Merge #859: Enable external change addresses

ad8cd74 Enable external/custom change addresses. (csH7KmCC9)

jmclient/jmclient/__init__.py

@@ -19,7 +19,7 @@ from .wallet import (Mnemonic, estimate_tx_fee, WalletError, BaseWallet, ImportW
 from .storage import (Argon2Hash, Storage, StorageError, RetryableStorageError,
                       StoragePasswordError, VolatileStorage)
 from .cryptoengine import (BTCEngine, BTC_P2PKH, BTC_P2SH_P2WPKH, BTC_P2WPKH, EngineError,
-                           TYPE_P2PKH, TYPE_P2SH_P2WPKH, TYPE_P2WPKH)
+                           TYPE_P2PKH, TYPE_P2SH_P2WPKH, TYPE_P2WPKH, detect_script_type)
 from .configure import (load_test_config, process_shutdown,
     load_program_config, jm_single, get_network, update_persist_config,
     validate_address, is_burn_destination, get_irc_mchannels,
@@ -36,7 +36,8 @@ from .podle import (set_commitment_file, get_commitment_file,
                     PoDLE, generate_podle, get_podle_commitments,
                     update_commitments)
 from .output import generate_podle_error_string, fmt_utxos, fmt_utxo,\
-    fmt_tx_data
+    fmt_tx_data, general_custom_change_warning, nonwallet_custom_change_warning,\
+    sweep_custom_change_warning
 from .schedule import (get_schedule, get_tumble_schedule, schedule_to_text,
                        tweak_tumble_schedule, human_readable_schedule_entry,
                        schedule_to_text, NO_ROUNDING)

jmclient/jmclient/cli_options.py

@@ -519,6 +519,13 @@ def get_sendpayment_parser():
                       'broadcasting the transaction. '
                       'Currently only works with direct '
                       'send (-N 0).')
+    parser.add_option('-u',
+                      '--custom-change',
+                      type="str",
+                      dest='customchange',
+                      default='',
+                      help='specify address to receive change '
+                      ', by default use in-wallet address.')
 
     add_common_options(parser)
     return parser

jmclient/jmclient/output.py

@@ -1,5 +1,36 @@
 from jmbase import utxo_to_utxostr
 
+general_custom_change_warning = """You are attempting to send change to a custom change
+address. Change outputs are usually directly linkable to
+your CoinJoin inputs, and incautious combination of
+custom change UTXOs can catastrophically compromise
+your CoinJoin privacy, especially if those UTXOs are from
+different mixdepths.
+
+Are you sure you know what you're doing?"""
+
+nonwallet_custom_change_warning ="""
+The custom change address type is different from your wallet
+address type.
+
+Be extremely careful here: It will be obvious to any blockchain
+observer that this output was disposed of by the taker (i.e.
+you) and is directly linkable to your CoinJoin inputs.
+
+Sending change in a one-off transaction to a party with a
+different address type than this wallet is otherwise probably
+OK.
+
+HOWEVER, if you regularly send your change to unusual
+address types, especially multisig P2(W)SH addresses, you
+seriously risk linking ALL of those CoinJoins to you,
+REGARDLESS of how carefully you spend those custom change
+UTXOs.
+
+Are you sure you want to continue?"""
+
+sweep_custom_change_warning = \
+    "Custom change cannot be set while doing a sweep (zero amount)."
 
 def fmt_utxos(utxos, wallet_service, prefix=''):
     output = []

jmclient/jmclient/taker.py

@@ -33,6 +33,7 @@ class Taker(object):
                  order_chooser=weighted_order_choose,
                  callbacks=None,
                  tdestaddrs=None,
+                 custom_change_address=None,
                  ignored_makers=None):
         """`schedule`` must be a list of tuples: (see sample_schedule_for_testnet
         for explanation of syntax, also schedule.py module in this directory),
@@ -84,6 +85,7 @@ class Taker(object):
         self.schedule = schedule
         self.order_chooser = order_chooser
         self.max_cj_fee = max_cj_fee
+        self.my_change_addr = custom_change_address
 
