Reticulum/RNS/Destination.py

# MIT License
#
# Copyright (c) 2016-2024 Mark Qvist / unsigned.io and contributors
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.

import os
import math
import time
import threading
import RNS

from RNS.Cryptography import Token
from .vendor import umsgpack as umsgpack

class Callbacks:
    def __init__(self):
        self.link_established = None
        self.packet = None
        self.proof_requested = None

class Destination:
    """
    A class used to describe endpoints in a Reticulum Network. Destination
    instances are used both to create outgoing and incoming endpoints. The
    destination type will decide if encryption, and what type, is used in
    communication with the endpoint. A destination can also announce its
    presence on the network, which will distribute necessary keys for
    encrypted communication with it.

    :param identity: An instance of :ref:`RNS.Identity<api-identity>`. Can hold only public keys for an outgoing destination, or holding private keys for an ingoing.
    :param direction: ``RNS.Destination.IN`` or ``RNS.Destination.OUT``.
    :param type: ``RNS.Destination.SINGLE``, ``RNS.Destination.GROUP`` or ``RNS.Destination.PLAIN``.
    :param app_name: A string specifying the app name.
    :param \\*aspects: Any non-zero number of string arguments.
    """

    # Constants
    SINGLE     = 0x00
    GROUP      = 0x01
    PLAIN      = 0x02
    LINK       = 0x03
    types      = [SINGLE, GROUP, PLAIN, LINK]

    PROVE_NONE = 0x21
    PROVE_APP  = 0x22
    PROVE_ALL  = 0x23
    proof_strategies = [PROVE_NONE, PROVE_APP, PROVE_ALL]

    ALLOW_NONE = 0x00
    ALLOW_ALL  = 0x01
    ALLOW_LIST = 0x02
    request_policies = [ALLOW_NONE, ALLOW_ALL, ALLOW_LIST]

    IN         = 0x11;
    OUT        = 0x12;
    directions = [IN, OUT]

    PR_TAG_WINDOW = 30

    RATCHET_COUNT    = 512
    """
    The default number of generated ratchet keys a destination will retain, if it has ratchets enabled.
    """

    RATCHET_INTERVAL = 30*60
    """
    The minimum interval between rotating ratchet keys, in seconds.
    """

    @staticmethod
    def expand_name(identity, app_name, *aspects):
        """
        :returns: A string containing the full human-readable name of the destination, for an app_name and a number of aspects.
        """

        # Check input values and build name string
        if "." in app_name: raise ValueError("Dots can't be used in app names")

        name = app_name
        for aspect in aspects:
            if "." in aspect: raise ValueError("Dots can't be used in aspects")
            name += "." + aspect

        if identity != None:
            name += "." + identity.hexhash

        return name


    @staticmethod
    def hash(identity, app_name, *aspects):
        """
        :returns: A destination name in adressable hash form, for an app_name and a number of aspects.
        """
        name_hash = RNS.Identity.full_hash(Destination.expand_name(None, app_name, *aspects).encode("utf-8"))[:(RNS.Identity.NAME_HASH_LENGTH//8)]
        addr_hash_material = name_hash
        if identity != None:
            if isinstance(identity, RNS.Identity):
                addr_hash_material += identity.hash
            elif isinstance(identity, bytes) and len(identity) == RNS.Reticulum.TRUNCATED_HASHLENGTH//8:
                addr_hash_material += identity
            else:
                raise TypeError("Invalid material supplied for destination hash calculation")

        return RNS.Identity.full_hash(addr_hash_material)[:RNS.Reticulum.TRUNCATED_HASHLENGTH//8]

    @staticmethod
    def app_and_aspects_from_name(full_name):
        """
        :returns: A tuple containing the app name and a list of aspects, for a full-name string.
        """
        components = full_name.split(".")
        return (components[0], components[1:])

