f shuffle inputs/outputs before sending to peer

Author: dunxen

lightning/src/ln/interactivetxs.rs

@@ -659,20 +659,27 @@ impl InteractiveTxConstructor {
 	{
 		let state_machine =
 			StateMachine::new(feerate_sat_per_kw, is_initiator, tx_locktime, to_remote_value);
-		let inputs_to_contribute = inputs_to_contribute
-			.into_iter()
-			.map(|(input, tx)| {
-				let serial_id = generate_local_serial_id(entropy_source, is_initiator);
-				(serial_id, input, tx)
-			})
-			.collect();
-		let outputs_to_contribute = outputs_to_contribute
+		let mut inputs_to_contribute: Vec<(SerialId, TxIn, TransactionU16LenLimited)> =
+			inputs_to_contribute
+				.into_iter()
+				.map(|(input, tx)| {
+					let serial_id = generate_local_serial_id(entropy_source, is_initiator);
+					(serial_id, input, tx)
+				})
+				.collect();
+		// We'll sort by the randomly generated serial IDs, effectively shuffling the order of the inputs
+		// as the user passed them to us to avoid leaking any potential categorization of transactions
+		// before we pass any of the inputs to the counterparty.
+		inputs_to_contribute.sort_unstable_by_key(|(serial_id, _, _)| *serial_id);
+		let mut outputs_to_contribute: Vec<(SerialId, TxOut)> = outputs_to_contribute
 			.into_iter()
 			.map(|output| {
 				let serial_id = generate_local_serial_id(entropy_source, is_initiator);
 				(serial_id, output)
 			})
 			.collect();
+		// In the same manner and for the same rationale as the inputs above, we'll shuffle the outputs.
+		outputs_to_contribute.sort_unstable_by_key(|(serial_id, _)| *serial_id);
 		let mut constructor =
 			Self { state_machine, channel_id, inputs_to_contribute, outputs_to_contribute };
 		let message_send = if is_initiator {