Merge branch 'uhoreg/e2e_cross-signing_merged' into develop

15 files changed, 693 insertions, 21 deletions

diff --git a/changelog.d/5759.misc b/changelog.d/5759.misc
new file mode 100644
index 0000000000..dc7e2c01bf
--- /dev/null
+++ b/changelog.d/5759.misc
@@ -0,0 +1,4 @@
+Allow devices to be marked as hidden, for use by features such as cross-signing.
+This adds a new field with a default value to the devices field in the database,
+and so the database upgrade may take a long time depending on how many devices
+are in the database.
diff --git a/changelog.d/5769.feature b/changelog.d/5769.feature
new file mode 100644
index 0000000000..bf994ca327
--- /dev/null
+++ b/changelog.d/5769.feature
@@ -0,0 +1 @@
+Allow uploading of cross-signing keys.
\ No newline at end of file
diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index 1bb2e86789..cca92c34ba 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -62,6 +62,7 @@ class Codes(object):
     INCOMPATIBLE_ROOM_VERSION = "M_INCOMPATIBLE_ROOM_VERSION"
     WRONG_ROOM_KEYS_VERSION = "M_WRONG_ROOM_KEYS_VERSION"
     EXPIRED_ACCOUNT = "ORG_MATRIX_EXPIRED_ACCOUNT"
+    INVALID_SIGNATURE = "M_INVALID_SIGNATURE"
     USER_DEACTIVATED = "M_USER_DEACTIVATED"
 
 
diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 71a8f33da3..5f23ee4488 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -1,5 +1,7 @@
 # -*- coding: utf-8 -*-
 # Copyright 2016 OpenMarket Ltd
+# Copyright 2019 New Vector Ltd
+# Copyright 2019 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -439,6 +441,21 @@ class DeviceHandler(DeviceWorkerHandler):
                 log_kv({"message": "sent device update to host", "host": host})
 
     @defer.inlineCallbacks
+    def notify_user_signature_update(self, from_user_id, user_ids):
+        """Notify a user that they have made new signatures of other users.
+
+        Args:
+            from_user_id (str): the user who made the signature
+            user_ids (list[str]): the users IDs that have new signatures
+        """
+
+        position = yield self.store.add_user_signature_change_to_streams(
+            from_user_id, user_ids
+        )
+
+        self.notifier.on_new_event("device_list_key", position, users=[from_user_id])
+
+    @defer.inlineCallbacks
     def on_federation_query_user_devices(self, user_id):
         stream_id, devices = yield self.store.get_devices_with_keys_by_user(user_id)
         return {"user_id": user_id, "stream_id": stream_id, "devices": devices}
diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index 0a84d0e2b0..be675197f1 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 # Copyright 2016 OpenMarket Ltd
-# Copyright 2018 New Vector Ltd
+# Copyright 2018-2019 New Vector Ltd
+# Copyright 2019 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -19,13 +20,18 @@ import logging
 from six import iteritems
 
 from canonicaljson import encode_canonical_json, json
+from signedjson.sign import SignatureVerifyException, verify_signed_json
 
 from twisted.internet import defer
 
-from synapse.api.errors import CodeMessageException, SynapseError
+from synapse.api.errors import CodeMessageException, Codes, SynapseError
 from synapse.logging.context import make_deferred_yieldable, run_in_background
 from synapse.logging.opentracing import log_kv, set_tag, tag_args, trace
-from synapse.types import UserID, get_domain_from_id
+from synapse.types import (
+    UserID,
+    get_domain_from_id,
+    get_verify_key_from_cross_signing_key,
+)
 from synapse.util import unwrapFirstError
 from synapse.util.retryutils import NotRetryingDestination
 
@@ -49,7 +55,7 @@ class E2eKeysHandler(object):
 
     @trace
     @defer.inlineCallbacks
-    def query_devices(self, query_body, timeout):
+    def query_devices(self, query_body, timeout, from_user_id):
         """ Handle a device key query from a client
 