    @staticmethod
    def hash_from_name_and_identity(full_name, identity):
        """
        :returns: A destination name in adressable hash form, for a full name string and Identity instance.
        """
        app_name, aspects = Destination.app_and_aspects_from_name(full_name)

        return Destination.hash(identity, app_name, *aspects)

    def __init__(self, identity, direction, type, app_name, *aspects):
        # Check input values and build name string
        if "." in app_name: raise ValueError("Dots can't be used in app names") 
        if not type in Destination.types: raise ValueError("Unknown destination type")
        if not direction in Destination.directions: raise ValueError("Unknown destination direction")

        self.accept_link_requests = True
        self.callbacks = Callbacks()
        self.request_handlers = {}
        self.type = type
        self.direction = direction
        self.proof_strategy = Destination.PROVE_NONE
        self.ratchets = None
        self.ratchets_path = None
        self.ratchet_interval = Destination.RATCHET_INTERVAL
        self.ratchet_file_lock = threading.Lock()
        self.retained_ratchets = Destination.RATCHET_COUNT
        self.latest_ratchet_time = None
        self.latest_ratchet_id = None
        self.__enforce_ratchets = False
        self.mtu = 0

        self.path_responses = {}
        self.links = []

        if identity == None and direction == Destination.IN and self.type != Destination.PLAIN:
            identity = RNS.Identity()
            aspects = aspects+(identity.hexhash,)

        if identity == None and direction == Destination.OUT and self.type != Destination.PLAIN:
            raise ValueError("Can't create outbound SINGLE destination without an identity")

        if identity != None and self.type == Destination.PLAIN:
            raise TypeError("Selected destination type PLAIN cannot hold an identity")

        self.identity = identity
        self.name = Destination.expand_name(identity, app_name, *aspects)

        # Generate the destination address hash
        self.hash = Destination.hash(self.identity, app_name, *aspects)
        self.name_hash = RNS.Identity.full_hash(self.expand_name(None, app_name, *aspects).encode("utf-8"))[:(RNS.Identity.NAME_HASH_LENGTH//8)]
        self.hexhash = self.hash.hex()

        self.default_app_data = None
        self.callback = None
        self.proofcallback = None

        RNS.Transport.register_destination(self)


    def __str__(self):
        """
        :returns: A human-readable representation of the destination including addressable hash and full name.
        """
        return "<"+self.name+"/"+self.hexhash+">"

    def _clean_ratchets(self):
        if self.ratchets != None:
            if len (self.ratchets) > self.retained_ratchets:
                self.ratchets = self.ratchets[:Destination.RATCHET_COUNT]

    def _persist_ratchets(self):
        try:
            with self.ratchet_file_lock:
                packed_ratchets = umsgpack.packb(self.ratchets)
                persisted_data = {"signature": self.sign(packed_ratchets), "ratchets": packed_ratchets}
                ratchets_file = open(self.ratchets_path, "wb")
                ratchets_file.write(umsgpack.packb(persisted_data))
                ratchets_file.close()
        except Exception as e:
            self.ratchets = None
            self.ratchets_path = None
            raise OSError("Could not write ratchet file contents for "+str(self)+". The contained exception was: "+str(e), RNS.LOG_ERROR)

    def rotate_ratchets(self):
        if self.ratchets != None:
            now = time.time()
            if now > self.latest_ratchet_time+self.ratchet_interval:
                RNS.log("Rotating ratchets for "+str(self), RNS.LOG_DEBUG)
                new_ratchet = RNS.Identity._generate_ratchet()
                self.ratchets.insert(0, new_ratchet)
                self.latest_ratchet_time = now
                self._clean_ratchets()
                self._persist_ratchets()
            return True
        else:
            raise SystemError("Cannot rotate ratchet on "+str(self)+", ratchets are not enabled")

        return False

    def announce(self, app_data=None, path_response=False, attached_interface=None, tag=None, send=True):
        """
        Creates an announce packet for this destination and broadcasts it on all
        relevant interfaces. Application specific data can be added to the announce.

