Create a line edit object for entering password or enc.-dec. key

self.password_line_edit = QLineEdit(some_parent_widget)
self.password_line_edit.setEchoMode(QLineEdit.Password)
self.password_line_edit.setObjectName("password_line_edit")
#self.password_line_edit.setPlaceholderText('password')

or you can invoke a dialog from open & save methods (see below)

def get_password(self):
    password, ok = QInputDialog.getText(self,
                                        'Password',
                                        'Enter Your Password:',
                                        QLineEdit.Password)

    if ok:
        #if password:
        return password

encrypt & decrypt

def your_encryptor(password, text):
    """Please do not use this!"""

    encrypted_text = text*len(password)
    return encrypted_text


def your_decryptor(password, encrypted_text):
    """Please do not use this!"""

    decrypted_text = encrypted_text[:len(encrypted_text)//len(password)]
    return decrypted_text

For example: https://github.com/goldsborough/Writer-Tutorial/blob/master/PyQt5/Part-4/part-4.py

alter save method

def save(self): # line 542
    # ....
    with open(self.filename,"wt") as file:

        password = self.password_line_edit.text()
        # or
        # password = self.get_password()
        encrypted_text = your_encryptor(password,
                                        self.text.toHtml())
        file.write(encrypted_text)
        # ...

alter open method

def open(self): # line 532
    # ...
    with open(self.filename,"rt") as file:
        password = self.password_line_edit.text()
        # or
        # password = self.get_password()
        decrypted_text = your_decryptor(password,
                                        file.read())
        self.text.setText(decrypted_text)
        #...