         {
@@ -67,6 +73,11 @@ class E2eKeysHandler(object):
                 }
             }
         }
+
+        Args:
+            from_user_id (str): the user making the query.  This is used when
+                adding cross-signing signatures to limit what signatures users
+                can see.
         """
 
         device_keys_query = query_body.get("device_keys", {})
@@ -125,6 +136,11 @@ class E2eKeysHandler(object):
                 r = remote_queries_not_in_cache.setdefault(domain, {})
                 r[user_id] = remote_queries[user_id]
 
+        # Get cached cross-signing keys
+        cross_signing_keys = yield self.get_cross_signing_keys_from_cache(
+            device_keys_query, from_user_id
+        )
+
         # Now fetch any devices that we don't have in our cache
         @trace
         @defer.inlineCallbacks
@@ -188,6 +204,14 @@ class E2eKeysHandler(object):
                     if user_id in destination_query:
                         results[user_id] = keys
 
+                for user_id, key in remote_result["master_keys"].items():
+                    if user_id in destination_query:
+                        cross_signing_keys["master_keys"][user_id] = key
+
+                for user_id, key in remote_result["self_signing_keys"].items():
+                    if user_id in destination_query:
+                        cross_signing_keys["self_signing_keys"][user_id] = key
+
             except Exception as e:
                 failure = _exception_to_failure(e)
                 failures[destination] = failure
@@ -204,7 +228,61 @@ class E2eKeysHandler(object):
             ).addErrback(unwrapFirstError)
         )
 
-        return {"device_keys": results, "failures": failures}
+        ret = {"device_keys": results, "failures": failures}
+
+        ret.update(cross_signing_keys)
+
+        return ret
+
+    @defer.inlineCallbacks
+    def get_cross_signing_keys_from_cache(self, query, from_user_id):
+        """Get cross-signing keys for users from the database
+
+        Args:
+            query (Iterable[string]) an iterable of user IDs.  A dict whose keys
+                are user IDs satisfies this, so the query format used for
+                query_devices can be used here.
+            from_user_id (str): the user making the query.  This is used when
+                adding cross-signing signatures to limit what signatures users
+                can see.
+
+        Returns:
+            defer.Deferred[dict[str, dict[str, dict]]]: map from
+                (master|self_signing|user_signing) -> user_id -> key
+        """
+        master_keys = {}
+        self_signing_keys = {}
+        user_signing_keys = {}
+
+        for user_id in query:
+            # XXX: consider changing the store functions to allow querying
+            # multiple users simultaneously.
+            key = yield self.store.get_e2e_cross_signing_key(
+                user_id, "master", from_user_id
+            )
+            if key:
+                master_keys[user_id] = key
+
+            key = yield self.store.get_e2e_cross_signing_key(
+                user_id, "self_signing", from_user_id
+            )
+            if key:
+                self_signing_keys[user_id] = key
+
+            # users can see other users' master and self-signing keys, but can
+            # only see their own user-signing keys
+            if from_user_id == user_id:
+                key = yield self.store.get_e2e_cross_signing_key(
+                    user_id, "user_signing", from_user_id
+                )
+                if key:
+                    user_signing_keys[user_id] = key
+
+        return {
+            "master_keys": master_keys,
+            "self_signing_keys": self_signing_keys,
+            "user_signing_keys": user_signing_keys,
+        }
 
     @trace
     @defer.inlineCallbacks
@@ -441,6 +519,116 @@ class E2eKeysHandler(object):
         log_kv({"message": "Inserting new one_time_keys.", "keys": new_keys})
         yield self.store.add_e2e_one_time_keys(user_id, device_id, time_now, new_keys)
 
