Telethon/telethon/network/mtprotostate.py

import logging
import os
import struct
import time
from hashlib import sha256

from ..crypto import AES
from ..errors import SecurityError, BrokenAuthKeyError
from ..extensions import BinaryReader
from ..tl.core import TLMessage
from ..tl.functions import InvokeAfterMsgRequest
from ..tl.core.gzippacked import GzipPacked

__log__ = logging.getLogger(__name__)


class MTProtoState:
    """
    `telethon.network.mtprotosender.MTProtoSender` needs to hold a state
    in order to be able to encrypt and decrypt incoming/outgoing messages,
    as well as generating the message IDs. Instances of this class hold
    together all the required information.

    It doesn't make sense to use `telethon.sessions.abstract.Session` for
    the sender because the sender should *not* be concerned about storing
    this information to disk, as one may create as many senders as they
    desire to any other data center, or some CDN. Using the same session
    for all these is not a good idea as each need their own authkey, and
    the concept of "copying" sessions with the unnecessary entities or
    updates state for these connections doesn't make sense.

    While it would be possible to have a `MTProtoPlainState` that does no
    encryption so that it was usable through the `MTProtoLayer` and thus
    avoid the need for a `MTProtoPlainSender`, the `MTProtoLayer` is more
    focused to efficiency and this state is also more advanced (since it
    supports gzipping and invoking after other message IDs). There are too
    many methods that would be needed to make it convenient to use for the
    authentication process, at which point the `MTProtoPlainSender` is better.
    """
    def __init__(self, auth_key):
        # Session IDs can be random on every connection
        self.id = struct.unpack('q', os.urandom(8))[0]
        self.auth_key = auth_key
        self.time_offset = 0
        self.salt = 0
        self._sequence = 0
        self._last_msg_id = 0

    def update_message_id(self, message):
        """
        Updates the message ID to a new one,
        used when the time offset changed.
        """
        message.msg_id = self._get_new_msg_id()

    @staticmethod
    def _calc_key(auth_key, msg_key, client):
        """
        Calculate the key based on Telegram guidelines for MTProto 2,
        specifying whether it's the client or not. See
        https://core.telegram.org/mtproto/description#defining-aes-key-and-initialization-vector
        """
        x = 0 if client else 8
        sha256a = sha256(msg_key + auth_key[x: x + 36]).digest()
        sha256b = sha256(auth_key[x + 40:x + 76] + msg_key).digest()

        aes_key = sha256a[:8] + sha256b[8:24] + sha256a[24:32]
        aes_iv = sha256b[:8] + sha256a[8:24] + sha256b[24:32]

        return aes_key, aes_iv

    def write_data_as_message(self, buffer, data, content_related,
                              *, after_id=None):
        """
        Writes a message containing the given data into buffer.

        Returns the message id.
        """
        msg_id = self._get_new_msg_id()
        seq_no = self._get_seq_no(content_related)
        if after_id is None:
            body = GzipPacked.gzip_if_smaller(content_related, data)
        else:
            body = GzipPacked.gzip_if_smaller(content_related,
                bytes(InvokeAfterMsgRequest(after_id, data)))

        buffer.write(struct.pack('<qii', msg_id, seq_no, len(body)))
        buffer.write(body)
        return msg_id

    def encrypt_message_data(self, data):
        """
        Encrypts the given message data using the current authorization key
        following MTProto 2.0 guidelines core.telegram.org/mtproto/description.
        """
        data = struct.pack('<qq', self.salt, self.id) + data
        padding = os.urandom(-(len(data) + 12) % 16 + 12)

        # Being substr(what, offset, length); x = 0 for client
        # "msg_key_large = SHA256(substr(auth_key, 88+x, 32) + pt + padding)"
        msg_key_large = sha256(
            self.auth_key.key[88:88 + 32] + data + padding).digest()

        # "msg_key = substr (msg_key_large, 8, 16)"
        msg_key = msg_key_large[8:24]
        aes_key, aes_iv = self._calc_key(self.auth_key.key, msg_key, True)

        key_id = struct.pack('<Q', self.auth_key.key_id)
        return (key_id + msg_key +
                AES.encrypt_ige(data + padding, aes_key, aes_iv))

    def decrypt_message_data(self, body):
        """
        Inverse of `encrypt_message_data` for incoming server messages.
        """
        if len(body) < 8:
            # TODO If len == 4, raise HTTPErrorCode(-little endian int)
            if body == b'l\xfe\xff\xff':
                raise BrokenAuthKeyError()
            else:
                raise BufferError("Can't decode packet ({})".format(body))

        # TODO Check salt, session_id and sequence_number
        key_id = struct.unpack('<Q', body[:8])[0]
        if key_id != self.auth_key.key_id:
            raise SecurityError('Server replied with an invalid auth key')

        msg_key = body[8:24]
        aes_key, aes_iv = self._calc_key(self.auth_key.key, msg_key, False)
        body = AES.decrypt_ige(body[24:], aes_key, aes_iv)

        # https://core.telegram.org/mtproto/security_guidelines
        # Sections "checking sha256 hash" and "message length"
        our_key = sha256(self.auth_key.key[96:96 + 32] + body)
        if msg_key != our_key.digest()[8:24]:
            raise SecurityError(
                "Received msg_key doesn't match with expected one")

        reader = BinaryReader(body)
        reader.read_long()  # remote_salt
        if reader.read_long() != self.id:
            raise SecurityError('Server replied with a wrong session ID')

        remote_msg_id = reader.read_long()
        remote_sequence = reader.read_int()
        reader.read_int()  # msg_len for the inner object, padding ignored

        # We could read msg_len bytes and use those in a new reader to read
        # the next TLObject without including the padding, but since the
        # reader isn't used for anything else after this, it's unnecessary.
        obj = reader.tgread_object()

        return TLMessage(remote_msg_id, remote_sequence, obj)

    def _get_new_msg_id(self):
        """
        Generates a new unique message ID based on the current
        time (in ms) since epoch, applying a known time offset.
        """
        now = time.time() + self.time_offset
        nanoseconds = int((now - int(now)) * 1e+9)
        new_msg_id = (int(now) << 32) | (nanoseconds << 2)

        if self._last_msg_id >= new_msg_id:
            new_msg_id = self._last_msg_id + 4

        self._last_msg_id = new_msg_id
        return new_msg_id

    def update_time_offset(self, correct_msg_id):
        """
        Updates the time offset to the correct
        one given a known valid message ID.
        """
        bad = self._get_new_msg_id()
        old = self.time_offset

        now = int(time.time())
        correct = correct_msg_id >> 32
        self.time_offset = correct - now

        if self.time_offset != old:
            self._last_msg_id = 0
            __log__.debug(
                'Updated time offset (old offset %d, bad %d, good %d, new %d)',
                old, bad, correct_msg_id, self.time_offset
            )

        return self.time_offset

    def _get_seq_no(self, content_related):
        """
        Generates the next sequence number depending on whether
        it should be for a content-related query or not.
        """
        if content_related:
            result = self._sequence * 2 + 1
            self._sequence += 1
            return result
        else:
            return self._sequence * 2