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Make ChannelSigner solely responsible for validating commitment sigs #3878

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b3773a7
Make `ChannelSigner` solely responsible for validating commitment sigs
tankyleo Jun 21, 2025
4005d59
Remove format skips in channel holder commitment validation functions
tankyleo Jun 21, 2025
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295 changes: 165 additions & 130 deletions lightning/src/ln/channel.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -21,10 +21,9 @@ use bitcoin::hashes::sha256::Hash as Sha256;
use bitcoin::hashes::sha256d::Hash as Sha256d;
use bitcoin::hashes::Hash;

use bitcoin::secp256k1::constants::PUBLIC_KEY_SIZE;
use bitcoin::secp256k1::{ecdsa::Signature, Secp256k1};
use bitcoin::secp256k1::{PublicKey, SecretKey};
use bitcoin::{secp256k1, sighash};
use bitcoin::secp256k1::{
self, constants::PUBLIC_KEY_SIZE, ecdsa::Signature, PublicKey, Secp256k1, SecretKey,
};

use crate::chain::chaininterface::{
fee_for_weight, ConfirmationTarget, FeeEstimator, LowerBoundedFeeEstimator,
Expand Down Expand Up @@ -2512,75 +2511,80 @@ where

fn received_msg(&self) -> &'static str;

#[rustfmt::skip]
fn check_counterparty_commitment_signature<L: Deref>(
&self, sig: &Signature, holder_commitment_point: &HolderCommitmentPoint, logger: &L
) -> Result<CommitmentTransaction, ChannelError> where L::Target: Logger {
let funding_script = self.funding().get_funding_redeemscript();

let commitment_data = self.context().build_commitment_transaction(self.funding(),
holder_commitment_point.transaction_number(), &holder_commitment_point.current_point(),
true, false, logger);
let initial_commitment_tx = commitment_data.tx;
let trusted_tx = initial_commitment_tx.trust();
let initial_commitment_bitcoin_tx = trusted_tx.built_transaction();
let sighash = initial_commitment_bitcoin_tx.get_sighash_all(&funding_script, self.funding().get_value_satoshis());
// They sign the holder commitment transaction...
log_trace!(logger, "Checking {} tx signature {} by key {} against tx {} (sighash {}) with redeemscript {} for channel {}.",
self.received_msg(), log_bytes!(sig.serialize_compact()[..]), log_bytes!(self.funding().counterparty_funding_pubkey().serialize()),
encode::serialize_hex(&initial_commitment_bitcoin_tx.transaction), log_bytes!(sighash[..]),
encode::serialize_hex(&funding_script), &self.context().channel_id());
secp_check!(self.context().secp_ctx.verify_ecdsa(&sighash, sig, self.funding().counterparty_funding_pubkey()), format!("Invalid {} signature from peer", self.received_msg()));

Ok(initial_commitment_tx)
}

#[rustfmt::skip]
fn initial_commitment_signed<L: Deref>(
&mut self, channel_id: ChannelId, counterparty_signature: Signature, holder_commitment_point: &mut HolderCommitmentPoint,
best_block: BestBlock, signer_provider: &SP, logger: &L,
) -> Result<(ChannelMonitor<<SP::Target as SignerProvider>::EcdsaSigner>, CommitmentTransaction), ChannelError>
&mut self, channel_id: ChannelId, counterparty_signature: Signature,
holder_commitment_point: &mut HolderCommitmentPoint, best_block: BestBlock,
signer_provider: &SP, logger: &L,
) -> Result<
(ChannelMonitor<<SP::Target as SignerProvider>::EcdsaSigner>, CommitmentTransaction),
ChannelError,
>
where
L::Target: Logger
L::Target: Logger,
{
let initial_commitment_tx = match self.check_counterparty_commitment_signature(&counterparty_signature, holder_commitment_point, logger) {
Ok(res) => res,
Err(ChannelError::Close(e)) => {
// TODO(dual_funding): Update for V2 established channels.
if !self.funding().is_outbound() {
self.funding_mut().channel_transaction_parameters.funding_outpoint = None;
}
return Err(ChannelError::Close(e));
},
Err(e) => {
// The only error we know how to handle is ChannelError::Close, so we fall over here
// to make sure we don't continue with an inconsistent state.
panic!("unexpected error type from check_counterparty_commitment_signature {:?}", e);
}
};
let context = self.context();
let commitment_data = context.build_commitment_transaction(self.funding(),
context.cur_counterparty_commitment_transaction_number,
&context.counterparty_cur_commitment_point.unwrap(), false, false, logger);
let counterparty_initial_commitment_tx = commitment_data.tx;
let counterparty_trusted_tx = counterparty_initial_commitment_tx.trust();
let counterparty_initial_bitcoin_tx = counterparty_trusted_tx.built_transaction();

