Provide funding key tweak when signing channel announcements

We'll need to sign a new channel announcement after a channel has a
splice locked, so provide the tweak necessary to generate a valid
signature.

Author: wpaulino

lightning/src/ln/channel.rs

@@ -8160,7 +8160,11 @@ impl<SP: Deref> FundedChannel<SP> where
 		};
 		match &self.context.holder_signer {
 			ChannelSignerType::Ecdsa(ecdsa) => {
-				let our_bitcoin_sig = match ecdsa.sign_channel_announcement_with_funding_key(&announcement, &self.context.secp_ctx) {
+				let funding_key_tweak =
+					self.funding.channel_transaction_parameters.holder_pubkeys.funding_key_tweak;
+				let our_bitcoin_sig = match ecdsa.sign_channel_announcement_with_funding_key(
+					&announcement, funding_key_tweak, &self.context.secp_ctx,
+				) {
 					Err(_) => {
 						log_error!(logger, "Signer rejected channel_announcement signing. Channel will not be announced!");
 						return None;
@@ -8201,8 +8205,11 @@ impl<SP: Deref> FundedChannel<SP> where
 				.map_err(|_| ChannelError::Ignore("Failed to generate node signature for channel_announcement".to_owned()))?;
 			match &self.context.holder_signer {
 				ChannelSignerType::Ecdsa(ecdsa) => {
-					let our_bitcoin_sig = ecdsa.sign_channel_announcement_with_funding_key(&announcement, &self.context.secp_ctx)
-						.map_err(|_| ChannelError::Ignore("Signer rejected channel_announcement".to_owned()))?;
+					let funding_key_tweak =
+						self.funding.channel_transaction_parameters.holder_pubkeys.funding_key_tweak;
+					let our_bitcoin_sig = ecdsa.sign_channel_announcement_with_funding_key(
+						&announcement, funding_key_tweak, &self.context.secp_ctx,
+					).map_err(|_| ChannelError::Ignore("Signer rejected channel_announcement".to_owned()))?;
 					Ok(msgs::ChannelAnnouncement {
 						node_signature_1: if were_node_one { our_node_sig } else { their_node_sig },
 						node_signature_2: if were_node_one { their_node_sig } else { our_node_sig },

lightning/src/sign/ecdsa.rs

@@ -4,7 +4,7 @@ use bitcoin::transaction::Transaction;
 
 use bitcoin::secp256k1;
 use bitcoin::secp256k1::ecdsa::Signature;
-use bitcoin::secp256k1::{PublicKey, Secp256k1, SecretKey};
+use bitcoin::secp256k1::{PublicKey, Scalar, Secp256k1, SecretKey};
 
 use crate::ln::chan_utils::{
 	ClosingTransaction, CommitmentTransaction, HTLCOutputInCommitment, HolderCommitmentTransaction,
@@ -227,7 +227,8 @@ pub trait EcdsaChannelSigner: ChannelSigner {
 		input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
 	) -> Result<Signature, ()>;
 	/// Signs a channel announcement message with our funding key proving it comes from one of the
-	/// channel participants.
+	/// channel participants. The `funding_key_tweak`, if set, must be added to the funding secret
+	/// key prior to signing in order to arrive at the key found in the 2-of-2 multisig.
 	///
 	/// Channel announcements also require a signature from each node's network key. Our node
 	/// signature is computed through [`NodeSigner::sign_gossip_message`].
@@ -238,7 +239,8 @@ pub trait EcdsaChannelSigner: ChannelSigner {
 	///
 	/// [`NodeSigner::sign_gossip_message`]: crate::sign::NodeSigner::sign_gossip_message
 	fn sign_channel_announcement_with_funding_key(
-		&self, msg: &UnsignedChannelAnnouncement, secp_ctx: &Secp256k1<secp256k1::All>,
+		&self, msg: &UnsignedChannelAnnouncement, funding_key_tweak: Option<Scalar>,
+		secp_ctx: &Secp256k1<secp256k1::All>,
 	) -> Result<Signature, ()>;
 
 	/// Signs the input of a splicing funding transaction with our funding key.

lightning/src/sign/mod.rs

@@ -1603,10 +1603,11 @@ impl EcdsaChannelSigner for InMemorySigner {
 	}
 
 	fn sign_channel_announcement_with_funding_key(
-		&self, msg: &UnsignedChannelAnnouncement, secp_ctx: &Secp256k1<secp256k1::All>,
+		&self, msg: &UnsignedChannelAnnouncement, funding_key_tweak: Option<Scalar>,
+		secp_ctx: &Secp256k1<secp256k1::All>,
 	) -> Result<Signature, ()> {
 		let msghash = hash_to_message!(&Sha256dHash::hash(&msg.encode()[..])[..]);
-		Ok(secp_ctx.sign_ecdsa(&msghash, &self.funding_key))
+		Ok(secp_ctx.sign_ecdsa(&msghash, &self.funding_key(funding_key_tweak)))
 	}
 
 	fn sign_splicing_funding_input(

lightning/src/util/test_channel_signer.rs

@@ -40,7 +40,7 @@ use bitcoin::secp256k1;
 #[cfg(taproot)]
 use bitcoin::secp256k1::All;
 use bitcoin::secp256k1::{ecdsa::Signature, Secp256k1};
-use bitcoin::secp256k1::{PublicKey, SecretKey};
+use bitcoin::secp256k1::{PublicKey, Scalar, SecretKey};
 #[cfg(taproot)]
 use musig2::types::{PartialSignature, PublicNonce};
 
@@ -466,9 +466,10 @@ impl EcdsaChannelSigner for TestChannelSigner {
 	}
 
 	fn sign_channel_announcement_with_funding_key(
-		&self, msg: &msgs::UnsignedChannelAnnouncement, secp_ctx: &Secp256k1<secp256k1::All>,
+		&self, msg: &msgs::UnsignedChannelAnnouncement, funding_key_tweak: Option<Scalar>,
+		secp_ctx: &Secp256k1<secp256k1::All>,
 	) -> Result<Signature, ()> {
-		self.inner.sign_channel_announcement_with_funding_key(msg, secp_ctx)
+		self.inner.sign_channel_announcement_with_funding_key(msg, funding_key_tweak, secp_ctx)
 	}
 
 	fn sign_splicing_funding_input(