Support HD derivation of master blinding key

src/wallet/wallet.cpp

@@ -123,6 +123,33 @@ CPubKey CWallet::GenerateNewKey()
     return pubkey;
 }
 
+void CWallet::DeriveBlindingKey(CKeyMetadata& metadata, CKey& secret)
+{
+    // Key is derived at m/0'/2' to avoid collision with hd split wallets
+    CKey key;                      //master key seed (256bit)
+    CExtKey masterKey;             //hd master key
+    CExtKey accountKey;            //key at m/0'
+    CExtKey externalChainChildKey; //key at m/0'/2'
+
+    // try to get the master key
+    if (!GetKey(hdChain.masterKeyID, key))
+        throw std::runtime_error(std::string(__func__) + ": Master key not found");
+
+    masterKey.SetMaster(key.begin(), key.size());
+
+    // derive m/0'
+    // use hardened derivation (child keys >= 0x80000000 are hardened after bip32)
+    masterKey.Derive(accountKey, BIP32_HARDENED_KEY_LIMIT);
+
+    // derive m/0'/2'
+    accountKey.Derive(externalChainChildKey, 2 | BIP32_HARDENED_KEY_LIMIT);
+
+    metadata.hdKeypath = "m/0'/2'";
+    metadata.hdMasterKeyID = hdChain.masterKeyID;
+    secret = externalChainChildKey.key;
+    return;
+}
+
 void CWallet::DeriveNewChildKey(CKeyMetadata& metadata, CKey& secret)
 {
     // for now we use a fixed keypath scheme of m/0'/0'/k

src/wallet/wallet.h

@@ -793,6 +793,8 @@ class CWallet : public CCryptoKeyStore, public CValidationInterface
      */
     CPubKey GenerateNewKey();
     void DeriveNewChildKey(CKeyMetadata& metadata, CKey& secret);
+    //! Derives static blinding key at m/0'/2'
+    void DeriveBlindingKey(CKeyMetadata& metadata, CKey& secret);
     //! Adds a key to the store, and saves it to disk.
     bool AddKeyPubKey(const CKey& key, const CPubKey &pubkey) override;
     //! Adds a key to the store, without saving it to disk (used by LoadWallet)

src/wallet/walletdb.cpp

@@ -686,7 +686,13 @@ DBErrors CWalletDB::LoadWallet(CWallet* pwallet)
 
     if (result == DB_LOAD_OK && pwallet->blinding_derivation_key.IsNull()) {
         CKey key;
-        key.MakeNewKey(true);
+        if (pwallet->IsHDEnabled()) {
+            int64_t nCreationTime = GetTime();
+            CKeyMetadata metadata(nCreationTime);
+            pwallet->DeriveBlindingKey(metadata, key);
+        } else {
+            key.MakeNewKey(true);
+        }
         uint256 keybin;
         memcpy(keybin.begin(), key.begin(), key.size());
         pwallet->blinding_derivation_key = keybin;