Generator: Output shuffling on final transaction

wallet/core/src/account/mod.rs

@@ -315,6 +315,7 @@ pub trait Account: AnySync + Send + Sync + 'static {
             self.clone().as_dyn_arc(),
             PaymentDestination::Change,
             fee_rate,
+            None,
             Fees::None,
             None,
         )?;
@@ -351,8 +352,14 @@ pub trait Account: AnySync + Send + Sync + 'static {
         let keydata = self.prv_key_data(wallet_secret).await?;
         let signer = Arc::new(Signer::new(self.clone().as_dyn_arc(), keydata, payment_secret));
 
-        let settings =
-            GeneratorSettings::try_new_with_account(self.clone().as_dyn_arc(), destination, fee_rate, priority_fee_sompi, payload)?;
+        let settings = GeneratorSettings::try_new_with_account(
+            self.clone().as_dyn_arc(),
+            destination,
+            fee_rate,
+            None,
+            priority_fee_sompi,
+            payload,
+        )?;
 
         let generator = Generator::try_new(settings, Some(signer), Some(abortable))?;
 
@@ -381,8 +388,14 @@ pub trait Account: AnySync + Send + Sync + 'static {
         payment_secret: Option<Secret>,
         abortable: &Abortable,
     ) -> Result<Bundle, Error> {
-        let settings =
-            GeneratorSettings::try_new_with_account(self.clone().as_dyn_arc(), destination, fee_rate, priority_fee_sompi, payload)?;
+        let settings = GeneratorSettings::try_new_with_account(
+            self.clone().as_dyn_arc(),
+            destination,
+            fee_rate,
+            None,
+            priority_fee_sompi,
+            payload,
+        )?;
         let keydata = self.prv_key_data(wallet_secret).await?;
         let signer = Arc::new(PSKBSigner::new(self.clone().as_dyn_arc(), keydata, payment_secret));
         let generator = Generator::try_new(settings, None, Some(abortable))?;
@@ -463,6 +476,7 @@ pub trait Account: AnySync + Send + Sync + 'static {
             self.clone().as_dyn_arc(),
             final_transaction_destination,
             fee_rate,
+            None,
             priority_fee_sompi,
             final_transaction_payload,
         )?
@@ -493,7 +507,8 @@ pub trait Account: AnySync + Send + Sync + 'static {
         payload: Option<Vec<u8>>,
         abortable: &Abortable,
     ) -> Result<GeneratorSummary> {
-        let settings = GeneratorSettings::try_new_with_account(self.as_dyn_arc(), destination, fee_rate, priority_fee_sompi, payload)?;
+        let settings =
+            GeneratorSettings::try_new_with_account(self.as_dyn_arc(), destination, fee_rate, None, priority_fee_sompi, payload)?;
 
         let generator = Generator::try_new(settings, None, Some(abortable))?;
 
@@ -620,6 +635,7 @@ pub trait DerivationCapableAccount: Account {
                         1,
                         PaymentDestination::Change,
                         fee_rate,
+                        None,
                         Fees::None,
                         None,
                         None,

wallet/core/src/account/pskb.rs

@@ -334,6 +334,7 @@ pub fn pskt_to_pending_transaction(
         source_utxo_context: None,
         destination_utxo_context: None,
         fee_rate: None,
+        shuffle_outputs: None,
         final_transaction_priority_fee: fee_u.into(),
         final_transaction_destination,
         final_transaction_payload: None,

wallet/core/src/tx/generator/generator.rs

@@ -70,6 +70,8 @@ use kaspa_consensus_core::mass::Kip9Version;
 use kaspa_consensus_core::subnets::SUBNETWORK_ID_NATIVE;
 use kaspa_consensus_core::tx::{Transaction, TransactionInput, TransactionOutpoint, TransactionOutput};
 use kaspa_txscript::pay_to_address_script;
+use rand::seq::SliceRandom;
+use rand::thread_rng;
 use std::collections::VecDeque;
 
