Update handling of `BumpTransactionEvent::ChannelClose` for 0FC channels
What the developer wrote
Authored by Leo Nash
Update handling of `BumpTransactionEvent::ChannelClose` for 0FC channels
Set an empty witness to spend the P2A anchor, and set the version of the
anchor transaction to 3.
We still check whether the commitment transaction has enough fees to be
broadcast on its own, as these transactions may be non-zero fee due to
trimmed / rounded outputs.
What changed, and why it matters
This commit updates how Lightning Dev Kit handles fee-bumping anchor transactions for a newer type of channel (zero-fee commitment channels, or 0FC). It makes two key changes: it uses a much smaller empty witness for the new-style P2A anchor output, and it sets the anchor transaction version to 3 instead of 2. These are protocol-specific adjustments needed so the new channel type can close and fee-bump correctly. The commit does not describe itself as a security fix, and the changes appear to be compatibility/correctness updates rather than a patch for an active vulnerability.
Treat as a protocol-compatibility and correctness update rather than an urgent security patch. Reviewers should verify that the empty-witness P2A spend and version-3 transaction format align with the latest Lightning spec for zero-fee commitment channels, and that the conditional logic correctly distinguishes 0FC from legacy anchor channels to avoid invalid transactions or failed force-closes.
Security signals we found
Protocol-specific transaction version change (v2 -> v3) for 0FC channels
Witness weight reduction for P2A anchor inputs
Conditional skipping of holder-keyed anchor signature for 0FC channels
Fee-sufficiency check retained for commitment transactions
Evidence from the diff
The patch modifies BumpTransactionEvent::ChannelClose handling in lightning/src/events/bump_transaction/mod.rs and adds a constant in lightning/src/ln/chan_utils.rs. For channels that support anchor zero-fee commitments (supports_anchor_zero_fee_commitments), it uses P2A_ANCHOR_INPUT_WITNESS_WEIGHT (1 weight unit) instead of the legacy ANCHOR_INPUT_WITNESS_WEIGHT (~114/115), and sets the anchor transaction version to a non-standard 3 instead of Version::TWO. For 0FC channels, it skips producing a signature witness for the P2A anchor, leaving the witness empty. The code still checks whether the commitment transaction has enough fees to be broadcast alone, because trimmed/rounded outputs can make these transactions non-zero fee.
Changed components
lightning/src/events/bump_transaction/mod.rslightning/src/ln/chan_utils.rsBumpTransactionEvent::ChannelClose handlingP2A anchor spending path0FC (zero-fee commitment) channel close flowInspect captured patch +39 / −16
diff --git a/lightning/src/events/bump_transaction/mod.rs b/lightning/src/events/bump_transaction/mod.rs
index c5b7885..f600397 100644
--- a/lightning/src/events/bump_transaction/mod.rs
+++ b/lightning/src/events/bump_transaction/mod.rs
@@ -25,6 +25,7 @@ use crate::ln::chan_utils;
use crate::ln::chan_utils::{
shared_anchor_script_pubkey, HTLCOutputInCommitment, ANCHOR_INPUT_WITNESS_WEIGHT,
HTLC_SUCCESS_INPUT_ANCHOR_WITNESS_WEIGHT, HTLC_TIMEOUT_INPUT_ANCHOR_WITNESS_WEIGHT,
+ P2A_ANCHOR_INPUT_WITNESS_WEIGHT,
};
use crate::ln::types::ChannelId;
use crate::prelude::*;
@@ -669,6 +670,16 @@ where
commitment_tx: &Transaction, commitment_tx_fee_sat: u64,
anchor_descriptor: &AnchorDescriptor,
) -> Result<(), ()> {
+ let channel_type = &anchor_descriptor
+ .channel_derivation_parameters
+ .transaction_parameters
+ .channel_type_features;
+ let anchor_input_witness_weight = if channel_type.supports_anchor_zero_fee_commitments() {
+ P2A_ANCHOR_INPUT_WITNESS_WEIGHT
+ } else {
+ ANCHOR_INPUT_WITNESS_WEIGHT
+ };
+
// First, check if the commitment transaction has sufficient fees on its own.
