Telethon/telethon/network/mtprotostate.py
2018-06-12 20:14:21 +02:00

164 lines
5.9 KiB
Python

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.tlobject import TLRequest
__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.
"""
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 create_message(self, obj, after=None):
"""
Creates a new `telethon.tl.tl_message.TLMessage` from
the given `telethon.tl.tlobject.TLObject` instance.
"""
return TLMessage(
msg_id=self._get_new_msg_id(),
seq_no=self._get_seq_no(isinstance(obj, TLRequest)),
obj=obj,
after_id=after.msg_id if after else None
)
@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 pack_message(self, message):
"""
Packs the given `telethon.tl.tl_message.TLMessage` using the
current authorization key following MTProto 2.0 guidelines.
See https://core.telegram.org/mtproto/description.
"""
data = struct.pack('<qq', self.salt, self.id) + bytes(message)
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 unpack_message(self, body):
"""
Inverse of `pack_message` for incoming server messages.
"""
if len(body) < 8:
if body == b'l\xfe\xff\xff':
raise BrokenAuthKeyError()
else:
raise BufferError("Can't decode packet ({})".format(body))
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
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.
"""
now = int(time.time())
correct = correct_msg_id >> 32
self.time_offset = correct - now
self._last_msg_id = 0
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