commands: add new cmd "getprivatekeyforpath" to export a WIF for a path

related: #6061

electrum/address_synchronizer.py

@@ -449,7 +449,7 @@ class AddressSynchronizer(Logger):
         domain = set(domain)
         # 1. Get the history of each address in the domain, maintain the
         #    delta of a tx as the sum of its deltas on domain addresses
-        tx_deltas = defaultdict(int)
+        tx_deltas = defaultdict(int)  # type: Dict[str, Optional[int]]
         for addr in domain:
             h = self.get_address_history(addr)
             for tx_hash, height in h:

electrum/bitcoin.py

@@ -565,7 +565,7 @@ def is_segwit_script_type(txin_type: str) -> bool:
     return txin_type in ('p2wpkh', 'p2wpkh-p2sh', 'p2wsh', 'p2wsh-p2sh')
 
 
-def serialize_privkey(secret: bytes, compressed: bool, txin_type: str,
+def serialize_privkey(secret: bytes, compressed: bool, txin_type: str, *,
                       internal_use: bool = False) -> str:
     # we only export secrets inside curve range
     secret = ecc.ECPrivkey.normalize_secret_bytes(secret)

electrum/commands.py

@@ -414,6 +414,13 @@ class Commands:
         domain = address
         return [wallet.export_private_key(address, password) for address in domain]
 
+    @command('wp')
+    async def getprivatekeyforpath(self, path, password=None, wallet: Abstract_Wallet = None):
+        """Get private key corresponding to derivation path (address index).
+        'path' can be either a str such as "m/0/50", or a list of ints such as [0, 50].
+        """
+        return wallet.export_private_key_for_path(path, password)
+
     @command('w')
     async def ismine(self, address, wallet: Abstract_Wallet = None):
         """Check if address is in wallet. Return true if and only address is in wallet"""

electrum/tests/test_commands.py

@@ -180,3 +180,17 @@ class TestCommandsTestnet(TestCaseForTestnet):
         }
         self.assertEqual("0200000000010139c5375fe9da7bd377c1783002b129f8c57d3e724d62f5eacb9739ca691a229d0100000000feffffff01301b0f0000000000160014ac0e2d229200bffb2167ed6fd196aef9d687d8bb0247304402206367fb2ddd723985f5f51e0f2435084c0a66f5c26f4403a75d3dd417b71a20450220545dc3637bcb49beedbbdf5063e05cad63be91af4f839886451c30ecd6edf1d20121021f110909ded653828a254515b58498a6bafc96799fb0851554463ed44ca7d9da00000000",
                          cmds._run('serialize', (jsontx,)))
+
+    @mock.patch.object(wallet.Abstract_Wallet, 'save_db')
+    def test_getprivatekeyforpath(self, mock_save_db):
+        wallet = restore_wallet_from_text('north rent dawn bunker hamster invest wagon market romance pig either squeeze',
+                                          gap_limit=2,
+                                          path='if_this_exists_mocking_failed_648151893',
+                                          config=self.config)['wallet']
+        cmds = Commands(config=self.config)
+        self.assertEqual("p2wpkh:cUzm7zPpWgLYeURgff4EsoMjhskCpsviBH4Y3aZcrBX8UJSRPjC2",
+                         cmds._run('getprivatekeyforpath', ([0, 10000],), wallet=wallet))
+        self.assertEqual("p2wpkh:cUzm7zPpWgLYeURgff4EsoMjhskCpsviBH4Y3aZcrBX8UJSRPjC2",
+                         cmds._run('getprivatekeyforpath', ("m/0/10000",), wallet=wallet))
+        self.assertEqual("p2wpkh:cQAj4WGf1socCPCJNMjXYCJ8Bs5JUAk5pbDr4ris44QdgAXcV24S",
+                         cmds._run('getprivatekeyforpath', ("m/5h/100000/88h/7",), wallet=wallet))

electrum/wallet.py

@@ -44,7 +44,7 @@ from abc import ABC, abstractmethod
 import itertools
 
 from .i18n import _
-from .bip32 import BIP32Node, convert_bip32_intpath_to_strpath
+from .bip32 import BIP32Node, convert_bip32_intpath_to_strpath, convert_bip32_path_to_list_of_uint32
 from .crypto import sha256
 from .util import (NotEnoughFunds, UserCancelled, profiler,
                    format_satoshis, format_fee_satoshis, NoDynamicFeeEstimates,
@@ -462,7 +462,7 @@ class Abstract_Wallet(AddressSynchronizer, ABC):
         """Return script type of wallet address."""
         pass
 
-    def export_private_key(self, address, password) -> str:
+    def export_private_key(self, address: str, password: Optional[str]) -> str:
         if self.is_watching_only():
             raise Exception(_("This is a watching-only wallet"))
         if not is_address(address):
@@ -475,6 +475,9 @@ class Abstract_Wallet(AddressSynchronizer, ABC):
         serialized_privkey = bitcoin.serialize_privkey(pk, compressed, txin_type)
         return serialized_privkey
 
+    def export_private_key_for_path(self, path: Union[Sequence[int], str], password: Optional[str]) -> str:
+        raise Exception("this wallet is not deterministic")
+
     @abstractmethod
     def get_public_keys(self, address: str) -> Sequence[str]:
         pass
@@ -2201,6 +2204,13 @@ class Deterministic_Wallet(Abstract_Wallet):
         pubkeys = self.derive_pubkeys(for_change, n)
         return self.pubkeys_to_address(pubkeys)
 
+    def export_private_key_for_path(self, path: Union[Sequence[int], str], password: Optional[str]) -> str:
+        if isinstance(path, str):
+            path = convert_bip32_path_to_list_of_uint32(path)
+        pk, compressed = self.keystore.get_private_key(path, password)
+        txin_type = self.get_txin_type()  # assumes no mixed-scripts in wallet
+        return bitcoin.serialize_privkey(pk, compressed, txin_type)
+
     def get_public_keys_with_deriv_info(self, address: str):
         der_suffix = self.get_address_index(address)
         der_suffix = [int(x) for x in der_suffix]
@@ -2301,7 +2311,7 @@ class Deterministic_Wallet(Abstract_Wallet):
     def get_fingerprint(self):
         return self.get_master_public_key()
 
-    def get_txin_type(self, address):
+    def get_txin_type(self, address=None):
         return self.txin_type