synapse/synapse/crypto/event_signing.py

# -*- coding: utf-8 -*-

# Copyright 2014-2016 OpenMarket Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.


import hashlib
import logging

from canonicaljson import encode_canonical_json
from signedjson.sign import sign_json
from unpaddedbase64 import decode_base64, encode_base64

from synapse.api.errors import Codes, SynapseError
from synapse.events.utils import prune_event, prune_event_dict

logger = logging.getLogger(__name__)


def check_event_content_hash(event, hash_algorithm=hashlib.sha256):
    """Check whether the hash for this PDU matches the contents"""
    name, expected_hash = compute_content_hash(event.get_pdu_json(), hash_algorithm)
    logger.debug(
        "Verifying content hash on %s (expecting: %s)",
        event.event_id,
        encode_base64(expected_hash),
    )

    # some malformed events lack a 'hashes'. Protect against it being missing
    # or a weird type by basically treating it the same as an unhashed event.
    hashes = event.get("hashes")
    if not isinstance(hashes, dict):
        raise SynapseError(400, "Malformed 'hashes'", Codes.UNAUTHORIZED)

    if name not in hashes:
        raise SynapseError(
            400,
            "Algorithm %s not in hashes %s" % (name, list(hashes)),
            Codes.UNAUTHORIZED,
        )
    message_hash_base64 = hashes[name]
    try:
        message_hash_bytes = decode_base64(message_hash_base64)
    except Exception:
        raise SynapseError(
            400, "Invalid base64: %s" % (message_hash_base64,), Codes.UNAUTHORIZED
        )
    return message_hash_bytes == expected_hash


def compute_content_hash(event_dict, hash_algorithm):
    """Compute the content hash of an event, which is the hash of the
    unredacted event.

    Args:
        event_dict (dict): The unredacted event as a dict
        hash_algorithm: A hasher from `hashlib`, e.g. hashlib.sha256, to use
            to hash the event

    Returns:
        tuple[str, bytes]: A tuple of the name of hash and the hash as raw
        bytes.
    """
    event_dict = dict(event_dict)
    event_dict.pop("age_ts", None)
    event_dict.pop("unsigned", None)
    event_dict.pop("signatures", None)
    event_dict.pop("hashes", None)
    event_dict.pop("outlier", None)
    event_dict.pop("destinations", None)

    event_json_bytes = encode_canonical_json(event_dict)

    hashed = hash_algorithm(event_json_bytes)
    return hashed.name, hashed.digest()


def compute_event_reference_hash(event, hash_algorithm=hashlib.sha256):
    """Computes the event reference hash. This is the hash of the redacted
    event.

    Args:
        event (FrozenEvent)
        hash_algorithm: A hasher from `hashlib`, e.g. hashlib.sha256, to use
            to hash the event

    Returns:
        tuple[str, bytes]: A tuple of the name of hash and the hash as raw
        bytes.
    """
    tmp_event = prune_event(event)
    event_dict = tmp_event.get_pdu_json()
    event_dict.pop("signatures", None)
    event_dict.pop("age_ts", None)
    event_dict.pop("unsigned", None)
    event_json_bytes = encode_canonical_json(event_dict)
    hashed = hash_algorithm(event_json_bytes)
    return hashed.name, hashed.digest()


def compute_event_signature(event_dict, signature_name, signing_key):
    """Compute the signature of the event for the given name and key.

    Args:
        event_dict (dict): The event as a dict
        signature_name (str): The name of the entity signing the event
            (typically the server's hostname).
        signing_key (syutil.crypto.SigningKey): The key to sign with

    Returns:
        dict[str, dict[str, str]]: Returns a dictionary in the same format of
        an event's signatures field.
    """
    redact_json = prune_event_dict(event_dict)
    redact_json.pop("age_ts", None)
    redact_json.pop("unsigned", None)
    logger.debug("Signing event: %s", encode_canonical_json(redact_json))
    redact_json = sign_json(redact_json, signature_name, signing_key)
    logger.debug("Signed event: %s", encode_canonical_json(redact_json))
    return redact_json["signatures"]


def add_hashes_and_signatures(
    event_dict, signature_name, signing_key, hash_algorithm=hashlib.sha256
):
    """Add content hash and sign the event

    Args:
        event_dict (dict): The event to add hashes to and sign
        signature_name (str): The name of the entity signing the event
            (typically the server's hostname).
        signing_key (syutil.crypto.SigningKey): The key to sign with
        hash_algorithm: A hasher from `hashlib`, e.g. hashlib.sha256, to use
            to hash the event
    """

    name, digest = compute_content_hash(event_dict, hash_algorithm=hash_algorithm)

    event_dict.setdefault("hashes", {})[name] = encode_base64(digest)

    event_dict["signatures"] = compute_event_signature(
        event_dict, signature_name=signature_name, signing_key=signing_key
    )