diff --git a/synapse/federation/federation_base.py b/synapse/federation/federation_base.py
new file mode 100644
index 0000000000..a990aec4fd
--- /dev/null
+++ b/synapse/federation/federation_base.py
@@ -0,0 +1,118 @@
+# -*- coding: utf-8 -*-
+# Copyright 2015 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.
+
+
+from twisted.internet import defer
+
+from synapse.events.utils import prune_event
+
+from syutil.jsonutil import encode_canonical_json
+
+from synapse.crypto.event_signing import check_event_content_hash
+
+from synapse.api.errors import SynapseError
+
+import logging
+
+
+logger = logging.getLogger(__name__)
+
+
+class FederationBase(object):
+ @defer.inlineCallbacks
+ def _check_sigs_and_hash_and_fetch(self, origin, pdus, outlier=False):
+ """Takes a list of PDUs and checks the signatures and hashs of each
+ one. If a PDU fails its signature check then we check if we have it in
+ the database and if not then request if from the originating server of
+ that PDU.
+
+ If a PDU fails its content hash check then it is redacted.
+
+ The given list of PDUs are not modified, instead the function returns
+ a new list.
+
+ Args:
+ pdu (list)
+ outlier (bool)
+
+ Returns:
+ Deferred : A list of PDUs that have valid signatures and hashes.
+ """
+ signed_pdus = []
+ for pdu in pdus:
+ try:
+ new_pdu = yield self._check_sigs_and_hash(pdu)
+ signed_pdus.append(new_pdu)
+ except SynapseError:
+ # FIXME: We should handle signature failures more gracefully.
+
+ # Check local db.
+ new_pdu = yield self.store.get_event(
+ pdu.event_id,
+ allow_rejected=True
+ )
+ if new_pdu:
+ signed_pdus.append(new_pdu)
+ continue
+
+ # Check pdu.origin
+ if pdu.origin != origin:
+ new_pdu = yield self.get_pdu(
+ destinations=[pdu.origin],
+ event_id=pdu.event_id,
+ outlier=outlier,
+ )
+
+ if new_pdu:
+ signed_pdus.append(new_pdu)
+ continue
+
+ logger.warn("Failed to find copy of %s with valid signature")
+
+ defer.returnValue(signed_pdus)
+
+ @defer.inlineCallbacks
+ def _check_sigs_and_hash(self, pdu):
+ """Throws a SynapseError if the PDU does not have the correct
+ signatures.
+
+ Returns:
+ FrozenEvent: Either the given event or it redacted if it failed the
+ content hash check.
+ """
+ # Check signatures are correct.
+ redacted_event = prune_event(pdu)
+ redacted_pdu_json = redacted_event.get_pdu_json()
+
+ try:
+ yield self.keyring.verify_json_for_server(
+ pdu.origin, redacted_pdu_json
+ )
+ except SynapseError:
+ logger.warn(
+ "Signature check failed for %s redacted to %s",
+ encode_canonical_json(pdu.get_pdu_json()),
+ encode_canonical_json(redacted_pdu_json),
+ )
+ raise
+
+ if not check_event_content_hash(pdu):
+ logger.warn(
+ "Event content has been tampered, redacting %s, %s",
+ pdu.event_id, encode_canonical_json(pdu.get_dict())
+ )
+ defer.returnValue(redacted_event)
+
+ defer.returnValue(pdu)
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