log_trace!(logger, "Initial counterparty tx for channel {} is: txid {} tx {}",
&context.channel_id(), counterparty_initial_bitcoin_tx.txid, encode::serialize_hex(&counterparty_initial_bitcoin_tx.transaction));

let initial_commitment_tx = context
.build_commitment_transaction(
self.funding(),
holder_commitment_point.transaction_number(),
&holder_commitment_point.current_point(),
true,
false,
logger,
)
.tx;
let holder_commitment_tx = HolderCommitmentTransaction::new(
initial_commitment_tx,
counterparty_signature,
Vec::new(),
&self.funding().get_holder_pubkeys().funding_pubkey,
&self.funding().counterparty_funding_pubkey()
&self.funding().counterparty_funding_pubkey(),
);

if context.holder_signer.as_ref().validate_holder_commitment(&holder_commitment_tx, Vec::new()).is_err() {
log_trace!(
logger,
"Validating {} commitment in channel {}",
self.received_msg(),
context.channel_id()
);
if context
.holder_signer
.as_ref()
.validate_holder_commitment(
&self.funding().channel_transaction_parameters,
&holder_commitment_tx,
Vec::new(),
&context.secp_ctx,
)
.is_err()
{
if !self.funding().is_outbound() {
self.funding_mut().channel_transaction_parameters.funding_outpoint = None;
}
return Err(ChannelError::close("Failed to validate our commitment".to_owned()));
}

let commitment_data = context.build_commitment_transaction(
self.funding(),
context.cur_counterparty_commitment_transaction_number,
&context.counterparty_cur_commitment_point.unwrap(),
false,
false,
logger,
);
let counterparty_initial_commitment_tx = commitment_data.tx;
let counterparty_trusted_tx = counterparty_initial_commitment_tx.trust();
let counterparty_initial_bitcoin_tx = counterparty_trusted_tx.built_transaction();

log_trace!(
logger,
"Initial counterparty tx for channel {} is: txid {} tx {}",
&context.channel_id(),
counterparty_initial_bitcoin_tx.txid,
encode::serialize_hex(&counterparty_initial_bitcoin_tx.transaction)
);

// Now that we're past error-generating stuff, update our local state:

