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Merge pull request lightningdevkit#3633 from jkczyz/2025-02-split-com…
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…mitment-signed

Split `commitment_signed` handling by check-accept
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@TheBlueMatt
TheBlueMatt authored Mar 6, 2025
2 parents 43de15e + 2be03ce commit c4d0560
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Showing 2 changed files with 154 additions and 124 deletions.
1 change: 1 addition & 0 deletions lightning/src/chain/channelmonitor.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -535,6 +535,7 @@ impl_writeable_tlv_based_enum_upgradable!(OnchainEvent,

#[derive(Clone, Debug, PartialEq, Eq)]
pub(crate) enum ChannelMonitorUpdateStep {
// Update LatestHolderCommitmentTXInfo in channel.rs if adding new fields to this variant.
LatestHolderCommitmentTXInfo {
commitment_tx: HolderCommitmentTransaction,
/// Note that LDK after 0.0.115 supports this only containing dust HTLCs (implying the
Expand Down
277 changes: 153 additions & 124 deletions lightning/src/ln/channel.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2032,7 +2032,7 @@ trait InitialRemoteCommitmentReceiver<SP: Deref> where SP::Target: SignerProvide
fn received_msg(&self) -> &'static str;

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

Expand Down Expand Up @@ -3457,6 +3457,132 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
!matches!(self.channel_state, ChannelState::AwaitingChannelReady(flags) if flags.is_set(AwaitingChannelReadyFlags::WAITING_FOR_BATCH))
}

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 keys = self.build_holder_transaction_keys(funding, holder_commitment_point.current_point());

let commitment_stats = self.build_commitment_transaction(funding, holder_commitment_point.transaction_number(), &keys, true, false, logger);
let commitment_txid = {
let trusted_tx = commitment_stats.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
};
let mut htlcs_cloned: Vec<_> = commitment_stats.htlcs_included.iter().map(|htlc| (htlc.0.clone(), htlc.1.map(|h| h.clone()))).collect();

// 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 };
if update_fee {
debug_assert!(!funding.is_outbound());
let counterparty_reserve_we_require_msat = funding.holder_selected_channel_reserve_satoshis * 1000;
if commitment_stats.remote_balance_msat < commitment_stats.total_fee_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();
*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()
+ 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_stats.total_fee_sat, info.fee / 1000);
}
}
}
}

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

// Up to LDK 0.0.115, HTLC information was required to be duplicated in the
// `htlcs_and_sigs` vec and in the `holder_commitment_tx` itself, both of which were passed
// in the `ChannelMonitorUpdate`. In 0.0.115, support for having a separate set of
// outbound-non-dust-HTLCSources in the `ChannelMonitorUpdate` was added, however for
// backwards compatibility, we never use it in production. To provide test coverage, here,
// we randomly decide (in test/fuzzing builds) to use the new vec sometimes.
#[allow(unused_assignments, unused_mut)]
let mut separate_nondust_htlc_sources = false;
#[cfg(all(feature = "std", any(test, fuzzing)))] {
use core::hash::{BuildHasher, Hasher};
// Get a random value using the only std API to do so - the DefaultHasher
let rand_val = std::collections::hash_map::RandomState::new().build_hasher().finish();
separate_nondust_htlc_sources = rand_val % 2 == 0;
}

let mut nondust_htlc_sources = Vec::with_capacity(htlcs_cloned.len());
let mut htlcs_and_sigs = Vec::with_capacity(htlcs_cloned.len());
for (idx, (htlc, mut source_opt)) in htlcs_cloned.drain(..).enumerate() {
if let Some(_) = htlc.transaction_output_index {
let htlc_tx = chan_utils::build_htlc_transaction(&commitment_txid, commitment_stats.feerate_per_kw,
funding.get_counterparty_selected_contest_delay().unwrap(), &htlc, &self.channel_type,
&keys.broadcaster_delayed_payment_key, &keys.revocation_key);

let htlc_redeemscript = chan_utils::get_htlc_redeemscript(&htlc, &self.channel_type, &keys);
let htlc_sighashtype = if self.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!(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], &keys.countersignatory_htlc_key.to_public_key()) {
return Err(ChannelError::close("Invalid HTLC tx signature from peer".to_owned()));
}
if !separate_nondust_htlc_sources {
htlcs_and_sigs.push((htlc, Some(msg.htlc_signatures[idx]), source_opt.take()));
}
} else {
htlcs_and_sigs.push((htlc, None, source_opt.take()));
}
if separate_nondust_htlc_sources {
if let Some(source) = source_opt.take() {
nondust_htlc_sources.push(source);
}
}
debug_assert!(source_opt.is_none(), "HTLCSource should have been put somewhere");
}

let holder_commitment_tx = HolderCommitmentTransaction::new(
commitment_stats.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_stats.outbound_htlc_preimages)
.map_err(|_| ChannelError::close("Failed to validate our commitment".to_owned()))?;

