feat(wallet): pad private transactions to a fixed maximum

integration_tests/tests/auth_transfer/private.rs

@@ -83,7 +83,6 @@ async fn private_transfer_to_foreign_account() -> Result<()> {
     let tx = fetch_privacy_preserving_tx(ctx.sequencer_client(), tx_hash).await;
     assert_eq!(tx.message.new_commitments[0], new_commitment1);
 
-    assert_eq!(tx.message.new_commitments.len(), 2);
     for commitment in tx.message.new_commitments {
         assert!(verify_commitment_is_in_state(commitment, ctx.sequencer_client()).await);
     }
@@ -172,7 +171,6 @@ async fn private_transfer_to_owned_account_using_claiming_path() -> Result<()> {
         .context("Failed to get private account commitment for sender")?;
     assert_eq!(tx.message.new_commitments[0], sender_commitment);
 
-    assert_eq!(tx.message.new_commitments.len(), 2);
     for commitment in tx.message.new_commitments {
         assert!(verify_commitment_is_in_state(commitment, ctx.sequencer_client()).await);
     }
@@ -307,7 +305,6 @@ async fn private_transfer_to_owned_account_continuous_run_path() -> Result<()> {
     tokio::time::sleep(Duration::from_secs(TIME_TO_WAIT_FOR_BLOCK_SECONDS)).await;
 
     // Verify commitments are in state
-    assert_eq!(tx.message.new_commitments.len(), 2);
     for commitment in tx.message.new_commitments {
         assert!(verify_commitment_is_in_state(commitment, ctx.sequencer_client()).await);
     }

integration_tests/tests/keys.rs

@@ -73,7 +73,6 @@ async fn sync_private_account_with_non_zero_chain_index() -> Result<()> {
         .context("Failed to get private account commitment for sender")?;
     assert_eq!(tx.message.new_commitments[0], new_commitment1);
 
-    assert_eq!(tx.message.new_commitments.len(), 2);
     for commitment in tx.message.new_commitments {
         assert!(verify_commitment_is_in_state(commitment, ctx.sequencer_client()).await);
     }

lez/wallet/src/account_manager.rs

@@ -193,6 +193,14 @@ pub struct AccountManager {
 }
 
 impl AccountManager {
+    /// The private-account count that every privacy-preserving transaction is padded up to with
+    /// dummy inputs via the default interface.
+    ///
+    /// The value is selected based on the largest account number per-tx currently supported
+    /// (it is 7 for AMM). It is recommended to reassess this value per new actively supported
+    /// application and that all users share the value for a larger anonymity set.
+    const MAX_PRIVATE_ACCOUNTS: usize = 7;
+
     pub async fn new(
         wallet: &WalletCore,
         accounts: Vec<AccountIdentity>,
@@ -410,6 +418,17 @@ impl AccountManager {
         .collect()
     }
 
