Merge pull request #4208 from jkczyz/2025-11-signature-weight

Use 72 WU instead of 73 WU for signature weight

Author: wpaulino

lightning/src/ln/chan_utils.rs

@@ -116,19 +116,27 @@ pub const HTLC_SUCCESS_INPUT_P2A_ANCHOR_WITNESS_WEIGHT: u64 = 324;
 /// The size of the 2-of-2 multisig script
 const MULTISIG_SCRIPT_SIZE: u64 = 1 + // OP_2
 	1 + // data len
-	33 + // pubkey1
+	crate::sign::COMPRESSED_PUBLIC_KEY_SIZE as u64 + // pubkey1
 	1 + // data len
-	33 + // pubkey2
+	crate::sign::COMPRESSED_PUBLIC_KEY_SIZE as u64 + // pubkey2
 	1 + // OP_2
 	1; // OP_CHECKMULTISIG
-/// The weight of a funding transaction input (2-of-2 P2WSH)
-/// See https://github.com/lightning/bolts/blob/master/03-transactions.md#expected-weight-of-the-commitment-transaction
+
+/// The weight of a funding transaction input (2-of-2 P2WSH).
+///
+/// Unlike in the [spec], 72 WU is used for the max signature size since 73 WU signatures are
+/// non-standard.
+///
+/// Note: If you have the `grind_signatures` feature enabled, this will be at least 1 byte
+/// shorter.
+///
+/// [spec]: https://github.com/lightning/bolts/blob/master/03-transactions.md#expected-weight-of-the-commitment-transaction
 pub const FUNDING_TRANSACTION_WITNESS_WEIGHT: u64 = 1 + // number_of_witness_elements
 	1 + // nil_len
 	1 + // sig len
-	73 + // sig1
+	crate::sign::MAX_STANDARD_SIGNATURE_SIZE as u64 + // sig1
 	1 + // sig len
-	73 + // sig2
+	crate::sign::MAX_STANDARD_SIGNATURE_SIZE as u64 + // sig2
 	1 + // witness_script_length
 	MULTISIG_SCRIPT_SIZE;
 

lightning/src/ln/channel.rs

@@ -6546,6 +6546,11 @@ fn estimate_v2_funding_transaction_fee(
 				.saturating_add(BASE_INPUT_WEIGHT)
 				.saturating_add(EMPTY_SCRIPT_SIG_WEIGHT)
 				.saturating_add(FUNDING_TRANSACTION_WITNESS_WEIGHT);
+			#[cfg(feature = "grind_signatures")]
+			{
+				// Guarantees a low R signature
+				weight -= 1;
+			}
 		}
 	}
 
@@ -17437,19 +17442,19 @@ mod tests {
 		// 2 inputs, initiator, 2000 sat/kw feerate
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&two_inputs, &[], true, false, 2000),
-			1520,
+			if cfg!(feature = "grind_signatures") { 1512 } else { 1516 },
 		);
 
 		// higher feerate
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&two_inputs, &[], true, false, 3000),
-			2280,
+			if cfg!(feature = "grind_signatures") { 2268 } else { 2274 },
 		);
 
 		// only 1 input
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&one_input, &[], true, false, 2000),
-			974,
+			if cfg!(feature = "grind_signatures") { 970 } else { 972 },
 		);
 
 		// 0 inputs
@@ -17467,13 +17472,13 @@ mod tests {
 		// splice initiator
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&one_input, &[], true, true, 2000),
-			1746,
+			if cfg!(feature = "grind_signatures") { 1736 } else { 1740 },
 		);
 
 		// splice acceptor
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&one_input, &[], false, true, 2000),
-			546,
+			if cfg!(feature = "grind_signatures") { 542 } else { 544 },
 		);
 	}
 
@@ -17497,40 +17502,46 @@ mod tests {
 		use crate::ln::channel::check_v2_funding_inputs_sufficient;
 
