Implement derivation of the internal Sapling spending key.

src/rust/include/librustzcash.h

@@ -299,6 +299,12 @@ extern "C" {
         unsigned char *xsk_i
     );
 
+    /// Derive a internal ExtendedSpendingKey from an external key
+    void librustzcash_zip32_xsk_derive_internal(
+        const unsigned char *xsk_external,
+        unsigned char *xsk_internal
+    );
+
     /// Derive a child ExtendedFullViewingKey from a parent.
     bool librustzcash_zip32_xfvk_derive(
         const unsigned char *xfvk_parent,

src/rust/src/rustzcash.rs

@@ -1059,6 +1059,23 @@ pub extern "C" fn librustzcash_zip32_xsk_derive(
         .expect("should be able to serialize an ExtendedSpendingKey");
 }
 
+/// Derive the Sapling internal spending key from the external extended
+/// spending key
+#[no_mangle]
+pub extern "C" fn librustzcash_zip32_xsk_derive_internal(
+    xsk_external: *const [c_uchar; 169],
+    xsk_internal_ret: *mut [c_uchar; 169],
+) {
+    let xsk_external = zip32::ExtendedSpendingKey::read(&unsafe { *xsk_external }[..])
+        .expect("valid ExtendedSpendingKey");
+
+    let xsk_internal = xsk_external.derive_internal();
+
+    xsk_internal
+        .write(&mut (unsafe { &mut *xsk_internal_ret })[..])
+        .expect("should be able to serialize an ExtendedSpendingKey");
+}
+
 /// Derive a child ExtendedFullViewingKey from a parent.
 #[no_mangle]
 pub extern "C" fn librustzcash_zip32_xfvk_derive(
@@ -1081,7 +1098,7 @@ pub extern "C" fn librustzcash_zip32_xfvk_derive(
     true
 }
 
-/// Derive the Sapling internal
+/// Derive the Sapling internal full viewing key from the corresponding external full viewing key
 #[no_mangle]
 pub extern "C" fn librustzcash_zip32_sapling_derive_internal_fvk(
     fvk: *const [c_uchar; 96],

src/wallet/rpcwallet.cpp

@@ -4013,6 +4013,15 @@ UniValue z_viewtransaction(const UniValue& params, bool fHelp)
         // Generate the common ovk for recovering t->z outputs.
         HDSeed seed = pwalletMain->GetHDSeedForRPC();
         ovks.insert(ovkForShieldingFromTaddr(seed));
+
+        auto legacyAcctUFVK = pwalletMain->GetUnifiedFullViewingKeyByAccount(ZCASH_LEGACY_ACCOUNT);
+        if (legacyAcctUFVK.has_value()) {
+            auto legacyAcctOVKs = legacyAcctUFVK.value().GetTransparentOVKsForShielding();
+            if (legacyAcctOVKs.has_value()) {
+                ovks.insert(legacyAcctOVKs.value().first);
+                ovks.insert(legacyAcctOVKs.value().second);
+            }
+        }
     }
 
     // Sapling spends

src/wallet/wallet.cpp

@@ -504,6 +504,13 @@ std::optional<libzcash::ZcashdUnifiedSpendingKey>
             throw std::runtime_error("CWalletDB::GenerateUnifiedSpendingKeyForAccount(): Unable to add Sapling key component to the wallet.");
         }
 
+        // Add the Sapling change key to the wallet
+        auto saplingChangeEsk = saplingEsk.DeriveInternalKey();
+        if (addSpendingKey(saplingChangeEsk) == KeyNotAdded) {
+            // If adding the Sapling change key to the wallet failed, abort the process.
+            throw std::runtime_error("CWalletDB::GenerateUnifiedSpendingKeyForAccount(): Unable to add Sapling key component to the wallet.");
+        }
+
         auto zufvk = ZcashdUnifiedFullViewingKey::FromUnifiedFullViewingKey(Params(), ufvk);
         if (!CCryptoKeyStore::AddUnifiedFullViewingKey(zufvk)) {
             throw std::runtime_error("CWalletDB::GenerateUnifiedSpendingKeyForAccount(): Failed to add UFVK to the keystore.");

src/zcash/address/zip32.cpp

@@ -232,7 +232,19 @@ SaplingExtendedFullViewingKey SaplingExtendedSpendingKey::ToXFVK() const
 }
 
 SaplingExtendedSpendingKey SaplingExtendedSpendingKey::DeriveInternalKey() const {
-    throw std::runtime_error("Not yet implemented");
+    CDataStream ss_p(SER_NETWORK, PROTOCOL_VERSION);
+    ss_p << *this;
+    CSerializeData external_key_bytes(ss_p.begin(), ss_p.end());
+
+    CSerializeData internal_key_bytes(ZIP32_XSK_SIZE);
+    librustzcash_zip32_xsk_derive_internal(
+        reinterpret_cast<unsigned char*>(external_key_bytes.data()),
+        reinterpret_cast<unsigned char*>(internal_key_bytes.data()));
+
+    CDataStream ss_i(internal_key_bytes, SER_NETWORK, PROTOCOL_VERSION);
+    SaplingExtendedSpendingKey xsk_internal;
+    ss_i >> xsk_internal;
+    return xsk_internal;
 }
 
 HDKeyPath Zip32AccountKeyPath(