let is_v2_established = self.is_v2_established();
Expand All @@ -2590,35 +2594,61 @@ where
assert!(!context.channel_state.is_monitor_update_in_progress()); // We have not had any monitor(s) yet to fail update!
if !is_v2_established {
if context.is_batch_funding() {
context.channel_state = ChannelState::AwaitingChannelReady(AwaitingChannelReadyFlags::WAITING_FOR_BATCH);
context.channel_state = ChannelState::AwaitingChannelReady(
AwaitingChannelReadyFlags::WAITING_FOR_BATCH,
);
} else {
context.channel_state = ChannelState::AwaitingChannelReady(AwaitingChannelReadyFlags::new());
context.channel_state =
ChannelState::AwaitingChannelReady(AwaitingChannelReadyFlags::new());
}
}
if holder_commitment_point.advance(&context.holder_signer, &context.secp_ctx, logger).is_err() {
if holder_commitment_point
.advance(&context.holder_signer, &context.secp_ctx, logger)
.is_err()
{
// We only fail to advance our commitment point/number if we're currently
// waiting for our signer to unblock and provide a commitment point.
// We cannot send accept_channel/open_channel before this has occurred, so if we
// err here by the time we receive funding_created/funding_signed, something has gone wrong.
debug_assert!(false, "We should be ready to advance our commitment point by the time we receive {}", self.received_msg());
return Err(ChannelError::close("Failed to advance holder commitment point".to_owned()));
debug_assert!(
false,
"We should be ready to advance our commitment point by the time we receive {}",
self.received_msg()
);
return Err(ChannelError::close(
"Failed to advance holder commitment point".to_owned(),
));
}

let context = self.context();
let funding = self.funding();
let obscure_factor = get_commitment_transaction_number_obscure_factor(&funding.get_holder_pubkeys().payment_point, &funding.get_counterparty_pubkeys().payment_point, funding.is_outbound());
let shutdown_script = context.shutdown_scriptpubkey.clone().map(|script| script.into_inner());
let obscure_factor = get_commitment_transaction_number_obscure_factor(
&funding.get_holder_pubkeys().payment_point,
&funding.get_counterparty_pubkeys().payment_point,
funding.is_outbound(),
);
let shutdown_script =
context.shutdown_scriptpubkey.clone().map(|script| script.into_inner());
let monitor_signer = signer_provider.derive_channel_signer(context.channel_keys_id);
// TODO(RBF): When implementing RBF, the funding_txo passed here must only update
// ChannelMonitorImp::first_confirmed_funding_txo during channel establishment, not splicing
let channel_monitor = ChannelMonitor::new(
context.secp_ctx.clone(), monitor_signer, shutdown_script,
funding.get_holder_selected_contest_delay(), &context.destination_script,
&funding.channel_transaction_parameters, funding.is_outbound(), obscure_factor,
holder_commitment_tx, best_block, context.counterparty_node_id, context.channel_id(),
context.secp_ctx.clone(),
monitor_signer,
shutdown_script,
funding.get_holder_selected_contest_delay(),
&context.destination_script,
&funding.channel_transaction_parameters,
funding.is_outbound(),
obscure_factor,
holder_commitment_tx,
best_block,
context.counterparty_node_id,
context.channel_id(),
);
channel_monitor.provide_initial_counterparty_commitment_tx(
counterparty_initial_commitment_tx.clone(), logger,
counterparty_initial_commitment_tx.clone(),
logger,
);

self.context_mut().cur_counterparty_commitment_transaction_number -= 1;
Expand Down Expand Up @@ -4199,86 +4229,102 @@ where
Ok(())
}

#[rustfmt::skip]
fn validate_commitment_signed<L: Deref>(
&self, funding: &FundingScope, holder_commitment_point: &HolderCommitmentPoint,
msg: &msgs::CommitmentSigned, logger: &L,
) -> Result<LatestHolderCommitmentTXInfo, ChannelError>
where
L::Target: Logger,
{
let funding_script = funding.get_funding_redeemscript();
let commitment_data = self.build_commitment_transaction(
funding,
holder_commitment_point.transaction_number(),
&holder_commitment_point.current_point(),
true,
false,
logger,
);

let commitment_data = self.build_commitment_transaction(funding,
holder_commitment_point.transaction_number(), &holder_commitment_point.current_point(),
true, false, logger);
let commitment_txid = {
let trusted_tx = commitment_data.tx.trust();
let bitcoin_tx = trusted_tx.built_transaction();
let sighash = bitcoin_tx.get_sighash_all(&funding_script, funding.get_value_satoshis());