+    @defer.inlineCallbacks
+    def upload_signing_keys_for_user(self, user_id, keys):
+        """Upload signing keys for cross-signing
+
+        Args:
+            user_id (string): the user uploading the keys
+            keys (dict[string, dict]): the signing keys
+        """
+
+        # if a master key is uploaded, then check it.  Otherwise, load the
+        # stored master key, to check signatures on other keys
+        if "master_key" in keys:
+            master_key = keys["master_key"]
+
+            _check_cross_signing_key(master_key, user_id, "master")
+        else:
+            master_key = yield self.store.get_e2e_cross_signing_key(user_id, "master")
+
+        # if there is no master key, then we can't do anything, because all the
+        # other cross-signing keys need to be signed by the master key
+        if not master_key:
+            raise SynapseError(400, "No master key available", Codes.MISSING_PARAM)
+
+        try:
+            master_key_id, master_verify_key = get_verify_key_from_cross_signing_key(
+                master_key
+            )
+        except ValueError:
+            if "master_key" in keys:
+                # the invalid key came from the request
+                raise SynapseError(400, "Invalid master key", Codes.INVALID_PARAM)
+            else:
+                # the invalid key came from the database
+                logger.error("Invalid master key found for user %s", user_id)
+                raise SynapseError(500, "Invalid master key")
+
+        # for the other cross-signing keys, make sure that they have valid
+        # signatures from the master key
+        if "self_signing_key" in keys:
+            self_signing_key = keys["self_signing_key"]
+
+            _check_cross_signing_key(
+                self_signing_key, user_id, "self_signing", master_verify_key
+            )
+
+        if "user_signing_key" in keys:
+            user_signing_key = keys["user_signing_key"]
+
+            _check_cross_signing_key(
+                user_signing_key, user_id, "user_signing", master_verify_key
+            )
+
+        # if everything checks out, then store the keys and send notifications
+        deviceids = []
+        if "master_key" in keys:
+            yield self.store.set_e2e_cross_signing_key(user_id, "master", master_key)
+            deviceids.append(master_verify_key.version)
+        if "self_signing_key" in keys:
+            yield self.store.set_e2e_cross_signing_key(
+                user_id, "self_signing", self_signing_key
+            )
+            try:
+                deviceids.append(
+                    get_verify_key_from_cross_signing_key(self_signing_key)[1].version
+                )
+            except ValueError:
+                raise SynapseError(400, "Invalid self-signing key", Codes.INVALID_PARAM)
+        if "user_signing_key" in keys:
+            yield self.store.set_e2e_cross_signing_key(
+                user_id, "user_signing", user_signing_key
+            )
+            # the signature stream matches the semantics that we want for
+            # user-signing key updates: only the user themselves is notified of
+            # their own user-signing key updates
+            yield self.device_handler.notify_user_signature_update(user_id, [user_id])
+
+        # master key and self-signing key updates match the semantics of device
+        # list updates: all users who share an encrypted room are notified
+        if len(deviceids):
+            yield self.device_handler.notify_device_update(user_id, deviceids)
+
+        return {}
+
+
+def _check_cross_signing_key(key, user_id, key_type, signing_key=None):
+    """Check a cross-signing key uploaded by a user.  Performs some basic sanity
+    checking, and ensures that it is signed, if a signature is required.
+
+    Args:
+        key (dict): the key data to verify
+        user_id (str): the user whose key is being checked
+        key_type (str): the type of key that the key should be
+        signing_key (VerifyKey): (optional) the signing key that the key should
+            be signed with.  If omitted, signatures will not be checked.
+    """
+    if (
+        key.get("user_id") != user_id
+        or key_type not in key.get("usage", [])
+        or len(key.get("keys", {})) != 1
+    ):
+        raise SynapseError(400, ("Invalid %s key" % (key_type,)), Codes.INVALID_PARAM)
+
+    if signing_key:
+        try:
+            verify_signed_json(key, user_id, signing_key)
+        except SignatureVerifyException:
+            raise SynapseError(
+                400, ("Invalid signature on %s key" % key_type), Codes.INVALID_SIGNATURE
+            )
+
 
 def _exception_to_failure(e):
     if isinstance(e, CodeMessageException):
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 19bca6717f..d99160e9d7 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2015, 2016 OpenMarket Ltd
-# Copyright 2018 New Vector Ltd
+# Copyright 2018, 2019 New Vector Ltd
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -1124,6 +1124,11 @@ class SyncHandler(object):
             # weren't in the previous sync *or* they left and rejoined.
             users_that_have_changed.update(newly_joined_or_invited_users)
 