         #List (which persists between transactions) of makers
         #who have not responded or behaved maliciously at any
@@ -288,13 +290,13 @@ class Taker(object):
         if not self.my_cj_addr:
             #previously used for donations; TODO reimplement?
             raise NotImplementedError
-        self.my_change_addr = None
         if self.cjamount != 0:
-            try:
-                self.my_change_addr = self.wallet_service.get_internal_addr(self.mixdepth)
-            except:
-                self.taker_info_callback("ABORT", "Failed to get a change address")
-                return False
+            if self.my_change_addr is None:
+                try:
+                    self.my_change_addr = self.wallet_service.get_internal_addr(self.mixdepth)
+                except:
+                    self.taker_info_callback("ABORT", "Failed to get a change address")
+                    return False
             #adjust the required amount upwards to anticipate an increase in
             #transaction fees after re-estimation; this is sufficiently conservative
             #to make failures unlikely while keeping the occurence of failure to
@@ -314,6 +316,7 @@ class Taker(object):
         else:
             #sweep
             self.input_utxos = self.wallet_service.get_utxos_by_mixdepth()[self.mixdepth]
+            self.my_change_addr = None
             #do our best to estimate the fee based on the number of
             #our own utxos; this estimate may be significantly higher
             #than the default set in option.txfee * makercount, where

jmclient/jmclient/taker_utils.py

@@ -24,7 +24,7 @@ Currently re-used by CLI script tumbler.py and joinmarket-qt
 def direct_send(wallet_service, amount, mixdepth, destination, answeryes=False,
                 accept_callback=None, info_callback=None, error_callback=None,
                 return_transaction=False, with_final_psbt=False,
-                optin_rbf=False):
+                optin_rbf=False, custom_change_addr=None):
     """Send coins directly from one mixdepth to one destination address;
     does not need IRC. Sweep as for normal sendpayment (set amount=0).
     If answeryes is True, callback/command line query is not performed.
@@ -34,7 +34,9 @@ def direct_send(wallet_service, amount, mixdepth, destination, answeryes=False,
     accept_callback:
     ====
     args:
-    deserialized tx, destination address, amount in satoshis, fee in satoshis
+    deserialized tx, destination address, amount in satoshis,
+    fee in satoshis, custom change address
+
     returns:
     True if accepted, False if not
     ====
@@ -52,6 +54,8 @@ def direct_send(wallet_service, amount, mixdepth, destination, answeryes=False,
     """
     #Sanity checks
     assert validate_address(destination)[0] or is_burn_destination(destination)
+    assert custom_change_addr is None or validate_address(custom_change_addr)[0]
+    assert amount > 0 or custom_change_addr is None
     assert isinstance(mixdepth, numbers.Integral)
     assert mixdepth >= 0
     assert isinstance(amount, numbers.Integral)
@@ -77,6 +81,7 @@ def direct_send(wallet_service, amount, mixdepth, destination, answeryes=False,
 
     txtype = wallet_service.get_txtype()
     if amount == 0:
+        #doing a sweep
         utxos = wallet_service.get_utxos_by_mixdepth()[mixdepth]
         if utxos == {}:
             log.error(
@@ -105,10 +110,11 @@ def direct_send(wallet_service, amount, mixdepth, destination, answeryes=False,
                 + wallet_service.wallet.get_path_repr(path) \
                 + "\n\nWARNING: This transaction if broadcasted will PERMANENTLY DESTROY your bitcoins\n"
         else:
-            #regular send (non-burn)
+            #regular sweep (non-burn)
             fee_est = estimate_tx_fee(len(utxos), 1, txtype=txtype)
             outs = [{"address": destination, "value": total_inputs_val - fee_est}]
     else:
+        #not doing a sweep; we will have change
         #8 inputs to be conservative
         initial_fee_est = estimate_tx_fee(8,2, txtype=txtype)
         utxos = wallet_service.select_utxos(mixdepth, amount + initial_fee_est)
@@ -119,7 +125,8 @@ def direct_send(wallet_service, amount, mixdepth, destination, answeryes=False,
         total_inputs_val = sum([va['value'] for u, va in utxos.items()])
         changeval = total_inputs_val - fee_est - amount
         outs = [{"value": amount, "address": destination}]
-        change_addr = wallet_service.get_internal_addr(mixdepth)
+        change_addr = wallet_service.get_internal_addr(mixdepth) if custom_change_addr is None \
+                      else custom_change_addr
         outs.append({"value": changeval, "address": change_addr})
 