log_trace!(logger, "Checking commitment tx signature {} by key {} against tx {} (sighash {}) with redeemscript {} in channel {}",
log_bytes!(msg.signature.serialize_compact()[..]),
log_bytes!(funding.counterparty_funding_pubkey().serialize()), encode::serialize_hex(&bitcoin_tx.transaction),
log_bytes!(sighash[..]), encode::serialize_hex(&funding_script), &self.channel_id());
if let Err(_) = self.secp_ctx.verify_ecdsa(&sighash, &msg.signature, &funding.counterparty_funding_pubkey()) {
return Err(ChannelError::close("Invalid commitment tx signature from peer".to_owned()));
}
bitcoin_tx.txid
};
if msg.htlc_signatures.len() != commitment_data.tx.nondust_htlcs().len() {
return Err(ChannelError::close(format!(
"Got wrong number of HTLC signatures ({}) from remote. It must be {}",
msg.htlc_signatures.len(),
commitment_data.tx.nondust_htlcs().len()
)));
}

let holder_commitment_tx = HolderCommitmentTransaction::new(
commitment_data.tx,
msg.signature,
msg.htlc_signatures.clone(),
&funding.get_holder_pubkeys().funding_pubkey,
funding.counterparty_funding_pubkey(),
);

log_trace!(
logger,
"Validating commitment_signed commitment in channel {}",
self.channel_id()
);
self.holder_signer
.as_ref()
.validate_holder_commitment(
&funding.channel_transaction_parameters,
&holder_commitment_tx,
commitment_data.outbound_htlc_preimages,
&self.secp_ctx,
)
.map_err(|_| ChannelError::close("Failed to validate our commitment".to_owned()))?;

// If our counterparty updated the channel fee in this commitment transaction, check that
// they can actually afford the new fee now.
let update_fee = if let Some((_, update_state)) = self.pending_update_fee {
update_state == FeeUpdateState::RemoteAnnounced
} else { false };
} else {
false
};
if update_fee {
debug_assert!(!funding.is_outbound());
let counterparty_reserve_we_require_msat = funding.holder_selected_channel_reserve_satoshis * 1000;
if commitment_data.stats.remote_balance_before_fee_anchors_msat < commitment_data.stats.total_fee_sat * 1000 + commitment_data.stats.total_anchors_sat * 1000 + counterparty_reserve_we_require_msat {
return Err(ChannelError::close("Funding remote cannot afford proposed new fee".to_owned()));
let counterparty_reserve_we_require_msat =
funding.holder_selected_channel_reserve_satoshis * 1000;
if commitment_data.stats.remote_balance_before_fee_anchors_msat
< commitment_data.stats.total_fee_sat * 1000
+ commitment_data.stats.total_anchors_sat * 1000
+ counterparty_reserve_we_require_msat
{
return Err(ChannelError::close(
"Funding remote cannot afford proposed new fee".to_owned(),
));
}
}
#[cfg(any(test, fuzzing))]
{
if funding.is_outbound() {
let projected_commit_tx_info = funding.next_local_commitment_tx_fee_info_cached.lock().unwrap().take();
let projected_commit_tx_info =
funding.next_local_commitment_tx_fee_info_cached.lock().unwrap().take();
*funding.next_remote_commitment_tx_fee_info_cached.lock().unwrap() = None;
if let Some(info) = projected_commit_tx_info {
let total_pending_htlcs = self.pending_inbound_htlcs.len() + self.pending_outbound_htlcs.len()
let total_pending_htlcs = self.pending_inbound_htlcs.len()
+ self.pending_outbound_htlcs.len()
+ self.holding_cell_htlc_updates.len();
if info.total_pending_htlcs == total_pending_htlcs
&& info.next_holder_htlc_id == self.next_holder_htlc_id
&& info.next_counterparty_htlc_id == self.next_counterparty_htlc_id
&& info.feerate == self.feerate_per_kw {
assert_eq!(commitment_data.stats.total_fee_sat, info.fee / 1000);
}
&& info.feerate == self.feerate_per_kw
{
assert_eq!(commitment_data.stats.total_fee_sat, info.fee / 1000);
}
}
}
}

