Use 72 WU instead of 73 WU for signature weight

jkczyz · jkczyz · commit 36ef03c5b722 · 2025-11-06T09:02:22.000-06:00
When estimating signature weight, 73 WU was used in some places while 72
WU was used in others. Consistently use 72 WU and replace hardcoded
values with constants. 73 WU signatures are non-standard and won't be
produced by LDK. Additionally, using 73 WU along with grind_signatures
adjustment is nonsensical.
diff --git a/lightning/src/ln/chan_utils.rs b/lightning/src/ln/chan_utils.rs
@@ -116,19 +116,24 @@ 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
+	bitcoin::secp256k1::constants::PUBLIC_KEY_SIZE as u64 + // pubkey1
 	1 + // data len
-	33 + // pubkey2
+	bitcoin::secp256k1::constants::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.
+///
+/// [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
+	bitcoin::secp256k1::constants::MAX_SIGNATURE_SIZE as u64 + // sig1
 	1 + // sig len
-	73 + // sig2
+	bitcoin::secp256k1::constants::MAX_SIGNATURE_SIZE as u64 + // sig2
 	1 + // witness_script_length
 	MULTISIG_SCRIPT_SIZE;
 
diff --git a/lightning/src/ln/channel.rs b/lightning/src/ln/channel.rs
@@ -17397,19 +17397,19 @@ mod tests {
 		// 2 inputs, initiator, 2000 sat/kw feerate
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&two_inputs, &[], true, false, 2000),
-			1520,
+			1516,
 		);
 
 		// higher feerate
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&two_inputs, &[], true, false, 3000),
-			2280,
+			2274,
 		);
 
 		// only 1 input
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&one_input, &[], true, false, 2000),
-			974,
+			972,
 		);
 
 		// 0 inputs
@@ -17427,13 +17427,13 @@ mod tests {
 		// splice initiator
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&one_input, &[], true, true, 2000),
-			1746,
+			1740,
 		);
 
 		// splice acceptor
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&one_input, &[], false, true, 2000),
-			546,
+			544,
 		);
 	}
 
@@ -17468,7 +17468,7 @@ mod tests {
 				true,
 				2000,
 			).unwrap(),
-			2292,
+			2284,
 		);
 
 		// negative case, inputs clearly insufficient
@@ -17484,13 +17484,13 @@ mod tests {
 			);
 			assert_eq!(
 				res.err().unwrap(),
-				"Total input amount 100000 is lower than needed for contribution 220000, considering fees of 1746. Need more inputs.",
+				"Total input amount 100000 is lower than needed for contribution 220000, considering fees of 1740. Need more inputs.",
 			);
 		}
 
 		// barely covers
 		{
-			let expected_fee: u64 = 2292;
+			let expected_fee: u64 = 2284;
 			assert_eq!(
 				check_v2_funding_inputs_sufficient(
 					(300_000 - expected_fee - 20) as i64,
@@ -17520,13 +17520,13 @@ mod tests {
 			);
 			assert_eq!(
 				res.err().unwrap(),
-				"Total input amount 300000 is lower than needed for contribution 298032, considering fees of 2522. Need more inputs.",
+				"Total input amount 300000 is lower than needed for contribution 298032, considering fees of 2513. Need more inputs.",
 			);
 		}
 
 		// barely covers, less fees (no extra weight, no init)
 		{
-			let expected_fee: u64 = 1092;
+			let expected_fee: u64 = 1088;
 			assert_eq!(
 				check_v2_funding_inputs_sufficient(
 					(300_000 - expected_fee - 20) as i64,
diff --git a/lightning/src/ln/interactivetxs.rs b/lightning/src/ln/interactivetxs.rs
@@ -3353,38 +3353,38 @@ mod tests {
 				FundingTxInput::new_p2wpkh(prevtx, 0).unwrap()
 			})
 			.collect();
-		let our_contributed = 110_000;
 		let txout = TxOut { value: Amount::from_sat(10_000), script_pubkey: ScriptBuf::new() };
 		let outputs = vec![txout];
 		let funding_feerate_sat_per_1000_weight = 3000;
 