     #compute transaction locktime, has special case for spending timelocked coins
@@ -175,7 +182,11 @@ def direct_send(wallet_service, amount, mixdepth, destination, answeryes=False,
         log.info("Got signed transaction:\n")
         log.info(human_readable_transaction(tx))
         actual_amount = amount if amount != 0 else total_inputs_val - fee_est
-        log.info("Sends: " + amount_to_str(actual_amount) + " to destination: " + destination)
+        sending_info = "Sends: " + amount_to_str(actual_amount) + \
+            " to destination: " + destination
+        if custom_change_addr:
+            sending_info += ", custom change to: " + custom_change_addr
+        log.info(sending_info)
         if not answeryes:
             if not accept_callback:
                 if input('Would you like to push to the network? (y/n):')[0] != 'y':
@@ -183,7 +194,8 @@ def direct_send(wallet_service, amount, mixdepth, destination, answeryes=False,
                     return False
             else:
                 accepted = accept_callback(human_readable_transaction(tx),
-                                           destination, actual_amount, fee_est)
+                                           destination, actual_amount, fee_est,
+                                           custom_change_addr)
                 if not accepted:
                     return False
         if jm_single().bc_interface.pushtx(tx.serialize()):

jmclient/test/commontest.py

@@ -26,7 +26,8 @@ PINL = '\r\n' if OS == 'Windows' else '\n'
 default_max_cj_fee = (1, float('inf'))
 
 # callbacks for making transfers in-script with direct_send:
-def dummy_accept_callback(tx, destaddr, actual_amount, fee_est):
+def dummy_accept_callback(tx, destaddr, actual_amount, fee_est,
+                          custom_change_addr):
     return True
 def dummy_info_callback(msg):
     pass

jmclient/test/test_taker.py

@@ -13,8 +13,8 @@ from base64 import b64encode
 from jmbase import utxostr_to_utxo, hextobin
 from jmclient import load_test_config, jm_single, set_commitment_file,\
     get_commitment_file, SegwitWallet, Taker, VolatileStorage,\
-    get_network, WalletService, NO_ROUNDING, BTC_P2PKH,\
-    NotEnoughFundsException
+    get_network, WalletService, NO_ROUNDING, NotEnoughFundsException,\
+    BTC_P2SH_P2WPKH, BTC_P2PKH, BTC_P2WPKH
 from taker_test_data import t_utxos_by_mixdepth, t_orderbook,\
     t_maker_response, t_chosen_orders, t_dummy_ext
 from commontest import default_max_cj_fee
@@ -146,7 +146,7 @@ def dummy_filter_orderbook(orders_fees, cjamount):
     return True
 
 def get_taker(schedule=None, schedule_len=0, on_finished=None,
-              filter_orders=None):
+              filter_orders=None, custom_change=None):
     if not schedule:
         #note, for taker.initalize() this will result in junk
         schedule = [['a', 'b', 'c', 'd', 'e', 'f']]*schedule_len
@@ -154,7 +154,8 @@ def get_taker(schedule=None, schedule_len=0, on_finished=None,
     on_finished_callback = on_finished if on_finished else taker_finished
     filter_orders_callback = filter_orders if filter_orders else dummy_filter_orderbook
     taker = Taker(WalletService(DummyWallet()), schedule, default_max_cj_fee,
-                  callbacks=[filter_orders_callback, None, on_finished_callback])
+                  callbacks=[filter_orders_callback, None, on_finished_callback],
+                  custom_change_address=custom_change)
     taker.wallet_service.current_blockheight = 10**6
     return taker
 