+    /// Generate the dummy inputs that pad this transaction's private-account count up to
+    /// `MAX_PRIVATE_ACCOUNTS`.
+    pub fn dummy_inputs_default(&self) -> Vec<DummyInput> {
+        let private_count = self
+            .states
+            .iter()
+            .filter(|state| matches!(state, State::Private(_)))
+            .count();
+        self.dummy_inputs(Self::MAX_PRIVATE_ACCOUNTS.saturating_sub(private_count))
+    }
+
     /// Build the per-account input vec for the privacy-preserving circuit. Each variant carries
     /// exactly the fields the circuit's code path for that account needs, with the ephemeral
     /// keys (`ssk`) drawn from the cached values that `private_account_keys` and the message
@@ -684,4 +703,66 @@ mod tests {
         assert!(acc.is_private());
         assert!(!acc.is_public());
     }
+
+    fn private_state() -> State {
+        let npk = NullifierPublicKey([0; 32]);
+        let vpk = ViewingPublicKey::from_seed(&[0; 32], &[0; 32]);
+        let pre_state = AccountWithMetadata::new(Account::default(), false, (&npk, &vpk, 0));
+        State::Private(AccountPreparedData {
+            nsk: None,
+            npk,
+            identifier: 0,
+            vpk,
+            pre_state,
+            proof: None,
+            random_seed: [0; 32],
+            is_pda: false,
+        })
+    }
+
+    fn public_state() -> State {
+        let npk = NullifierPublicKey([0; 32]);
+        let vpk = ViewingPublicKey::from_seed(&[0; 32], &[0; 32]);
+        let account = AccountWithMetadata::new(Account::default(), false, (&npk, &vpk, 0));
+        State::Public { account, sk: None }
+    }
+
+    fn manager(states: Vec<State>) -> AccountManager {
+        AccountManager {
+            states,
+            pin: None,
+            dummy_commitment_root: [0; 32],
+        }
+    }
+
+    #[test]
+    fn dummy_inputs_default_pads_private_count_to_max() {
+        let max = AccountManager::MAX_PRIVATE_ACCOUNTS;
+
+        // Empty txs get padded to the max.
+        assert_eq!(manager(vec![]).dummy_inputs_default().len(), max);
+        // In a padded transaction, the padding amount depends on
+        // the amount of private accounts used.
+        assert_eq!(
+            manager(vec![private_state(), private_state()])
+                .dummy_inputs_default()
+                .len(),
+            max - 2
+        );
+        assert_eq!(
+            manager(vec![private_state(), public_state(), private_state()])
+                .dummy_inputs_default()
+                .len(),
+            max - 2
+        );
+
+        // If the private accounts in the transaction exceed the max, no padding
+        // is done.
+        let full: Vec<State> = std::iter::repeat_with(private_state).take(max).collect();
+        assert_eq!(manager(full).dummy_inputs_default().len(), 0);
+        let over: Vec<State> = std::iter::repeat_with(private_state)
+            .take(max + 2)
+            .collect();
+        assert_eq!(manager(over).dummy_inputs_default().len(), 0);
+    }
 }

lez/wallet/src/lib.rs

@@ -557,13 +557,9 @@ impl WalletCore {
         instruction_data: InstructionData,
         program: &ProgramWithDependencies,
     ) -> Result<(HashType, Vec<SharedSecretKey>), ExecutionFailureKind> {
-        self.send_privacy_preserving_tx_with_pre_check(
-            accounts,
-            instruction_data,
-            program,
-            |_| Ok(()),
-            0,
-        )
+        self.send_privacy_preserving_tx_with_pre_check(accounts, instruction_data, program, |_| {
+            Ok(())
+        })
         .await
     }
 
@@ -573,7 +569,6 @@ impl WalletCore {
         instruction_data: InstructionData,
         program: &ProgramWithDependencies,
         tx_pre_check: impl FnOnce(&[&Account]) -> Result<(), ExecutionFailureKind>,
-        dummy_count: usize,
     ) -> Result<(HashType, Vec<SharedSecretKey>), ExecutionFailureKind> {
         let acc_manager = account_manager::AccountManager::new(self, accounts).await?;
 
@@ -592,7 +587,7 @@ impl WalletCore {
                 pre_states,
                 instruction_data,
                 acc_manager.account_identities(),
-                acc_manager.dummy_inputs(dummy_count),
+                acc_manager.dummy_inputs_default(),
                 &program.to_owned(),
             )?;
 

lez/wallet/src/program_facades/native_token_transfer/deshielded.rs

@@ -24,7 +24,6 @@ impl NativeTokenTransfer<'_> {
                 instruction_data,
                 &program.into(),
                 tx_pre_check,
-                0,
             )
             .await
             .map(|(resp, secrets)| {

lez/wallet/src/program_facades/native_token_transfer/private.rs

@@ -58,7 +58,6 @@ impl NativeTokenTransfer<'_> {
                 instruction_data,
                 &program.into(),
                 tx_pre_check,
-                0,
             )
             .await
             .map(|(resp, secrets)| {
@@ -92,7 +91,6 @@ impl NativeTokenTransfer<'_> {
                 instruction_data,
                 &program.into(),
                 tx_pre_check,
-                0,
             )
             .await
             .map(|(resp, secrets)| {

lez/wallet/src/program_facades/native_token_transfer/shielded.rs

@@ -24,7 +24,6 @@ impl NativeTokenTransfer<'_> {
                 instruction_data,
                 &program.into(),
                 tx_pre_check,
-                0,
             )
             .await
             .map(|(resp, secrets)| {
@@ -58,7 +57,6 @@ impl NativeTokenTransfer<'_> {
                 instruction_data,
                 &program.into(),
                 tx_pre_check,
-                0,
             )
             .await
             .map(|(resp, secrets)| {