+            user_signatures_changed = yield self.store.get_users_whose_signatures_changed(
+                user_id, since_token.device_list_key
+            )
+            users_that_have_changed.update(user_signatures_changed)
+
             # Now find users that we no longer track
             for room_id in newly_left_rooms:
                 left_users = yield self.state.get_current_users_in_room(room_id)
diff --git a/synapse/replication/slave/storage/devices.py b/synapse/replication/slave/storage/devices.py
index d9300fce33..f045e1b937 100644
--- a/synapse/replication/slave/storage/devices.py
+++ b/synapse/replication/slave/storage/devices.py
@@ -33,6 +33,9 @@ class SlavedDeviceStore(EndToEndKeyWorkerStore, DeviceWorkerStore, BaseSlavedSto
         self._device_list_stream_cache = StreamChangeCache(
             "DeviceListStreamChangeCache", device_list_max
         )
+        self._user_signature_stream_cache = StreamChangeCache(
+            "UserSignatureStreamChangeCache", device_list_max
+        )
         self._device_list_federation_stream_cache = StreamChangeCache(
             "DeviceListFederationStreamChangeCache", device_list_max
         )
diff --git a/synapse/rest/client/v2_alpha/keys.py b/synapse/rest/client/v2_alpha/keys.py
index 2e680134a0..151a70d449 100644
--- a/synapse/rest/client/v2_alpha/keys.py
+++ b/synapse/rest/client/v2_alpha/keys.py
@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2015, 2016 OpenMarket Ltd
+# Copyright 2019 New Vector Ltd
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -27,7 +28,7 @@ from synapse.http.servlet import (
 from synapse.logging.opentracing import log_kv, set_tag, trace
 from synapse.types import StreamToken
 
-from ._base import client_patterns
+from ._base import client_patterns, interactive_auth_handler
 
 logger = logging.getLogger(__name__)
 
@@ -155,10 +156,11 @@ class KeyQueryServlet(RestServlet):
 
     @defer.inlineCallbacks
     def on_POST(self, request):
-        yield self.auth.get_user_by_req(request, allow_guest=True)
+        requester = yield self.auth.get_user_by_req(request, allow_guest=True)
+        user_id = requester.user.to_string()
         timeout = parse_integer(request, "timeout", 10 * 1000)
         body = parse_json_object_from_request(request)
-        result = yield self.e2e_keys_handler.query_devices(body, timeout)
+        result = yield self.e2e_keys_handler.query_devices(body, timeout, user_id)
         return 200, result
 
 
@@ -238,8 +240,46 @@ class OneTimeKeyServlet(RestServlet):
         return 200, result
 
 
+class SigningKeyUploadServlet(RestServlet):
+    """
+    POST /keys/device_signing/upload HTTP/1.1
+    Content-Type: application/json
+
+    {
+    }
+    """
+
+    PATTERNS = client_patterns("/keys/device_signing/upload$", releases=())
+
+    def __init__(self, hs):
+        """
+        Args:
+            hs (synapse.server.HomeServer): server
+        """
+        super(SigningKeyUploadServlet, self).__init__()
+        self.hs = hs
+        self.auth = hs.get_auth()
+        self.e2e_keys_handler = hs.get_e2e_keys_handler()
+        self.auth_handler = hs.get_auth_handler()
+
+    @interactive_auth_handler
+    @defer.inlineCallbacks
+    def on_POST(self, request):
+        requester = yield self.auth.get_user_by_req(request)
+        user_id = requester.user.to_string()
+        body = parse_json_object_from_request(request)
+
+        yield self.auth_handler.validate_user_via_ui_auth(
+            requester, body, self.hs.get_ip_from_request(request)
+        )
+
+        result = yield self.e2e_keys_handler.upload_signing_keys_for_user(user_id, body)
+        return (200, result)
+
+
 def register_servlets(hs, http_server):
     KeyUploadServlet(hs).register(http_server)
     KeyQueryServlet(hs).register(http_server)
     KeyChangesServlet(hs).register(http_server)
     OneTimeKeyServlet(hs).register(http_server)
+    SigningKeyUploadServlet(hs).register(http_server)
diff --git a/synapse/storage/__init__.py b/synapse/storage/__init__.py
index e7f6ea7286..e9a9c2cd8d 100644
--- a/synapse/storage/__init__.py
+++ b/synapse/storage/__init__.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2014-2016 OpenMarket Ltd
-# Copyright 2018 New Vector Ltd
+# Copyright 2018,2019 New Vector Ltd
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -136,6 +136,9 @@ class DataStore(
         self._device_list_id_gen = StreamIdGenerator(
             db_conn, "device_lists_stream", "stream_id"
         )
+        self._cross_signing_id_gen = StreamIdGenerator(
+            db_conn, "e2e_cross_signing_keys", "stream_id"
+        )
 