@@ -438,10 +439,46 @@ def test_taker_init(setup_taker, schedule, highfee, toomuchcoins, minmakers,
         a = taker.coinjoin_address()
     taker.wallet_service.wallet.inject_addr_get_failure = True
     taker.my_cj_addr = "dummy"
+    taker.my_change_addr = None
     assert not taker.prepare_my_bitcoin_data()
     #clean up
     return clean_up()
 
+def test_custom_change(setup_taker):
+    # create three random custom change addresses, one of each
+    # known type in Joinmarket.
+    privs = [x*32 + b"\x01" for x in [struct.pack(b'B', y) for y in range(1,4)]]
+    scripts = [a.key_to_script(i) for a, i in zip([BTC_P2PKH, BTC_P2SH_P2WPKH, BTC_P2WPKH], privs)]
+    addrs = [a.privkey_to_address(i) for a, i in zip([BTC_P2PKH, BTC_P2SH_P2WPKH, BTC_P2WPKH], privs)]
+    schedule = [(0, 20000000, 3, "mnsquzxrHXpFsZeL42qwbKdCP2y1esN3qw", 0, NO_ROUNDING)]
+    for script, addr in zip(scripts, addrs):
+        taker = get_taker(schedule, custom_change=addr)
+        orderbook = copy.deepcopy(t_orderbook)
+        res = taker.initialize(orderbook)
+        taker.orderbook = copy.deepcopy(t_chosen_orders)
+        maker_response = copy.deepcopy(t_maker_response)
+        res = taker.receive_utxos(maker_response)
+        assert res[0]
+        # ensure that the transaction created for signing has
+        # the address we intended with the right amount:
+        custom_change_found = False
+        for out in taker.latest_tx.vout:
+            # input utxo is 20M; amount is 2M; as per logs:
+            # totalin=200000000
+            # my_txfee=12930
+            # makers_txfee=3000
+            # cjfee_total=12000 => changevalue=179975070
+            # note that there is a small variation in the size of
+            # the transaction (a few bytes) for the different scriptPubKey
+            # type, but this is currently ignored by the Taker, who makes
+            # fee estimate purely based on the number of ins and outs;
+            # this will never be too far off anyway.
+            if out.scriptPubKey == script and out.nValue == 179975070:
+                # must be only one
+                assert not custom_change_found
+                custom_change_found = True
+        assert custom_change_found
+
 @pytest.mark.parametrize(
     "schedule_len",
     [

scripts/joinmarket-qt.py

@@ -70,7 +70,10 @@ from jmclient import load_program_config, get_network, update_persist_config,\
     NO_ROUNDING, get_max_cj_fee_values, get_default_max_absolute_fee, \
     get_default_max_relative_fee, RetryableStorageError, add_base_options, \
     BTCEngine, FidelityBondMixin, wallet_change_passphrase, \
-    parse_payjoin_setup, send_payjoin, JMBIP78ReceiverManager
+    parse_payjoin_setup, send_payjoin, JMBIP78ReceiverManager, \
+    detect_script_type, general_custom_change_warning, \
+    nonwallet_custom_change_warning, sweep_custom_change_warning, EngineError
+
 from qtsupport import ScheduleWizard, TumbleRestartWizard, config_tips,\
     config_types, QtHandler, XStream, Buttons, OkButton, CancelButton,\
     PasswordDialog, MyTreeWidget, JMQtMessageBox, BLUE_FG,\
@@ -298,9 +301,11 @@ class SpendTab(QWidget):
         self.pjEndpointLabel.setVisible(True)
         self.pjEndpointInput.setVisible(True)
 
