def sign_transaction(self, tx, password):
        if tx.is_complete():
            return

        try:
            p2pkhTransaction = True
            derivations = self.get_tx_derivations(tx)
            inputhasharray = []
            hasharray = []
            pubkeyarray = []

            # Build hasharray from inputs
            for i, txin in enumerate(tx.inputs()):
                if txin['type'] == 'coinbase':
                    self.give_error(
                        "Coinbase not supported")  # should never happen

                if txin['type'] != 'p2pkh':
                    p2pkhTransaction = False

                for x_pubkey in txin['x_pubkeys']:
                    if x_pubkey in derivations:
                        index = derivations.get(x_pubkey)
                        inputPath = "%s/%d/%d" % (self.get_derivation(),
                                                  index[0], index[1])
                        inputHash = Hash(
                            binascii.unhexlify(tx.serialize_preimage(i)))
                        hasharray_i = {
                            'hash': to_hexstr(inputHash),
                            'keypath': inputPath
                        }
                        hasharray.append(hasharray_i)
                        inputhasharray.append(inputHash)
                        break
                else:
                    self.give_error("No matching x_key for sign_transaction"
                                    )  # should never happen

            # Build pubkeyarray from outputs
            for _type, address, amount in tx.outputs():
                assert _type == TYPE_ADDRESS
                info = tx.output_info.get(address)
                if info is not None:
                    index, xpubs, m = info
                    changePath = self.get_derivation() + "/%d/%d" % index
                    changePubkey = self.derive_pubkey(index[0], index[1])
                    pubkeyarray_i = {
                        'pubkey': changePubkey,
                        'keypath': changePath
                    }
                    pubkeyarray.append(pubkeyarray_i)

            # Special serialization of the unsigned transaction for
            # the mobile verification app.
            # At the moment, verification only works for p2pkh transactions.
            if p2pkhTransaction:

                class CustomTXSerialization(Transaction):
                    @classmethod
                    def input_script(self, txin, estimate_size=False):
                        if txin['type'] == 'p2pkh':
                            return Transaction.get_preimage_script(txin)
                        if txin['type'] == 'p2sh':
                            # Multisig verification has partial support, but is disabled. This is the
                            # expected serialization though, so we leave it here until we activate it.
                            return '00' + push_script(
                                Transaction.get_preimage_script(txin))
                        raise Exception("unsupported type %s" % txin['type'])

                tx_dbb_serialized = CustomTXSerialization(
                    tx.serialize()).serialize()
            else:
                # We only need this for the signing echo / verification.
                tx_dbb_serialized = None

            # Build sign command
            dbb_signatures = []
            steps = math.ceil(1.0 * len(hasharray) / self.maxInputs)
            for step in range(int(steps)):
                hashes = hasharray[step * self.maxInputs:(step + 1) *
                                   self.maxInputs]

                msg = {
                    "sign": {
                        "data": hashes,
                        "checkpub": pubkeyarray,
                    },
                }
                if tx_dbb_serialized is not None:
                    msg["sign"]["meta"] = to_hexstr(Hash(tx_dbb_serialized))
                msg = json.dumps(msg).encode('ascii')
                dbb_client = self.plugin.get_client(self)

                if not dbb_client.is_paired():
                    raise Exception("Could not sign transaction.")

                reply = dbb_client.hid_send_encrypt(msg)
                if 'error' in reply:
                    raise Exception(reply['error']['message'])

                if 'echo' not in reply:
                    raise Exception("Could not sign transaction.")

                if self.plugin.is_mobile_paired(
                ) and tx_dbb_serialized is not None:
                    reply['tx'] = tx_dbb_serialized
                    self.plugin.comserver_post_notification(reply)

                if steps > 1:
                    self.handler.show_message(
                        _("Signing large transaction. Please be patient ...") +
                        "\n\n" +
                        _("To continue, touch the Digital Bitbox's blinking light for 3 seconds."
                          ) + " " +
                        _("(Touch {} of {})").format((step + 1), steps) +
                        "\n\n" +
                        _("To cancel, briefly touch the blinking light or wait for the timeout."
                          ) + "\n\n")
                else:
                    self.handler.show_message(
                        _("Signing transaction...") + "\n\n" +
                        _("To continue, touch the Digital Bitbox's blinking light for 3 seconds."
                          ) + "\n\n" +
                        _("To cancel, briefly touch the blinking light or wait for the timeout."
                          ))

                # Send twice, first returns an echo for smart verification
                reply = dbb_client.hid_send_encrypt(msg)
                self.handler.finished()

                if 'error' in reply:
                    if reply["error"].get('code') in (600, 601):
                        # aborted via LED short touch or timeout
                        raise UserCancelled()
                    raise Exception(reply['error']['message'])

                if 'sign' not in reply:
                    raise Exception("Could not sign transaction.")

                dbb_signatures.extend(reply['sign'])

            # Fill signatures
            if len(dbb_signatures) != len(tx.inputs()):
                raise Exception("Incorrect number of transactions signed."
                                )  # Should never occur
            for i, txin in enumerate(tx.inputs()):
                num = txin['num_sig']
                for pubkey in txin['pubkeys']:
                    signatures = list(filter(None, txin['signatures']))
                    if len(signatures) == num:
                        break  # txin is complete
                    ii = txin['pubkeys'].index(pubkey)
                    signed = dbb_signatures[i]
                    if 'recid' in signed:
                        # firmware > v2.1.1
                        recid = int(signed['recid'], 16)
                        s = binascii.unhexlify(signed['sig'])
                        h = inputhasharray[i]
                        pk = MyVerifyingKey.from_signature(s,
                                                           recid,
                                                           h,
                                                           curve=SECP256k1)
                        pk = to_hexstr(point_to_ser(pk.pubkey.point, True))
                    elif 'pubkey' in signed:
                        # firmware <= v2.1.1
                        pk = signed['pubkey']
                    if pk != pubkey:
                        continue
                    sig_r = int(signed['sig'][:64], 16)
                    sig_s = int(signed['sig'][64:], 16)
                    sig = sigencode_der(sig_r, sig_s,
                                        generator_secp256k1.order())
                    txin['signatures'][ii] = to_hexstr(sig) + int_to_hex(
                        Transaction.nHashType() & 255, 1)
                    tx._inputs[i] = txin
        except UserCancelled:
            raise
        except BaseException as e:
            self.give_error(e, True)
        else:
            print_error("Transaction is_complete", tx.is_complete())
            tx.raw = tx.serialize()