From 09d45900e294782aacfdc087e063e19815fdf979 Mon Sep 17 00:00:00 2001
From: optout <13562139+optout21@users.noreply.github.com>
Date: Wed, 9 Oct 2024 15:27:13 +0200
Subject: [PATCH] Signer extended with method to sign prev funding transaction
 input

---
 lightning/src/sign/ecdsa.rs               | 11 +++++++++++
 lightning/src/sign/mod.rs                 | 21 +++++++++++++++++++++
 lightning/src/util/test_channel_signer.rs |  7 +++++++
 3 files changed, 39 insertions(+)

diff --git a/lightning/src/sign/ecdsa.rs b/lightning/src/sign/ecdsa.rs
index ecdd45aa3f5..2bbd6ea4381 100644
--- a/lightning/src/sign/ecdsa.rs
+++ b/lightning/src/sign/ecdsa.rs
@@ -209,4 +209,15 @@ pub trait EcdsaChannelSigner: ChannelSigner {
 	fn sign_channel_announcement_with_funding_key(
 		&self, msg: &UnsignedChannelAnnouncement, secp_ctx: &Secp256k1<secp256k1::All>,
 	) -> Result<Signature, ()>;
+	/// Sign an input of a transaction with our funding key.
+	/// Used for splicing, when signing the previous funding transaction.
+	/// The previous funding transaction becomes an input to the new funding transaction,
+	/// and it is a multisig, which we also need to sign.
+	/// [`input_index`]: The index of the input that was the previous funding transaction,
+	///     within the new funding transaction.
+	/// [`input_value`]: The value of the previous funding transaction.
+	fn sign_splicing_funding_input(
+		&self, tx: &Transaction, input_index: u16, input_value: u64,
+		secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()>;
 }
diff --git a/lightning/src/sign/mod.rs b/lightning/src/sign/mod.rs
index 4b8a9e0255a..7265e25eff3 100644
--- a/lightning/src/sign/mod.rs
+++ b/lightning/src/sign/mod.rs
@@ -1695,6 +1695,27 @@ impl EcdsaChannelSigner for InMemorySigner {
 		let msghash = hash_to_message!(&Sha256dHash::hash(&msg.encode()[..])[..]);
 		Ok(secp_ctx.sign_ecdsa(&msghash, &self.funding_key))
 	}
+
+	fn sign_splicing_funding_input(
+		&self, tx: &Transaction, input_index: u16, input_value: u64,
+		secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()> {
+		let funding_pubkey = PublicKey::from_secret_key(secp_ctx, &self.funding_key);
+		let counterparty_funding_key =
+			&self.counterparty_pubkeys().expect(MISSING_PARAMS_ERR).funding_pubkey;
+		let funding_redeemscript =
+			make_funding_redeemscript(&funding_pubkey, counterparty_funding_key);
+		let sighash = &sighash::SighashCache::new(tx)
+			.p2wsh_signature_hash(
+				input_index as usize,
+				&funding_redeemscript,
+				Amount::from_sat(input_value),
+				EcdsaSighashType::All,
+			)
+			.unwrap()[..];
+		let msg = hash_to_message!(sighash);
+		Ok(sign(secp_ctx, &msg, &self.funding_key))
+	}
 }
 
 #[cfg(taproot)]
diff --git a/lightning/src/util/test_channel_signer.rs b/lightning/src/util/test_channel_signer.rs
index 9aa34fd4ca6..163b03e7fbb 100644
--- a/lightning/src/util/test_channel_signer.rs
+++ b/lightning/src/util/test_channel_signer.rs
@@ -353,6 +353,13 @@ impl EcdsaChannelSigner for TestChannelSigner {
 	) -> Result<Signature, ()> {
 		self.inner.sign_channel_announcement_with_funding_key(msg, secp_ctx)
 	}
+
+	fn sign_splicing_funding_input(
+		&self, tx: &Transaction, input_index: u16, input_value: u64,
+		secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()> {
+		self.inner.sign_splicing_funding_input(tx, input_index, input_value, secp_ctx)
+	}
 }
 
 #[cfg(taproot)]