-        # while user is attempting a payjoin, address
+        # while user is attempting a payjoin, address/change
         # cannot be edited; to back out, they hit Abort.
         self.addressInput.setEnabled(False)
+        self.changeInput.setEnabled(False)
+        self.changeInput.clear()
         self.abortButton.setEnabled(True)
 
     def switchToJoinmarket(self):
@@ -314,14 +319,24 @@ class SpendTab(QWidget):
         self.switchToJoinmarket()
         self.addressInput.setText('')
         self.amountInput.setText('')
+        self.changeInput.setText('')
         self.addressInput.setEnabled(True)
         self.pjEndpointInput.setEnabled(True)
         self.mixdepthInput.setEnabled(True)
         self.amountInput.setEnabled(True)
+        self.changeInput.setEnabled(True)
         self.startButton.setEnabled(True)
         self.abortButton.setEnabled(False)
 
     def checkAddress(self, addr):
+        valid, errmsg = validate_address(str(addr))
+        if not valid and len(addr) > 0:
+            JMQtMessageBox(self,
+                       "Bitcoin address not valid.\n" + errmsg,
+                       mbtype='warn',
+                       title="Error")
+
+    def parseURIAndValidateAddress(self, addr):
         addr = addr.strip()
         if btc.is_bip21_uri(addr):
             try:
@@ -344,12 +359,7 @@ class SpendTab(QWidget):
             self.bip21_uri = None
 
         self.addressInput.setText(addr)
-        valid, errmsg = validate_address(str(addr))
-        if not valid and len(addr) > 0:
-            JMQtMessageBox(self,
-                       "Bitcoin address not valid.\n" + errmsg,
-                       mbtype='warn',
-                       title="Error")
+        self.checkAddress(addr)
 
     def checkAmount(self, amount_str):
         if not amount_str:
@@ -519,6 +529,7 @@ class SpendTab(QWidget):
         sch_buttons_box.setLayout(sch_buttons_layout)
         sch_layout.addWidget(sch_buttons_box, 0, 1, 1, 1)
 
+        #construct layout for single joins
         innerTopLayout = QGridLayout()
         innerTopLayout.setSpacing(4)
         self.single_join_tab.setLayout(innerTopLayout)
@@ -531,7 +542,7 @@ class SpendTab(QWidget):
             'The address or bitcoin: URI you want to send the payment to')
         self.addressInput = QLineEdit()
         self.addressInput.editingFinished.connect(
-            lambda: self.checkAddress(self.addressInput.text()))
+            lambda: self.parseURIAndValidateAddress(self.addressInput.text()))
         innerTopLayout.addWidget(recipientLabel, 1, 0)
         innerTopLayout.addWidget(self.addressInput, 1, 1, 1, 2)
 
@@ -570,6 +581,17 @@ class SpendTab(QWidget):
         innerTopLayout.addWidget(amountLabel, 4, 0)
         innerTopLayout.addWidget(self.amountInput, 4, 1, 1, 2)
 
+        changeLabel = QLabel('Custom change address')
+        changeLabel.setToolTip(
+            'Specify an address to receive change, rather ' +
+            'than sending it to the internal wallet.')
+        self.changeInput = QLineEdit()
+        self.changeInput.editingFinished.connect(
+            lambda: self.checkAddress(self.changeInput.text().strip()))
+        self.changeInput.setPlaceholderText("(optional)")
+        innerTopLayout.addWidget(changeLabel, 5, 0)
+        innerTopLayout.addWidget(self.changeInput, 5, 1, 1, 2)
+
         self.startButton = QPushButton('Start')
         self.startButton.setToolTip(
             'If "checktx" is selected in the Settings, you will be \n'
@@ -585,7 +607,7 @@ class SpendTab(QWidget):
         buttons.addWidget(self.startButton)
         buttons.addWidget(self.abortButton)
         self.abortButton.clicked.connect(self.abortTransactions)
-        innerTopLayout.addLayout(buttons, 5, 0, 1, 2)
+        innerTopLayout.addLayout(buttons, 6, 0, 1, 2)
         splitter1 = QSplitter(QtCore.Qt.Vertical)
         self.textedit = QTextEdit()
         self.textedit.verticalScrollBar().rangeChanged.connect(
@@ -676,14 +698,18 @@ class SpendTab(QWidget):
             self.updateSchedView()
         self.startJoin()
 
