src/ln/channelmanager.rs

+		if let Some(signing_session) = &channel.interactive_tx_signing_session {
+			if signing_session.has_received_commitment_signed() &&
+				signing_session.local_inputs_count() > 0 &&
+				!channel.is_their_tx_signatures_sent()


Shouldn't we just need to check if the session hasn't had the signatures provided?

Also, since handle_channel_resumption handles resumption from both a reestablish and a monitor update, it seems like we don't actually need to push an event for the received commitment_signed case at all? Once the corresponding monitor update for that message completes, we'll come here and push the event. It does mean that we can't provide signatures while the monitor update hasn't completed, but that seems fine? We'd be able to get rid of monitor_pending_tx_signatures as well.

Working on this.

lightning/src/ln/channel.rs

wpaulino · 2025-07-23T18:00:21Z

lightning/src/ln/channel.rs

@@ -7605,6 +7586,188 @@ where
 		}
 	}

+	fn verify_interactive_tx_signatures(


Is test coverage for this actually feasible yet?

lightning/src/ln/channel.rs

The return type of commitment_signed() started groing into a really long and unwieldy tuple. We introduce a dedicated enum return type for it here.

…hecks In a following commit, We'll use the contained scriptPubKeys to validate P2WPKH and P2TR key path spends and to assist in checking that signatures in provided holder witnesses use SIGHASH_ALL to prevent funds being frozen or held ransom.

LDK checks the following: * Each input spends an output that is one of P2WPKH, P2WSH, or P2TR. These were already checked by LDK when the inputs to be contributed were provided. * All signatures use the `SIGHASH_ALL` sighash type. * P2WPKH and P2TR key path spends are valid (verifies signatures) NOTE: * When checking P2WSH spends, LDK tries to decode 70-72 byte witness elements as ECDSA signatures with a sighash flag. If the internal DER-decoding fails, then LDK just assumes it wasn't a signature and carries with checks. If the element can be decoded as an ECDSA signature, the the sighash flag must be `SIGHASH_ALL`. * When checking P2TR script-path spends, LDK assumes all elements of exactly 65 bytes with the last byte matching any valid sighash flag byte are schnorr signatures and checks that the sighash type is `SIGHASH_ALL`. If the last byte is not any valid sighash flag, the element is assumed not to be a signature and is ignored. Elements of 64 bytes are not checked because if they were schnorr signatures then they would implicitly be `SIGHASH_DEFAULT` which is an alias of `SIGHASH_ALL`.

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

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dunxen added 5 commits July 24, 2025 09:55

Generate FundingTransactionReadyForSigning on reestablish

d64112c

Create CommitmentSignedResult enum for Channel::commitment_signed();

c638579

The return type of commitment_signed() started groing into a really long and unwieldy tuple. We introduce a dedicated enum return type for it here.

Use bool for monitor_pending_tx_signatures over Option<TxSignatures>

ca2d5e0

We directly get the holder `TxSignatures` when necessary.

dunxen force-pushed the 2025-06-readyforsigningevent branch from 50e7cf4 to ca2d5e0 Compare July 25, 2025 10:56

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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