 		// positive case, inputs well over intended contribution
-		assert_eq!(
-			check_v2_funding_inputs_sufficient(
-				220_000,
-				&[
-					funding_input_sats(200_000),
-					funding_input_sats(100_000),
-				],
-				true,
-				true,
-				2000,
-			).unwrap(),
-			2292,
-		);
+		{
+			let expected_fee = if cfg!(feature = "grind_signatures") { 2278 } else { 2284 };
+			assert_eq!(
+				check_v2_funding_inputs_sufficient(
+					220_000,
+					&[
+						funding_input_sats(200_000),
+						funding_input_sats(100_000),
+					],
+					true,
+					true,
+					2000,
+				).unwrap(),
+				expected_fee,
+			);
+		}
 
 		// negative case, inputs clearly insufficient
 		{
-			let res = check_v2_funding_inputs_sufficient(
-				220_000,
-				&[
-					funding_input_sats(100_000),
-				],
-				true,
-				true,
-				2000,
-			);
+			let expected_fee = if cfg!(feature = "grind_signatures") { 1736 } else { 1740 };
 			assert_eq!(
-				res.err().unwrap(),
-				"Total input amount 100000 is lower than needed for contribution 220000, considering fees of 1746. Need more inputs.",
+				check_v2_funding_inputs_sufficient(
+					220_000,
+					&[
+						funding_input_sats(100_000),
+					],
+					true,
+					true,
+					2000,
+				),
+				Err(format!(
+					"Total input amount 100000 is lower than needed for contribution 220000, considering fees of {}. Need more inputs.",
+					expected_fee,
+				)),
 			);
 		}
 
 		// barely covers
 		{
-			let expected_fee: u64 = 2292;
+			let expected_fee = if cfg!(feature = "grind_signatures") { 2278 } else { 2284 };
 			assert_eq!(
 				check_v2_funding_inputs_sufficient(
 					(300_000 - expected_fee - 20) as i64,
@@ -17548,25 +17559,28 @@ mod tests {
 
 		// higher fee rate, does not cover
 		{
-			let res = check_v2_funding_inputs_sufficient(
-				298032,
-				&[
-					funding_input_sats(200_000),
-					funding_input_sats(100_000),
-				],
-				true,
-				true,
-				2200,
-			);
+			let expected_fee = if cfg!(feature = "grind_signatures") { 2506 } else { 2513 };
 			assert_eq!(
-				res.err().unwrap(),
-				"Total input amount 300000 is lower than needed for contribution 298032, considering fees of 2522. Need more inputs.",
+				check_v2_funding_inputs_sufficient(
+					298032,
+					&[
+						funding_input_sats(200_000),
+						funding_input_sats(100_000),
+					],
+					true,
+					true,
+					2200,
+				),
+				Err(format!(
+					"Total input amount 300000 is lower than needed for contribution 298032, considering fees of {}. Need more inputs.",
+					expected_fee
+				)),
 			);
 		}
 
-		// barely covers, less fees (no extra weight, no init)
+		// barely covers, less fees (no extra weight, not initiator)
 		{
-			let expected_fee: u64 = 1092;
+			let expected_fee = if cfg!(feature = "grind_signatures") { 1084 } else { 1088 };
 			assert_eq!(
 				check_v2_funding_inputs_sufficient(
 					(300_000 - expected_fee - 20) as i64,

lightning/src/ln/funding.rs

@@ -143,7 +143,13 @@ impl FundingTxInput {
 	/// [`TxIn::sequence`]: bitcoin::TxIn::sequence
 	/// [`set_sequence`]: Self::set_sequence
 	pub fn new_p2wpkh(prevtx: Transaction, vout: u32) -> Result<Self, ()> {
-		let witness_weight = Weight::from_wu(P2WPKH_WITNESS_WEIGHT);
+		let witness_weight = Weight::from_wu(P2WPKH_WITNESS_WEIGHT)
+			- if cfg!(feature = "grind_signatures") {
+				// Guarantees a low R signature
+				Weight::from_wu(1)
+			} else {
+				Weight::ZERO
+			};
 		FundingTxInput::new(prevtx, vout, witness_weight, Script::is_p2wpkh)
 	}
 
@@ -224,4 +230,9 @@ impl FundingTxInput {
 	pub fn set_sequence(&mut self, sequence: Sequence) {
 		self.sequence = sequence;
 	}
+
+	/// Converts the [`FundingTxInput`] into a [`Utxo`] for coin selection.
+	pub fn into_utxo(self) -> Utxo {
+		self.utxo
+	}
 }