-    def checkDirectSend(self, dtx, destaddr, amount, fee):
+    def checkDirectSend(self, dtx, destaddr, amount, fee, custom_change_addr):
         """Give user info to decide whether to accept a direct send;
         note the callback includes the full prettified transaction,
         but currently not printing it for space reasons.
         """
         mbinfo = ["Sending " + btc.amount_to_str(amount) + ",",
-                  "to: " + destaddr + ",",
-                  "Fee: " + btc.amount_to_str(fee) + ".",
+                  "to: " + destaddr + ","]
+
+        if custom_change_addr:
+            mbinfo.append("change to: " + custom_change_addr + ",")
+
+        mbinfo += ["fee: " + btc.amount_to_str(fee) + ".",
                   "Accept?"]
         reply = JMQtMessageBox(self, '\n'.join([m + '<p>' for m in mbinfo]),
                                mbtype='question', title="Direct send")
@@ -716,11 +742,15 @@ class SpendTab(QWidget):
         bip78url = self.pjEndpointInput.text()
 
         if makercount == 0 and not bip78url:
+            custom_change = None
+            if len(self.changeInput.text().strip()) > 0:
+                custom_change = str(self.changeInput.text().strip())
             try:
                 txid = direct_send(mainWindow.wallet_service, amount, mixdepth,
                                   destaddr, accept_callback=self.checkDirectSend,
                                   info_callback=self.infoDirectSend,
-                                  error_callback=self.errorDirectSend)
+                                  error_callback=self.errorDirectSend,
+                                  custom_change_addr=custom_change)
             except Exception as e:
                 JMQtMessageBox(self, e.args[0], title="Error", mbtype="warn")
                 return
@@ -761,6 +791,7 @@ class SpendTab(QWidget):
             self.pjEndpointInput.setEnabled(False)
             self.mixdepthInput.setEnabled(False)
             self.amountInput.setEnabled(False)
+            self.changeInput.setEnabled(False)
             self.startButton.setEnabled(False)
             d = task.deferLater(reactor, 0.0, send_payjoin, manager,
                     accept_callback=self.checkDirectSend,
@@ -826,6 +857,9 @@ class SpendTab(QWidget):
             check_offers_callback = None
 
         destaddrs = self.tumbler_destaddrs if self.tumbler_options else []
+        custom_change = None
+        if len(self.changeInput.text().strip()) > 0:
+            custom_change = str(self.changeInput.text().strip())
         maxcjfee = get_max_cj_fee_values(jm_single().config, None,
                                          user_callback=self.getMaxCJFees)
         log.info("Using maximum coinjoin fee limits per maker of {:.4%}, {} "
@@ -838,6 +872,7 @@ class SpendTab(QWidget):
                                       self.takerInfo,
                                       self.takerFinished],
                            tdestaddrs=destaddrs,
+                           custom_change_address=custom_change,
                            ignored_makers=ignored_makers)
         if not self.clientfactory:
             #First run means we need to start: create clientfactory
@@ -1117,6 +1152,55 @@ class SpendTab(QWidget):
                 "Amount, in bitcoins, must be provided.",
                 mbtype='warn', title="Error")
             return False
+        if len(self.changeInput.text().strip()) != 0:
+            dest_addr = str(self.addressInput.text().strip())
+            change_addr = str(self.changeInput.text().strip())
+            makercount = int(self.numCPInput.text())
+            try:
+                amount = btc.amount_to_sat(self.amountInput.text())
+            except ValueError as e:
+                JMQtMessageBox(self, e.args[0], title="Error", mbtype="warn")
+                return False
+            valid, errmsg = validate_address(change_addr)
+            if not valid:
+                JMQtMessageBox(self,
+                               "Custom change address is invalid: \"%s\"" % errmsg,
+                               mbtype='warn', title="Error")
+                return False
+
+            if change_addr == dest_addr:
+                msg = ''.join(["Custom change address cannot be the ",
+                               "same as the recipient address."])
+                JMQtMessageBox(self,
+                               msg,
+                               mbtype='warn', title="Error")
+                return False
+            if amount == 0:
+                JMQtMessageBox(self, sweep_custom_change_warning,
+                               mbtype='warn', title="Error")
+                return False
+            if makercount > 0:
+                reply = JMQtMessageBox(self, general_custom_change_warning,
+                                       mbtype='question', title="Warning")
+                if reply == QMessageBox.No:
+                    return False
+
+            change_spk = mainWindow.wallet_service.addr_to_script(change_addr)
+            engine_recognized = True
+            try:
+                change_addr_type = detect_script_type(change_spk)
+            except EngineError:
+                engine_recognized = False
+            wallet_type = mainWindow.wallet_service.TYPE
+            if (not engine_recognized) or (
+                change_addr_type != wallet_type and makercount > 0):
+                reply = JMQtMessageBox(self,
+                                       nonwallet_custom_change_warning,
+                                       mbtype='question',
+                                       title="Warning")
+                if reply == QMessageBox.No:
+                    return False
+
         return True
 