        :param app_data: *bytes* containing the app_data.
        :param path_response: Internal flag used by :ref:`RNS.Transport<api-transport>`. Ignore.
        """
        if self.type != Destination.SINGLE:
            raise TypeError("Only SINGLE destination types can be announced")

        if self.direction != Destination.IN:
            raise TypeError("Only IN destination types can be announced")
        
        ratchet = b""
        now = time.time()
        stale_responses = []
        for entry_tag in self.path_responses:
            entry = self.path_responses[entry_tag]
            if now > entry[0]+Destination.PR_TAG_WINDOW:
                stale_responses.append(entry_tag)

        for entry_tag in stale_responses:
            self.path_responses.pop(entry_tag)

        if (path_response == True and tag != None) and tag in self.path_responses:
            # This code is currently not used, since Transport will block duplicate
            # path requests based on tags. When multi-path support is implemented in
            # Transport, this will allow Transport to detect redundant paths to the
            # same destination, and select the best one based on chosen criteria,
            # since it will be able to detect that a single emitted announce was
            # received via multiple paths. The difference in reception time will
            # potentially also be useful in determining characteristics of the
            # multiple available paths, and to choose the best one.
            RNS.log("Using cached announce data for answering path request with tag "+RNS.prettyhexrep(tag), RNS.LOG_EXTREME)
            announce_data = self.path_responses[tag][1]
        
        else:
            destination_hash = self.hash
            random_hash = RNS.Identity.get_random_hash()[0:5]+int(time.time()).to_bytes(5, "big")

            if self.ratchets != None:
                self.rotate_ratchets()
                ratchet = RNS.Identity._ratchet_public_bytes(self.ratchets[0])
                RNS.Identity._remember_ratchet(self.hash, ratchet)

            if app_data == None and self.default_app_data != None:
                if isinstance(self.default_app_data, bytes):
                    app_data = self.default_app_data
                elif callable(self.default_app_data):
                    returned_app_data = self.default_app_data()
                    if isinstance(returned_app_data, bytes):
                        app_data = returned_app_data
            
            signed_data = self.hash+self.identity.get_public_key()+self.name_hash+random_hash+ratchet
            if app_data != None:
                signed_data += app_data

            signature = self.identity.sign(signed_data)
            announce_data = self.identity.get_public_key()+self.name_hash+random_hash+ratchet+signature

            if app_data != None:
                announce_data += app_data

            self.path_responses[tag] = [time.time(), announce_data]

        if path_response:
            announce_context = RNS.Packet.PATH_RESPONSE
        else:
            announce_context = RNS.Packet.NONE

        if ratchet:
            context_flag = RNS.Packet.FLAG_SET
        else:
            context_flag = RNS.Packet.FLAG_UNSET

        announce_packet = RNS.Packet(self, announce_data, RNS.Packet.ANNOUNCE, context = announce_context,
                                     attached_interface = attached_interface, context_flag=context_flag)
        if send:
            announce_packet.send()
        else:
            return announce_packet

    def accepts_links(self, accepts = None):
        """
        Set or query whether the destination accepts incoming link requests.

        :param accepts: If ``True`` or ``False``, this method sets whether the destination accepts incoming link requests. If not provided or ``None``, the method returns whether the destination currently accepts link requests.
        :returns: ``True`` or ``False`` depending on whether the destination accepts incoming link requests, if the *accepts* parameter is not provided or ``None``.
        """
        if accepts == None:
            return self.accept_link_requests

        if accepts:
            self.accept_link_requests = True
        else:
            self.accept_link_requests = False

    def set_link_established_callback(self, callback):
        """
        Registers a function to be called when a link has been established to
        this destination.

        :param callback: A function or method with the signature *callback(link)* to be called when a new link is established with this destination.
        """
        self.callbacks.link_established = callback

    def set_packet_callback(self, callback):
        """
        Registers a function to be called when a packet has been received by
        this destination.

        :param callback: A function or method with the signature *callback(data, packet)* to be called when this destination receives a packet.
        """
        self.callbacks.packet = callback

    def set_proof_requested_callback(self, callback):
        """
        Registers a function to be called when a proof has been requested for
        a packet sent to this destination. Allows control over when and if
        proofs should be returned for received packets.

        :param callback: A function or method to with the signature *callback(packet)* be called when a packet that requests a proof is received. The callback must return one of True or False. If the callback returns True, a proof will be sent. If it returns False, a proof will not be sent.
        """
        self.callbacks.proof_requested = callback

    def set_proof_strategy(self, proof_strategy):
        """
        Sets the destinations proof strategy.