 use super::SignerT;
@@ -296,6 +298,8 @@ struct Inner {
     signature_mass_per_input: u64,
     // fee rate
     fee_rate: Option<f64>,
+    // shuffle outputs of final transaction
+    shuffle_outputs: Option<bool>,
     // final transaction amount and fees
     // `None` is used for sweep transactions
     final_transaction: Option<FinalTransaction>,
@@ -362,6 +366,7 @@ impl Generator {
             minimum_signatures,
             change_address,
             fee_rate,
+            shuffle_outputs,
             final_transaction_priority_fee,
             final_transaction_destination,
             final_transaction_payload,
@@ -469,6 +474,7 @@ impl Generator {
             standard_change_output_compute_mass: standard_change_output_mass,
             signature_mass_per_input,
             fee_rate,
+            shuffle_outputs,
             final_transaction,
             final_transaction_priority_fee,
             final_transaction_outputs,
@@ -1069,7 +1075,7 @@ impl Generator {
 
                 let change_output_value = change_output_value.unwrap_or(0);
 
-                let mut final_outputs = self.inner.final_transaction_outputs.clone();
+                let mut final_outputs: Vec<TransactionOutput> = self.inner.final_transaction_outputs.clone();
 
                 if self.inner.final_transaction_priority_fee.receiver_pays() {
                     let output = final_outputs.get_mut(0).expect("include fees requires one output");
@@ -1080,10 +1086,23 @@ impl Generator {
                     }
                 }
 
-                let change_output_index = if change_output_value > 0 {
-                    let change_output_index = Some(final_outputs.len());
-                    final_outputs.push(TransactionOutput::new(change_output_value, pay_to_address_script(&self.inner.change_address)));
-                    change_output_index
+                // Cache the change output (if any) before shuffling so we can find its index.
+                let change_output = if change_output_value > 0 {
+                    let change_output = TransactionOutput::new(change_output_value, pay_to_address_script(&self.inner.change_address));
+                    final_outputs.push(change_output.clone());
+                    Some(change_output)
+                } else {
+                    None
+                };
+
+                // Shuffle the outputs if required for extra privacy.
+                if self.inner.shuffle_outputs.unwrap_or(true) {
+                    final_outputs.shuffle(&mut thread_rng());
+                }
+
+                // Find the new change_output_index after shuffling if there was a change output.
+                let change_output_index = if let Some(change_output) = change_output {
+                    final_outputs.iter().position(|output| output == &change_output)
                 } else {
                     None
                 };

wallet/core/src/tx/generator/settings.rs

@@ -30,6 +30,8 @@ pub struct GeneratorSettings {
     pub change_address: Address,
     // fee rate
     pub fee_rate: Option<f64>,
+    // Whether to shuffle the outputs of the final transaction for privacy reasons.
+    pub shuffle_outputs: Option<bool>,
     // applies only to the final transaction
     pub final_transaction_priority_fee: Fees,
     // final transaction outputs
@@ -63,6 +65,7 @@ impl GeneratorSettings {
         account: Arc<dyn Account>,
         final_transaction_destination: PaymentDestination,
         fee_rate: Option<f64>,
+        shuffle_outputs: Option<bool>,
         final_priority_fee: Fees,
         final_transaction_payload: Option<Vec<u8>>,
     ) -> Result<Self> {
@@ -83,8 +86,8 @@ impl GeneratorSettings {
             utxo_iterator: Box::new(utxo_iterator),
             source_utxo_context: Some(account.utxo_context().clone()),
             priority_utxo_entries: None,
-
             fee_rate,
+            shuffle_outputs,
             final_transaction_priority_fee: final_priority_fee,
             final_transaction_destination,
             final_transaction_payload,
@@ -94,6 +97,7 @@ impl GeneratorSettings {
         Ok(settings)
     }
 