if msg.htlc_signatures.len() != commitment_data.tx.nondust_htlcs().len() {
return Err(ChannelError::close(format!("Got wrong number of HTLC signatures ({}) from remote. It must be {}", msg.htlc_signatures.len(), commitment_data.tx.nondust_htlcs().len())));
}

let holder_keys = commitment_data.tx.trust().keys();
let mut nondust_htlc_sources = Vec::with_capacity(commitment_data.tx.nondust_htlcs().len());
let mut dust_htlcs = Vec::with_capacity(commitment_data.htlcs_included.len() - commitment_data.tx.nondust_htlcs().len());
for (idx, (htlc, mut source_opt)) in commitment_data.htlcs_included.into_iter().enumerate() {
let mut nondust_htlc_sources =
Vec::with_capacity(holder_commitment_tx.nondust_htlcs().len());
let mut dust_htlcs = Vec::with_capacity(
commitment_data.htlcs_included.len() - holder_commitment_tx.nondust_htlcs().len(),
);
for (htlc, mut source_opt) in commitment_data.htlcs_included.into_iter() {
if let Some(_) = htlc.transaction_output_index {
let htlc_tx = chan_utils::build_htlc_transaction(&commitment_txid, commitment_data.tx.feerate_per_kw(),
funding.get_counterparty_selected_contest_delay().unwrap(), &htlc, funding.get_channel_type(),
&holder_keys.broadcaster_delayed_payment_key, &holder_keys.revocation_key);

let htlc_redeemscript = chan_utils::get_htlc_redeemscript(&htlc, funding.get_channel_type(), &holder_keys);
let htlc_sighashtype = if funding.get_channel_type().supports_anchors_zero_fee_htlc_tx() { EcdsaSighashType::SinglePlusAnyoneCanPay } else { EcdsaSighashType::All };
let htlc_sighash = hash_to_message!(&sighash::SighashCache::new(&htlc_tx).p2wsh_signature_hash(0, &htlc_redeemscript, htlc.to_bitcoin_amount(), htlc_sighashtype).unwrap()[..]);
log_trace!(logger, "Checking HTLC tx signature {} by key {} against tx {} (sighash {}) with redeemscript {} in channel {}.",
log_bytes!(msg.htlc_signatures[idx].serialize_compact()[..]), log_bytes!(holder_keys.countersignatory_htlc_key.to_public_key().serialize()),
encode::serialize_hex(&htlc_tx), log_bytes!(htlc_sighash[..]), encode::serialize_hex(&htlc_redeemscript), &self.channel_id());
if let Err(_) = self.secp_ctx.verify_ecdsa(&htlc_sighash, &msg.htlc_signatures[idx], &holder_keys.countersignatory_htlc_key.to_public_key()) {
return Err(ChannelError::close("Invalid HTLC tx signature from peer".to_owned()));
}
if htlc.offered {
if let Some(source) = source_opt.take() {
nondust_htlc_sources.push(source.clone());
Expand All @@ -4292,17 +4338,6 @@ where
debug_assert!(source_opt.is_none(), "HTLCSource should have been put somewhere");
}

let holder_commitment_tx = HolderCommitmentTransaction::new(
commitment_data.tx,
msg.signature,
msg.htlc_signatures.clone(),
&funding.get_holder_pubkeys().funding_pubkey,
funding.counterparty_funding_pubkey()
);

self.holder_signer.as_ref().validate_holder_commitment(&holder_commitment_tx, commitment_data.outbound_htlc_preimages)
.map_err(|_| ChannelError::close("Failed to validate our commitment".to_owned()))?;

Ok(LatestHolderCommitmentTXInfo {
commitment_tx: holder_commitment_tx,
htlc_outputs: dust_htlcs,
Expand Down
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