        :param proof_strategy: One of ``RNS.Destination.PROVE_NONE``, ``RNS.Destination.PROVE_ALL`` or ``RNS.Destination.PROVE_APP``. If ``RNS.Destination.PROVE_APP`` is set, the `proof_requested_callback` will be called to determine whether a proof should be sent or not.
        """
        if not proof_strategy in Destination.proof_strategies:
            raise TypeError("Unsupported proof strategy")
        else:
            self.proof_strategy = proof_strategy

    def register_request_handler(self, path, response_generator = None, allow = ALLOW_NONE, allowed_list = None):
        """
        Registers a request handler.

        :param path: The path for the request handler to be registered.
        :param response_generator: A function or method with the signature *response_generator(path, data, request_id, link_id, remote_identity, requested_at)* to be called. Whatever this funcion returns will be sent as a response to the requester. If the function returns ``None``, no response will be sent.
        :param allow: One of ``RNS.Destination.ALLOW_NONE``, ``RNS.Destination.ALLOW_ALL`` or ``RNS.Destination.ALLOW_LIST``. If ``RNS.Destination.ALLOW_LIST`` is set, the request handler will only respond to requests for identified peers in the supplied list.
        :param allowed_list: A list of *bytes-like* :ref:`RNS.Identity<api-identity>` hashes.
        :raises: ``ValueError`` if any of the supplied arguments are invalid.
        """
        if path == None or path == "":
            raise ValueError("Invalid path specified")
        elif not callable(response_generator):
            raise ValueError("Invalid response generator specified")
        elif not allow in Destination.request_policies:
            raise ValueError("Invalid request policy")
        else:
            path_hash = RNS.Identity.truncated_hash(path.encode("utf-8"))
            request_handler = [path, response_generator, allow, allowed_list]
            self.request_handlers[path_hash] = request_handler

    def deregister_request_handler(self, path):
        """
        Deregisters a request handler.

        :param path: The path for the request handler to be deregistered.
        :returns: True if the handler was deregistered, otherwise False.
        """
        path_hash = RNS.Identity.truncated_hash(path.encode("utf-8"))
        if path_hash in self.request_handlers:
            self.request_handlers.pop(path_hash)
            return True
        else:
            return False

    def receive(self, packet):
        if packet.packet_type == RNS.Packet.LINKREQUEST:
            plaintext = packet.data
            self.incoming_link_request(plaintext, packet)
        else:
            plaintext = self.decrypt(packet.data)
            packet.ratchet_id = self.latest_ratchet_id
            if plaintext != None:
                if packet.packet_type == RNS.Packet.DATA:
                    if self.callbacks.packet != None:
                        try:
                            self.callbacks.packet(plaintext, packet)
                        except Exception as e:
                            RNS.log("Error while executing receive callback from "+str(self)+". The contained exception was: "+str(e), RNS.LOG_ERROR)

    def incoming_link_request(self, data, packet):
        if self.accept_link_requests:
            link = RNS.Link.validate_request(self, data, packet)
            if link != None:
                self.links.append(link)

    def _reload_ratchets(self, ratchets_path):
        if os.path.isfile(ratchets_path):
            with self.ratchet_file_lock:
                try:
                    ratchets_file = open(ratchets_path, "rb")
                    persisted_data = umsgpack.unpackb(ratchets_file.read())
                    if "signature" in persisted_data and "ratchets" in persisted_data:
                        if self.identity.validate(persisted_data["signature"], persisted_data["ratchets"]):
                            self.ratchets = umsgpack.unpackb(persisted_data["ratchets"])
                            self.ratchets_path = ratchets_path
                        else:
                            raise KeyError("Invalid ratchet file signature")

                except Exception as e:
                    self.ratchets = None
                    self.ratchets_path = None
                    raise OSError("Could not read ratchet file contents for "+str(self)+". The contained exception was: "+str(e), RNS.LOG_ERROR)
        else:
            RNS.log("No existing ratchet data found, initialising new ratchet file for "+str(self), RNS.LOG_DEBUG)
            self.ratchets = []
            self.ratchets_path = ratchets_path
            self._persist_ratchets()

    def enable_ratchets(self, ratchets_path):
        """
        Enables ratchets on the destination. When ratchets are enabled, Reticulum will automatically rotate
        the keys used to encrypt packets to this destination, and include the latest ratchet key in announces.

