Draft: Add PIR spendability support to zcash_client_sqlite

zcash_client_backend/Cargo.toml

@@ -266,6 +266,10 @@ non-standard-fees = ["zcash_primitives/non-standard-fees"]
 
 #! ### Experimental features
 
+## Enables PIR-based Orchard note spendability: nullifier spend detection and
+## witness construction via Private Information Retrieval.
+spendability-pir = ["orchard"]
+
 ## Exposes unstable APIs. Their behaviour may change at any time.
 unstable = ["dep:byteorder", "zcash_keys/unstable"]
 

zcash_client_backend/src/data_api.rs

@@ -3105,6 +3105,14 @@ pub trait WalletWrite: WalletRead {
     }
 }
 
+/// The result of a PIR Orchard witness lookup: Merkle path, anchor height, and anchor root.
+#[cfg(feature = "orchard")]
+pub type PirOrchardWitness = (
+    incrementalmerkletree::MerklePath<orchard::tree::MerkleHashOrchard, 32>,
+    u64,
+    [u8; 32],
+);
+
 /// This trait describes a capability for manipulating wallet note commitment trees.
 #[cfg_attr(feature = "test-dependencies", delegatable_trait)]
 pub trait WalletCommitmentTrees {
@@ -3168,4 +3176,19 @@ pub trait WalletCommitmentTrees {
         start_index: u64,
         roots: &[CommitmentTreeRoot<orchard::tree::MerkleHashOrchard>],
     ) -> Result<(), ShardTreeError<Self::Error>>;
+
+    /// Retrieves a PIR-provided Orchard Merkle authentication path for the note at the
+    /// given commitment tree position. Returns the path, anchor height, and anchor root.
+    ///
+    /// The default implementation returns `Ok(None)`, indicating no PIR witness is
+    /// available. See `zcash_client_sqlite::WalletDb` for the production implementation
+    /// backed by the `pir_witness_data` table.
+    #[cfg(feature = "orchard")]
+    fn get_pir_orchard_merkle_path(
+        &self,
+        position: incrementalmerkletree::Position,
+    ) -> Result<Option<PirOrchardWitness>, Self::Error> {
+        let _ = position;
+        Ok(None)
+    }
 }

zcash_client_backend/src/data_api/testing.rs

@@ -977,6 +977,8 @@ where
             &SpendingKeys::from_unified_spending_key(usk.clone()),
             ovk_policy,
             &proposal,
+            #[cfg(feature = "spendability-pir")]
+            false,
         )
     }
 
@@ -1141,6 +1143,35 @@ where
             &SpendingKeys::from_unified_spending_key(usk.clone()),
             ovk_policy,
             proposal,
+            #[cfg(feature = "spendability-pir")]
+            false,
+        )
+    }
+
+    /// Like [`Self::create_proposed_transactions`] but uses PIR-stored witnesses
+    /// instead of ShardTree witnesses for Orchard spends.
+    #[cfg(feature = "spendability-pir")]
+    #[allow(clippy::type_complexity)]
+    pub fn create_proposed_transactions_pir<InputsErrT, FeeRuleT, ChangeErrT, N>(
+        &mut self,
+        usk: &UnifiedSpendingKey,
+        ovk_policy: OvkPolicy,
+        proposal: &Proposal<FeeRuleT, N>,
+    ) -> Result<NonEmpty<TxId>, super::wallet::CreateErrT<DbT, InputsErrT, FeeRuleT, ChangeErrT, N>>
+    where
+        FeeRuleT: FeeRule,
+    {
+        let prover = LocalTxProver::bundled();
+        let network = self.network().clone();
+        create_proposed_transactions(
+            self.wallet_mut(),
+            &network,
+            &prover,
+            &prover,
+            &SpendingKeys::from_unified_spending_key(usk.clone()),
+            ovk_policy,
+            proposal,
+            true,
         )
     }