Ok(LatestHolderCommitmentTXInfo {
commitment_tx: holder_commitment_tx,
htlc_outputs: htlcs_and_sigs,
nondust_htlc_sources,
})
}

/// Transaction nomenclature is somewhat confusing here as there are many different cases - a
/// transaction is referred to as "a's transaction" implying that a will be able to broadcast
/// the transaction. Thus, b will generally be sending a signature over such a transaction to
Expand Down Expand Up @@ -4711,6 +4837,14 @@ struct CommitmentTxInfoCached {
feerate: u32,
}

/// Partial data from ChannelMonitorUpdateStep::LatestHolderCommitmentTXInfo used to simplify the
/// return type of `ChannelContext::validate_commitment_signed`.
struct LatestHolderCommitmentTXInfo {
pub commitment_tx: HolderCommitmentTransaction,
pub htlc_outputs: Vec<(HTLCOutputInCommitment, Option<Signature>, Option<HTLCSource>)>,
pub nondust_htlc_sources: Vec<HTLCSource>,
}

/// Contents of a wire message that fails an HTLC backwards. Useful for [`FundedChannel::fail_htlc`] to
/// fail with either [`msgs::UpdateFailMalformedHTLC`] or [`msgs::UpdateFailHTLC`] as needed.
trait FailHTLCContents {
Expand Down Expand Up @@ -5515,118 +5649,22 @@ impl<SP: Deref> FundedChannel<SP> where
return Err(ChannelError::close("Peer sent commitment_signed after we'd started exchanging closing_signeds".to_owned()));
}

let funding_script = self.funding.get_funding_redeemscript();

let keys = self.context.build_holder_transaction_keys(&self.funding, self.holder_commitment_point.current_point());

let commitment_stats = self.context.build_commitment_transaction(&self.funding, self.holder_commitment_point.transaction_number(), &keys, true, false, logger);
let commitment_txid = {
let trusted_tx = commitment_stats.tx.trust();
let bitcoin_tx = trusted_tx.built_transaction();
let sighash = bitcoin_tx.get_sighash_all(&funding_script, self.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!(self.funding.counterparty_funding_pubkey().serialize()), encode::serialize_hex(&bitcoin_tx.transaction),
log_bytes!(sighash[..]), encode::serialize_hex(&funding_script), &self.context.channel_id());
if let Err(_) = self.context.secp_ctx.verify_ecdsa(&sighash, &msg.signature, &self.funding.counterparty_funding_pubkey()) {
return Err(ChannelError::close("Invalid commitment tx signature from peer".to_owned()));
}
bitcoin_tx.txid
};
let mut htlcs_cloned: Vec<_> = commitment_stats.htlcs_included.iter().map(|htlc| (htlc.0.clone(), htlc.1.map(|h| h.clone()))).collect();

// 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.context.pending_update_fee {
update_state == FeeUpdateState::RemoteAnnounced
} else { false };
if update_fee {
debug_assert!(!self.funding.is_outbound());
let counterparty_reserve_we_require_msat = self.funding.holder_selected_channel_reserve_satoshis * 1000;
if commitment_stats.remote_balance_msat < commitment_stats.total_fee_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 self.funding.is_outbound() {
let projected_commit_tx_info = self.funding.next_local_commitment_tx_fee_info_cached.lock().unwrap().take();
*self.funding.next_remote_commitment_tx_fee_info_cached.lock().unwrap() = None;
if let Some(info) = projected_commit_tx_info {
let total_pending_htlcs = self.context.pending_inbound_htlcs.len() + self.context.pending_outbound_htlcs.len()
+ self.context.holding_cell_htlc_updates.len();
if info.total_pending_htlcs == total_pending_htlcs
&& info.next_holder_htlc_id == self.context.next_holder_htlc_id
&& info.next_counterparty_htlc_id == self.context.next_counterparty_htlc_id
&& info.feerate == self.context.feerate_per_kw {
assert_eq!(commitment_stats.total_fee_sat, info.fee / 1000);
}
}
}
}

if msg.htlc_signatures.len() != commitment_stats.num_nondust_htlcs {
return Err(ChannelError::close(format!("Got wrong number of HTLC signatures ({}) from remote. It must be {}", msg.htlc_signatures.len(), commitment_stats.num_nondust_htlcs)));
}
let commitment_tx_info = self.context.validate_commitment_signed(&self.funding, &self.holder_commitment_point, msg, logger)?;