-		let total_inputs: u64 = input_prevouts.iter().map(|o| o.value.to_sat()).sum();
-		let total_outputs: u64 = outputs.iter().map(|o| o.value.to_sat()).sum();
-		let gross_change = total_inputs - total_outputs - our_contributed;
-		let fees = 1746;
-		let common_fees = 234;
+		let total_inputs: Amount = input_prevouts.iter().map(|o| o.value).sum();
+		let total_outputs: Amount = outputs.iter().map(|o| o.value).sum();
+		let fees = Amount::from_sat(1740);
+		let common_fees = Amount::from_sat(234);
 
 		// There is leftover for change
 		let context = FundingNegotiationContext {
 			is_initiator: true,
-			our_funding_contribution: SignedAmount::from_sat(our_contributed as i64),
+			our_funding_contribution: SignedAmount::from_sat(110_000),
 			funding_tx_locktime: AbsoluteLockTime::ZERO,
 			funding_feerate_sat_per_1000_weight,
 			shared_funding_input: None,
 			our_funding_inputs: inputs,
 			our_funding_outputs: outputs,
 			change_script: None,
 		};
+		let gross_change =
+			total_inputs - total_outputs - context.our_funding_contribution.to_unsigned().unwrap();
 		assert_eq!(
 			calculate_change_output_value(&context, false, &ScriptBuf::new(), 300),
-			Ok(Some(gross_change - fees - common_fees)),
+			Ok(Some((gross_change - fees - common_fees).to_sat())),
 		);
 
 		// There is leftover for change, without common fees
 		let context = FundingNegotiationContext { is_initiator: false, ..context };
 		assert_eq!(
 			calculate_change_output_value(&context, false, &ScriptBuf::new(), 300),
-			Ok(Some(gross_change - fees)),
+			Ok(Some((gross_change - fees).to_sat())),
 		);
 
 		// Insufficient inputs, no leftover
@@ -3415,21 +3415,25 @@ mod tests {
 			our_funding_contribution: SignedAmount::from_sat(117_992),
 			..context
 		};
+		let gross_change =
+			total_inputs - total_outputs - context.our_funding_contribution.to_unsigned().unwrap();
 		assert_eq!(
 			calculate_change_output_value(&context, false, &ScriptBuf::new(), 100),
-			Ok(Some(262)),
+			Ok(Some((gross_change - fees).to_sat())),
 		);
 
 		// Larger fee, smaller change
 		let context = FundingNegotiationContext {
 			is_initiator: true,
-			our_funding_contribution: SignedAmount::from_sat(our_contributed as i64),
+			our_funding_contribution: SignedAmount::from_sat(110_000),
 			funding_feerate_sat_per_1000_weight: funding_feerate_sat_per_1000_weight * 3,
 			..context
 		};
+		let gross_change =
+			total_inputs - total_outputs - context.our_funding_contribution.to_unsigned().unwrap();
 		assert_eq!(
 			calculate_change_output_value(&context, false, &ScriptBuf::new(), 300),
-			Ok(Some(4060)),
+			Ok(Some((gross_change - fees * 3 - common_fees * 3).to_sat())),
 		);
 	}
 
diff --git a/lightning/src/sign/mod.rs b/lightning/src/sign/mod.rs
@@ -114,10 +114,12 @@ impl DelayedPaymentOutputDescriptor {
 	/// The maximum length a well-formed witness spending one of these should have.
 	/// Note: If you have the grind_signatures feature enabled, this will be at least 1 byte
 	/// shorter.
-	// Calculated as 1 byte length + 73 byte signature, 1 byte empty vec push, 1 byte length plus
-	// redeemscript push length.
-	pub const MAX_WITNESS_LENGTH: u64 =
-		1 + 73 + 1 + chan_utils::REVOKEABLE_REDEEMSCRIPT_MAX_LENGTH as u64 + 1;
+	pub const MAX_WITNESS_LENGTH: u64 = (1 /* witness items */
+		+ 1 /* sig push */
+		+ bitcoin::secp256k1::constants::MAX_SIGNATURE_SIZE
+		+ 1 /* empty vec push */
+		+ 1 /* redeemscript push */
+		+ chan_utils::REVOKEABLE_REDEEMSCRIPT_MAX_LENGTH) as u64;
 }
 