        Enabling ratchets on a destination will provide forward secrecy for packets sent to that destination,
        even when sent outside a ``Link``. The normal Reticulum ``Link`` establishment procedure already performs
        its own ephemeral key exchange for each link establishment, which means that ratchets are not necessary
        to provide forward secrecy for links.

        Enabling ratchets will have a small impact on announce size, adding 32 bytes to every sent announce.

        :param ratchets_path: The path to a file to store ratchet data in.
        :returns: True if the operation succeeded, otherwise False.
        """
        if ratchets_path != None:
            self.latest_ratchet_time = 0
            self._reload_ratchets(ratchets_path)

            # TODO: Remove at some point
            RNS.log("Ratchets enabled on "+str(self), RNS.LOG_DEBUG)
            return True

        else:
            raise ValueError("No ratchet file path specified for "+str(self))

    def enforce_ratchets(self):
        """
        When ratchet enforcement is enabled, this destination will never accept packets that use its
        base Identity key for encryption, but only accept packets encrypted with one of the retained
        ratchet keys.
        """
        if self.ratchets != None:
            self.__enforce_ratchets = True
            RNS.log("Ratchets enforced on "+str(self), RNS.LOG_DEBUG)
            return True
        else:
            return False

    def set_retained_ratchets(self, retained_ratchets):
        """
        Sets the number of previously generated ratchet keys this destination will retain,
        and try to use when decrypting incoming packets. Defaults to ``Destination.RATCHET_COUNT``.

        :param retained_ratchets: The number of generated ratchets to retain.
        :returns: True if the operation succeeded, False if not.
        """
        if isinstance(retained_ratchets, int) and retained_ratchets > 0:
            self.retained_ratchets = retained_ratchets
            self._clean_ratchets()
            return True
        else:
            return False

    def set_ratchet_interval(self, interval):
        """
        Sets the minimum interval in seconds between ratchet key rotation.
        Defaults to ``Destination.RATCHET_INTERVAL``.

        :param interval: The minimum interval in seconds.
        :returns: True if the operation succeeded, False if not.
        """
        if isinstance(interval, int) and interval > 0:
            self.ratchet_interval = interval
            return True
        else:
            return False

    def create_keys(self):
        """
        For a ``RNS.Destination.GROUP`` type destination, creates a new symmetric key.

        :raises: ``TypeError`` if called on an incompatible type of destination.
        """
        if self.type == Destination.PLAIN:
            raise TypeError("A plain destination does not hold any keys")

        if self.type == Destination.SINGLE:
            raise TypeError("A single destination holds keys through an Identity instance")

        if self.type == Destination.GROUP:
            self.prv_bytes = Token.generate_key()
            self.prv = Token(self.prv_bytes)

    def get_private_key(self):
        """
        For a ``RNS.Destination.GROUP`` type destination, returns the symmetric private key.

        :raises: ``TypeError`` if called on an incompatible type of destination.
        """
        if self.type == Destination.PLAIN:
            raise TypeError("A plain destination does not hold any keys")
        elif self.type == Destination.SINGLE:
            raise TypeError("A single destination holds keys through an Identity instance")
        else:
            return self.prv_bytes

    def load_private_key(self, key):
        """
        For a ``RNS.Destination.GROUP`` type destination, loads a symmetric private key.

        :param key: A *bytes-like* containing the symmetric key.
        :raises: ``TypeError`` if called on an incompatible type of destination.
        """
        if self.type == Destination.PLAIN:
            raise TypeError("A plain destination does not hold any keys")

        if self.type == Destination.SINGLE:
            raise TypeError("A single destination holds keys through an Identity instance")

        if self.type == Destination.GROUP:
            self.prv_bytes = key
            self.prv = Token(self.prv_bytes)

    def load_public_key(self, key):
        if self.type != Destination.SINGLE:
            raise TypeError("Only the \"single\" destination type can hold a public key")
        else:
            raise TypeError("A single destination holds keys through an Identity instance")

    def encrypt(self, plaintext):
        """
        Encrypts information for ``RNS.Destination.SINGLE`` or ``RNS.Destination.GROUP`` type destination.

