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# Copyright (c) 2022 Tulir Asokan
#
# This Source Code Form is subject to the terms of the Mozilla Public
# License, v. 2.0. If a copy of the MPL was not distributed with this
# file, You can obtain one at http://mozilla.org/MPL/2.0/.
import json
import olm
from mautrix.errors import (
DecryptedPayloadError,
DecryptionError,
DuplicateMessageIndex,
MismatchingRoomError,
SessionNotFound,
VerificationError,
)
from mautrix.types import (
EncryptedEvent,
EncryptedMegolmEventContent,
EncryptionAlgorithm,
Event,
SessionID,
TrustState,
)
from .device_lists import DeviceListMachine
from .sessions import InboundGroupSession
class MegolmDecryptionMachine(DeviceListMachine):
async def decrypt_megolm_event(self, evt: EncryptedEvent) -> Event:
"""
Decrypt an event that was encrypted using Megolm.
Args:
evt: The whole encrypted event.
Returns:
The decrypted event, including some unencrypted metadata from the input event.
Raises:
DecryptionError: If decryption failed.
"""
if not isinstance(evt.content, EncryptedMegolmEventContent):
raise DecryptionError("Unsupported event content class")
elif evt.content.algorithm != EncryptionAlgorithm.MEGOLM_V1:
raise DecryptionError("Unsupported event encryption algorithm")
async with self._megolm_decrypt_lock:
session = await self.crypto_store.get_group_session(
evt.room_id, evt.content.session_id
)
if session is None:
# TODO check if olm session is wedged
raise SessionNotFound(evt.content.session_id, evt.content.sender_key)
try:
plaintext, index = session.decrypt(evt.content.ciphertext)
except olm.OlmGroupSessionError as e:
raise DecryptionError("Failed to decrypt megolm event") from e
if not await self.crypto_store.validate_message_index(
session.sender_key, SessionID(session.id), evt.event_id, index, evt.timestamp
):
raise DuplicateMessageIndex()
await self._ratchet_session(session, index)
forwarded_keys = False
if (
evt.content.device_id == self.client.device_id
and session.signing_key == self.account.signing_key
and session.sender_key == self.account.identity_key
and not session.forwarding_chain
):
trust_level = TrustState.VERIFIED
else:
device = await self.get_or_fetch_device_by_key(evt.sender, session.sender_key)
if not session.forwarding_chain or (
len(session.forwarding_chain) == 1
and session.forwarding_chain[0] == session.sender_key
):
if not device:
self.log.debug(
f"Couldn't resolve trust level of session {session.id}: "
f"sent by unknown device {evt.sender}/{session.sender_key}"
)
trust_level = TrustState.UNKNOWN_DEVICE
elif (
device.signing_key != session.signing_key
or device.identity_key != session.sender_key
):
raise VerificationError()
else:
trust_level = await self.resolve_trust(device)
else:
forwarded_keys = True
last_chain_item = session.forwarding_chain[-1]
received_from = await self.crypto_store.find_device_by_key(
evt.sender, last_chain_item
)
if received_from:
trust_level = await self.resolve_trust(received_from)
else:
self.log.debug(
f"Couldn't resolve trust level of session {session.id}: "
f"forwarding chain ends with unknown device {last_chain_item}"
)
trust_level = TrustState.FORWARDED
try:
data = json.loads(plaintext)
room_id = data["room_id"]
event_type = data["type"]
content = data["content"]
except json.JSONDecodeError as e:
raise DecryptedPayloadError("Failed to parse megolm payload") from e
except KeyError as e:
raise DecryptedPayloadError("Megolm payload is missing fields") from e
if room_id != evt.room_id:
raise MismatchingRoomError()
if evt.content.relates_to and "m.relates_to" not in content:
content["m.relates_to"] = evt.content.relates_to.serialize()
result = Event.deserialize(
{
"room_id": evt.room_id,
"event_id": evt.event_id,
"sender": evt.sender,
"origin_server_ts": evt.timestamp,
"type": event_type,
"content": content,
}
)
result.unsigned = evt.unsigned
result.type = result.type.with_class(evt.type.t_class)
result["mautrix"] = {
"trust_state": trust_level,
"forwarded_keys": forwarded_keys,
"was_encrypted": True,
}
return result
async def _ratchet_session(self, sess: InboundGroupSession, index: int) -> None:
expected_message_index = sess.ratchet_safety.next_index
did_modify = True
if index > expected_message_index:
sess.ratchet_safety.missed_indices += list(range(expected_message_index, index))
sess.ratchet_safety.next_index = index + 1
elif index == expected_message_index:
sess.ratchet_safety.next_index = index + 1
else:
try:
sess.ratchet_safety.missed_indices.remove(index)
except ValueError:
did_modify = False
# Use presence of received_at as a sign that this is a recent megolm session,
# and therefore it's safe to drop missed indices entirely.
if (
sess.received_at
and sess.ratchet_safety.missed_indices
and sess.ratchet_safety.missed_indices[0] < expected_message_index - 10
):
i = 0
for i, lost_index in enumerate(sess.ratchet_safety.missed_indices):
if lost_index < expected_message_index - 10:
sess.ratchet_safety.lost_indices.append(lost_index)
else:
break
sess.ratchet_safety.missed_indices = sess.ratchet_safety.missed_indices[i + 1 :]
ratchet_target_index = sess.ratchet_safety.next_index
if len(sess.ratchet_safety.missed_indices) > 0:
ratchet_target_index = min(sess.ratchet_safety.missed_indices)
self.log.debug(
f"Ratchet safety info for {sess.id}: {sess.ratchet_safety}, {ratchet_target_index=}"
)
sess_id = SessionID(sess.id)
if (
sess.max_messages
and ratchet_target_index >= sess.max_messages
and not sess.ratchet_safety.missed_indices
and self.delete_fully_used_keys_on_decrypt
):
self.log.info(f"Deleting fully used session {sess.id}")
await self.crypto_store.redact_group_session(
sess.room_id, sess_id, reason="maximum messages reached"
)
return
elif sess.first_known_index < ratchet_target_index and self.ratchet_keys_on_decrypt:
self.log.info(f"Ratcheting session {sess.id} to {ratchet_target_index}")
sess = sess.ratchet_to(ratchet_target_index)
elif not did_modify:
return
await self.crypto_store.put_group_session(sess.room_id, sess.sender_key, sess_id, sess)
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