+    #[allow(clippy::too_many_arguments)]
     pub fn try_new_with_context(
         utxo_context: UtxoContext,
         priority_utxo_entries: Option<Vec<UtxoEntryReference>>,
@@ -102,6 +106,7 @@ impl GeneratorSettings {
         minimum_signatures: u16,
         final_transaction_destination: PaymentDestination,
         fee_rate: Option<f64>,
+        shuffle_outputs: Option<bool>,
         final_priority_fee: Fees,
         final_transaction_payload: Option<Vec<u8>>,
         multiplexer: Option<Multiplexer<Box<Events>>>,
@@ -118,8 +123,8 @@ impl GeneratorSettings {
             utxo_iterator: Box::new(utxo_iterator),
             source_utxo_context: Some(utxo_context),
             priority_utxo_entries,
-
             fee_rate,
+            shuffle_outputs,
             final_transaction_priority_fee: final_priority_fee,
             final_transaction_destination,
             final_transaction_payload,
@@ -139,6 +144,7 @@ impl GeneratorSettings {
         minimum_signatures: u16,
         final_transaction_destination: PaymentDestination,
         fee_rate: Option<f64>,
+        shuffle_outputs: Option<bool>,
         final_priority_fee: Fees,
         final_transaction_payload: Option<Vec<u8>>,
         multiplexer: Option<Multiplexer<Box<Events>>>,
@@ -152,8 +158,8 @@ impl GeneratorSettings {
             utxo_iterator: Box::new(utxo_iterator),
             source_utxo_context: None,
             priority_utxo_entries,
-
             fee_rate,
+            shuffle_outputs,
             final_transaction_priority_fee: final_priority_fee,
             final_transaction_destination,
             final_transaction_payload,

wallet/core/src/tx/generator/test.rs

@@ -436,6 +436,7 @@ where
         priority_utxo_entries,
         destination_utxo_context,
         fee_rate,
+        shuffle_outputs: None,
         final_transaction_priority_fee: final_priority_fee,
         final_transaction_destination,
         final_transaction_payload,

wallet/core/src/wasm/tx/generator/generator.rs

@@ -169,6 +169,7 @@ impl Generator {
             final_transaction_destination,
             change_address,
             fee_rate,
+            shuffle_outputs,
             final_priority_fee,
             sig_op_count,
             minimum_signatures,
@@ -192,6 +193,7 @@ impl Generator {
                     minimum_signatures,
                     final_transaction_destination,
                     fee_rate,
+                    shuffle_outputs,
                     final_priority_fee,
                     payload,
                     multiplexer,
@@ -209,6 +211,7 @@ impl Generator {
                     minimum_signatures,
                     final_transaction_destination,
                     fee_rate,
+                    shuffle_outputs,
                     final_priority_fee,
                     payload,
                     multiplexer,
@@ -272,6 +275,7 @@ struct GeneratorSettings {
     pub final_transaction_destination: PaymentDestination,
     pub change_address: Option<Address>,
     pub fee_rate: Option<f64>,
+    pub shuffle_outputs: Option<bool>,
     pub final_priority_fee: Fees,
     pub sig_op_count: u8,
     pub minimum_signatures: u16,
@@ -292,6 +296,8 @@ impl TryFrom<IGeneratorSettingsObject> for GeneratorSettings {
 
         let fee_rate = args.get_f64("feeRate").ok().and_then(|v| (v.is_finite() && !v.is_nan() && v >= 1e-8).then_some(v));
 
+        let shuffle_outputs = args.get_bool("shuffleOutputs").ok();
+
         let final_priority_fee = args.get::<IFees>("priorityFee")?.try_into()?;
 
         let generator_source = if let Ok(Some(context)) = args.try_cast_into::<UtxoContext>("entries") {
@@ -325,6 +331,7 @@ impl TryFrom<IGeneratorSettingsObject> for GeneratorSettings {
             final_transaction_destination,
             change_address,
             fee_rate,
+            shuffle_outputs,
             final_priority_fee,
             sig_op_count,
             minimum_signatures,