// Up to LDK 0.0.115, HTLC information was required to be duplicated in the
// `htlcs_and_sigs` vec and in the `holder_commitment_tx` itself, both of which were passed
// in the `ChannelMonitorUpdate`. In 0.0.115, support for having a separate set of
// outbound-non-dust-HTLCSources in the `ChannelMonitorUpdate` was added, however for
// backwards compatibility, we never use it in production. To provide test coverage, here,
// we randomly decide (in test/fuzzing builds) to use the new vec sometimes.
#[allow(unused_assignments, unused_mut)]
let mut separate_nondust_htlc_sources = false;
#[cfg(all(feature = "std", any(test, fuzzing)))] {
use core::hash::{BuildHasher, Hasher};
// Get a random value using the only std API to do so - the DefaultHasher
let rand_val = std::collections::hash_map::RandomState::new().build_hasher().finish();
separate_nondust_htlc_sources = rand_val % 2 == 0;
}

let mut nondust_htlc_sources = Vec::with_capacity(htlcs_cloned.len());
let mut htlcs_and_sigs = Vec::with_capacity(htlcs_cloned.len());
for (idx, (htlc, mut source_opt)) in htlcs_cloned.drain(..).enumerate() {
if let Some(_) = htlc.transaction_output_index {
let htlc_tx = chan_utils::build_htlc_transaction(&commitment_txid, commitment_stats.feerate_per_kw,
self.funding.get_counterparty_selected_contest_delay().unwrap(), &htlc, &self.context.channel_type,
&keys.broadcaster_delayed_payment_key, &keys.revocation_key);

let htlc_redeemscript = chan_utils::get_htlc_redeemscript(&htlc, &self.context.channel_type, &keys);
let htlc_sighashtype = if self.context.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!(keys.countersignatory_htlc_key.to_public_key().serialize()),
encode::serialize_hex(&htlc_tx), log_bytes!(htlc_sighash[..]), encode::serialize_hex(&htlc_redeemscript), &self.context.channel_id());
if let Err(_) = self.context.secp_ctx.verify_ecdsa(&htlc_sighash, &msg.htlc_signatures[idx], &keys.countersignatory_htlc_key.to_public_key()) {
return Err(ChannelError::close("Invalid HTLC tx signature from peer".to_owned()));
}
if !separate_nondust_htlc_sources {
htlcs_and_sigs.push((htlc, Some(msg.htlc_signatures[idx]), source_opt.take()));
}
} else {
htlcs_and_sigs.push((htlc, None, source_opt.take()));
}
if separate_nondust_htlc_sources {
if let Some(source) = source_opt.take() {
nondust_htlc_sources.push(source);
}
}
debug_assert!(source_opt.is_none(), "HTLCSource should have been put somewhere");
if self.holder_commitment_point.advance(&self.context.holder_signer, &self.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.
// During post-funding channel operation, we only advance our point upon
// receiving a commitment_signed, and our counterparty cannot send us
// another commitment signed until we've provided a new commitment point
// in revoke_and_ack, which requires unblocking our signer and completing
// the advance to the next point. This should be unreachable since
// a new commitment_signed should fail at our signature checks in
// validate_commitment_signed.
debug_assert!(false, "We should be ready to advance our commitment point by the time we receive commitment_signed");
return Err(ChannelError::close("Failed to advance our commitment point".to_owned()));
}

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

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

// Update state now that we've passed all the can-fail calls...
let mut need_commitment = false;
if let &mut Some((_, ref mut update_state)) = &mut self.context.pending_update_fee {
Expand Down Expand Up @@ -5666,31 +5704,22 @@ impl<SP: Deref> FundedChannel<SP> where
}
}

let LatestHolderCommitmentTXInfo {
commitment_tx, htlc_outputs, nondust_htlc_sources,
} = commitment_tx_info;
self.context.latest_monitor_update_id += 1;
let mut monitor_update = ChannelMonitorUpdate {
update_id: self.context.latest_monitor_update_id,
counterparty_node_id: Some(self.context.counterparty_node_id),
updates: vec![ChannelMonitorUpdateStep::LatestHolderCommitmentTXInfo {
commitment_tx: holder_commitment_tx,
htlc_outputs: htlcs_and_sigs,
commitment_tx,
htlc_outputs,
claimed_htlcs,
nondust_htlc_sources,
}],
channel_id: Some(self.context.channel_id()),
};

if self.holder_commitment_point.advance(&self.context.holder_signer, &self.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.
// During post-funding channel operation, we only advance our point upon
// receiving a commitment_signed, and our counterparty cannot send us
// another commitment signed until we've provided a new commitment point
// in revoke_and_ack, which requires unblocking our signer and completing
// the advance to the next point. This should be unreachable since
// a new commitment_signed should fail at our signature checks above.
debug_assert!(false, "We should be ready to advance our commitment point by the time we receive commitment_signed");
return Err(ChannelError::close("Failed to advance our commitment point".to_owned()));
}
self.context.expecting_peer_commitment_signed = false;
// Note that if we need_commitment & !AwaitingRemoteRevoke we'll call
// build_commitment_no_status_check() next which will reset this to RAAFirst.
Expand Down

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