let commitment_tx_feerate_sat_per_1000_weight = compute_feerate_sat_per_1000_weight(
commitment_tx_fee_sat,
@@ -689,7 +700,7 @@ where
let commitment_tx_fee_sat = Amount::from_sat(commitment_tx_fee_sat);
anchor_utxo.value += commitment_tx_fee_sat;
let starting_package_and_fixed_input_satisfaction_weight =
- commitment_tx.weight().to_wu() + ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT;
+ commitment_tx.weight().to_wu() + anchor_input_witness_weight + EMPTY_SCRIPT_SIG_WEIGHT;
let mut package_and_fixed_input_satisfaction_weight =
starting_package_and_fixed_input_satisfaction_weight;
@@ -714,8 +725,14 @@ where
)
.await?;
+ let version = if channel_type.supports_anchor_zero_fee_commitments() {
+ Version::non_standard(3)
+ } else {
+ Version::TWO
+ };
+
let mut anchor_tx = Transaction {
- version: Version::TWO,
+ version,
lock_time: LockTime::ZERO, // TODO: Use next best height.
input: vec![anchor_descriptor.unsigned_tx_input()],
output: vec![],
@@ -724,7 +741,7 @@ where
let input_satisfaction_weight: u64 =
coin_selection.confirmed_utxos.iter().map(|utxo| utxo.satisfaction_weight).sum();
let total_satisfaction_weight =
- ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT + input_satisfaction_weight;
+ anchor_input_witness_weight + EMPTY_SCRIPT_SIG_WEIGHT + input_satisfaction_weight;
let total_input_amount = must_spend_amount
+ coin_selection.confirmed_utxos.iter().map(|utxo| utxo.output.value).sum();
@@ -780,18 +797,21 @@ where
log_debug!(self.logger, "Signing anchor transaction {}", anchor_txid);
anchor_tx = self.utxo_source.sign_psbt(anchor_psbt).await?;
- let signer = self
- .signer_provider
- .derive_channel_signer(anchor_descriptor.channel_derivation_parameters.keys_id);
- let channel_parameters =
- &anchor_descriptor.channel_derivation_parameters.transaction_parameters;
- let anchor_sig = signer.sign_holder_keyed_anchor_input(
- channel_parameters,
- &anchor_tx,
- 0,
- &self.secp,
- )?;
- anchor_tx.input[0].witness = anchor_descriptor.tx_input_witness(&anchor_sig);
+ // No need to produce any witness to spend P2A anchors
+ if channel_type.supports_anchors_zero_fee_htlc_tx() {
+ let signer = self
+ .signer_provider
+ .derive_channel_signer(anchor_descriptor.channel_derivation_parameters.keys_id);
+ let channel_parameters =
+ &anchor_descriptor.channel_derivation_parameters.transaction_parameters;
+ let anchor_sig = signer.sign_holder_keyed_anchor_input(
+ channel_parameters,
+ &anchor_tx,
+ 0,
+ &self.secp,
+ )?;
+ anchor_tx.input[0].witness = anchor_descriptor.tx_input_witness(&anchor_sig);
+ }
#[cfg(debug_assertions)]
{
diff --git a/lightning/src/ln/chan_utils.rs b/lightning/src/ln/chan_utils.rs
index f7aaf39..0a5e372 100644
--- a/lightning/src/ln/chan_utils.rs
+++ b/lightning/src/ln/chan_utils.rs
@@ -89,7 +89,10 @@ pub const ANCHOR_INPUT_WITNESS_WEIGHT: u64 = 114;
#[cfg(not(feature = "grind_signatures"))]
pub const ANCHOR_INPUT_WITNESS_WEIGHT: u64 = 115;
-/// The maximum value of the P2A anchor
+/// The weight of a P2A anchor witness.
+pub const P2A_ANCHOR_INPUT_WITNESS_WEIGHT: u64 = 1;
+
+/// The maximum value of a P2A anchor.
pub const P2A_MAX_VALUE: u64 = 240;
/// The upper bound weight of an HTLC timeout input from a commitment transaction with anchor
Why this scored 42/100
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