 impl_writeable_tlv_based!(DelayedPaymentOutputDescriptor, {
@@ -131,15 +133,19 @@ impl_writeable_tlv_based!(DelayedPaymentOutputDescriptor, {
 	(13, channel_transaction_parameters, (option: ReadableArgs, Some(channel_value_satoshis.0.unwrap()))),
 });
 
-pub(crate) const P2WPKH_WITNESS_WEIGHT: u64 = 1 /* num stack items */ +
-	1 /* sig length */ +
-	73 /* sig including sighash flag */ +
-	1 /* pubkey length */ +
-	33 /* pubkey */;
+/// Witness weight for satisfying a P2WPKH spend.
+pub(crate) const P2WPKH_WITNESS_WEIGHT: u64 = (1 /* witness items */
+	+ 1 /* sig push */
+	+ bitcoin::secp256k1::constants::MAX_SIGNATURE_SIZE
+	+ 1 /* pubkey push */
+	+ bitcoin::secp256k1::constants::PUBLIC_KEY_SIZE)
+	as u64;
 
-/// Witness weight for satisying a P2TR key-path spend.
-pub(crate) const P2TR_KEY_PATH_WITNESS_WEIGHT: u64 = 1 /* witness items */
-	+ 1 /* schnorr sig len */ + 64 /* schnorr sig */;
+/// Witness weight for satisfying a P2TR key-path spend.
+pub(crate) const P2TR_KEY_PATH_WITNESS_WEIGHT: u64 = (1 /* witness items */
+	+ 1 /* sig push */
+	+ bitcoin::secp256k1::constants::SCHNORR_SIGNATURE_SIZE)
+	as u64;
 
 /// If a [`KeysManager`] is built with [`KeysManager::new`] with `v2_remote_key_derivation` set
 /// (and for all channels after they've been spliced), the script which we receive funds to on-chain
@@ -192,10 +198,16 @@ impl StaticPaymentOutputDescriptor {
 	/// shorter.
 	pub fn max_witness_length(&self) -> u64 {
 		if self.needs_csv_1_for_spend() {
-			let witness_script_weight = 1 /* pubkey push */ + 33 /* pubkey */ +
-				1 /* OP_CHECKSIGVERIFY */ + 1 /* OP_1 */ + 1 /* OP_CHECKSEQUENCEVERIFY */;
-			1 /* num witness items */ + 1 /* sig push */ + 73 /* sig including sighash flag */ +
-				1 /* witness script push */ + witness_script_weight
+			let witness_script_weight = 1 /* pubkey push */
+				+ bitcoin::secp256k1::constants::PUBLIC_KEY_SIZE
+				+ 1 /* OP_CHECKSIGVERIFY */
+				+ 1 /* OP_1 */
+				+ 1 /* OP_CHECKSEQUENCEVERIFY */;
+			(1 /* num witness items */
+				+ 1 /* sig push */
+				+ bitcoin::secp256k1::constants::MAX_SIGNATURE_SIZE
+				+ 1 /* witness script push */
+				+ witness_script_weight) as u64
 		} else {
 			P2WPKH_WITNESS_WEIGHT
 		}
@@ -511,7 +523,7 @@ impl SpendableOutputDescriptor {
 						sequence: Sequence::ZERO,
 						witness: Witness::new(),
 					});
-					witness_weight += 1 + 73 + 34;
+					witness_weight += P2WPKH_WITNESS_WEIGHT;
 					#[cfg(feature = "grind_signatures")]
 					{
 						// Guarantees a low R signature