        :param plaintext: A *bytes-like* containing the plaintext to be encrypted.
        :raises: ``ValueError`` if destination does not hold a necessary key for encryption.
        """
        if self.type == Destination.PLAIN:
            return plaintext

        if self.type == Destination.SINGLE and self.identity != None:
            selected_ratchet = RNS.Identity.get_ratchet(self.hash)
            if selected_ratchet:
                self.latest_ratchet_id = RNS.Identity._get_ratchet_id(selected_ratchet)
            return self.identity.encrypt(plaintext, ratchet=selected_ratchet)

        if self.type == Destination.GROUP:
            if hasattr(self, "prv") and self.prv != None:
                try:
                    return self.prv.encrypt(plaintext)
                except Exception as e:
                    RNS.log("The GROUP destination could not encrypt data", RNS.LOG_ERROR)
                    RNS.log("The contained exception was: "+str(e), RNS.LOG_ERROR)
            else:
                raise ValueError("No private key held by GROUP destination. Did you create or load one?")

    def decrypt(self, ciphertext):
        """
        Decrypts information for ``RNS.Destination.SINGLE`` or ``RNS.Destination.GROUP`` type destination.

        :param ciphertext: *Bytes* containing the ciphertext to be decrypted.
        :raises: ``ValueError`` if destination does not hold a necessary key for decryption.
        """
        if self.type == Destination.PLAIN:
            return ciphertext

        if self.type == Destination.SINGLE and self.identity != None:
            if self.ratchets:
                decrypted = None
                try:
                    decrypted = self.identity.decrypt(ciphertext, ratchets=self.ratchets, enforce_ratchets=self.__enforce_ratchets, ratchet_id_receiver=self)
                except:
                    decrypted = None

                if not decrypted:
                    try:
                        RNS.log(f"Decryption with ratchets failed on {self}, reloading ratchets from storage and retrying", RNS.LOG_ERROR)
                        self._reload_ratchets(self.ratchets_path)
                        decrypted = self.identity.decrypt(ciphertext, ratchets=self.ratchets, enforce_ratchets=self.__enforce_ratchets, ratchet_id_receiver=self)
                    except Exception as e:
                        RNS.log(f"Decryption still failing after ratchet reload. The contained exception was: {e}", RNS.LOG_ERROR)
                        raise e

                    RNS.log("Decryption succeeded after ratchet reload", RNS.LOG_NOTICE)

                return decrypted

            else:
                return self.identity.decrypt(ciphertext, ratchets=None, enforce_ratchets=self.__enforce_ratchets, ratchet_id_receiver=self)

        if self.type == Destination.GROUP:
            if hasattr(self, "prv") and self.prv != None:
                try:
                    return self.prv.decrypt(ciphertext)
                except Exception as e:
                    RNS.log("The GROUP destination could not decrypt data", RNS.LOG_ERROR)
                    RNS.log("The contained exception was: "+str(e), RNS.LOG_ERROR)
            else:
                raise ValueError("No private key held by GROUP destination. Did you create or load one?")

    def sign(self, message):
        """
        Signs information for ``RNS.Destination.SINGLE`` type destination.

        :param message: *Bytes* containing the message to be signed.
        :returns: A *bytes-like* containing the message signature, or *None* if the destination could not sign the message.
        """
        if self.type == Destination.SINGLE and self.identity != None:
            return self.identity.sign(message)
        else:
            return None

    def set_default_app_data(self, app_data=None):
        """
        Sets the default app_data for the destination. If set, the default
        app_data will be included in every announce sent by the destination,
        unless other app_data is specified in the *announce* method.

        :param app_data: A *bytes-like* containing the default app_data, or a *callable* returning a *bytes-like* containing the app_data.
        """
        self.default_app_data = app_data

    def clear_default_app_data(self):
        """
        Clears default app_data previously set for the destination.
        """
        self.set_default_app_data(app_data=None)