 class TxHistoryTab(QWidget):

scripts/sendpayment.py

@@ -16,7 +16,9 @@ from jmclient import Taker, load_program_config, get_schedule,\
     jm_single, estimate_tx_fee, direct_send, WalletService,\
     open_test_wallet_maybe, get_wallet_path, NO_ROUNDING, \
     get_sendpayment_parser, get_max_cj_fee_values, check_regtest, \
-    parse_payjoin_setup, send_payjoin
+    parse_payjoin_setup, send_payjoin, general_custom_change_warning, \
+    nonwallet_custom_change_warning, sweep_custom_change_warning, \
+    detect_script_type, EngineError
 from twisted.python.log import startLogging
 from jmbase.support import get_log, jmprint, \
     EXIT_FAILURE, EXIT_ARGERROR, DUST_THRESHOLD
@@ -202,9 +204,46 @@ def main():
             log.info("Estimated miner/tx fees for this coinjoin amount: {:.1%}"
                 .format(exp_tx_fees_ratio))
 
+    custom_change = None
+    if options.customchange != '':
+        addr_valid, errormsg = validate_address(options.customchange)
+        if not addr_valid:
+            parser.error(
+                "The custom change address provided is not valid\n{}".format(
+                errormsg))
+            sys.exit(EXIT_ARGERROR)
+        custom_change = options.customchange
+        if destaddr and custom_change == destaddr:
+            parser.error("The custom change address cannot be the same as the "
+                         "destination address.")
+            sys.exit(EXIT_ARGERROR)
+        if sweeping:
+            parser.error(sweep_custom_change_warning)
+            sys.exit(EXIT_ARGERROR)
+        if bip78url:
+            parser.error("Custom change is not currently supported "
+                "with Payjoin. Please retry without a custom change address.")
+            sys.exit(EXIT_ARGERROR)
+        if options.makercount > 0:
+            if not options.answeryes and input(
+                general_custom_change_warning + " (y/n):")[0] != "y":
+                sys.exit(EXIT_ARGERROR)
+            change_spk = wallet_service.addr_to_script(custom_change)
+            engine_recognized = True
+            try:
+                change_addr_type = detect_script_type(change_spk)
+            except EngineError:
+                engine_recognized = False
+            if (not engine_recognized) or (
+                change_addr_type != wallet_service.TYPE):
+                if not options.answeryes and input(
+                    nonwallet_custom_change_warning + " (y/n):")[0] != "y":
+                    sys.exit(EXIT_ARGERROR)
+
     if options.makercount == 0 and not bip78url:
         tx = direct_send(wallet_service, amount, mixdepth, destaddr,
-                         options.answeryes, with_final_psbt=options.with_psbt)
+                         options.answeryes, with_final_psbt=options.with_psbt,
+                         custom_change_addr=custom_change)
         if options.with_psbt:
             log.info("This PSBT is fully signed and can be sent externally for "
                      "broadcasting:")
@@ -315,7 +354,8 @@ def main():
                       schedule,
                       order_chooser=chooseOrdersFunc,
                       max_cj_fee=maxcjfee,
-                      callbacks=(filter_orders_callback, None, taker_finished))
+                      callbacks=(filter_orders_callback, None, taker_finished),
+                      custom_change_address=custom_change)
     clientfactory = JMClientProtocolFactory(taker)
 
     if jm_single().config.get("BLOCKCHAIN", "network") == "regtest":