         self._access_tokens_id_gen = IdGenerator(db_conn, "access_tokens", "id")
         self._event_reports_id_gen = IdGenerator(db_conn, "event_reports", "id")
@@ -207,6 +210,9 @@ class DataStore(
         self._device_list_stream_cache = StreamChangeCache(
             "DeviceListStreamChangeCache", device_list_max
         )
+        self._user_signature_stream_cache = StreamChangeCache(
+            "UserSignatureStreamChangeCache", device_list_max
+        )
         self._device_list_federation_stream_cache = StreamChangeCache(
             "DeviceListFederationStreamChangeCache", device_list_max
         )
diff --git a/synapse/storage/devices.py b/synapse/storage/devices.py
index ac5239e50a..f7a3542348 100644
--- a/synapse/storage/devices.py
+++ b/synapse/storage/devices.py
@@ -1,5 +1,7 @@
 # -*- coding: utf-8 -*-
 # Copyright 2016 OpenMarket Ltd
+# Copyright 2019 New Vector Ltd
+# Copyright 2019 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -20,7 +22,7 @@ from canonicaljson import json
 
 from twisted.internet import defer
 
-from synapse.api.errors import StoreError
+from synapse.api.errors import Codes, StoreError
 from synapse.logging.opentracing import (
     get_active_span_text_map,
     set_tag,
@@ -47,7 +49,8 @@ DROP_DEVICE_LIST_STREAMS_NON_UNIQUE_INDEXES = (
 
 class DeviceWorkerStore(SQLBaseStore):
     def get_device(self, user_id, device_id):
-        """Retrieve a device.
+        """Retrieve a device. Only returns devices that are not marked as
+        hidden.
 
         Args:
             user_id (str): The ID of the user which owns the device
@@ -59,14 +62,15 @@ class DeviceWorkerStore(SQLBaseStore):
         """
         return self._simple_select_one(
             table="devices",
-            keyvalues={"user_id": user_id, "device_id": device_id},
+            keyvalues={"user_id": user_id, "device_id": device_id, "hidden": False},
             retcols=("user_id", "device_id", "display_name"),
             desc="get_device",
         )
 
     @defer.inlineCallbacks
     def get_devices_by_user(self, user_id):
-        """Retrieve all of a user's registered devices.
+        """Retrieve all of a user's registered devices. Only returns devices
+        that are not marked as hidden.
 
         Args:
             user_id (str):
@@ -77,7 +81,7 @@ class DeviceWorkerStore(SQLBaseStore):
         """
         devices = yield self._simple_select_list(
             table="devices",
-            keyvalues={"user_id": user_id},
+            keyvalues={"user_id": user_id, "hidden": False},
             retcols=("user_id", "device_id", "display_name"),
             desc="get_devices_by_user",
         )
@@ -324,6 +328,41 @@ class DeviceWorkerStore(SQLBaseStore):
         """
         txn.execute(sql, (destination, stream_id))
 
+    @defer.inlineCallbacks
+    def add_user_signature_change_to_streams(self, from_user_id, user_ids):
+        """Persist that a user has made new signatures
+
+        Args:
+            from_user_id (str): the user who made the signatures
+            user_ids (list[str]): the users who were signed
+        """
+
+        with self._device_list_id_gen.get_next() as stream_id:
+            yield self.runInteraction(
+                "add_user_sig_change_to_streams",
+                self._add_user_signature_change_txn,
+                from_user_id,
+                user_ids,
+                stream_id,
+            )
+        return stream_id
+
+    def _add_user_signature_change_txn(self, txn, from_user_id, user_ids, stream_id):
+        txn.call_after(
+            self._user_signature_stream_cache.entity_has_changed,
+            from_user_id,
+            stream_id,
+        )
+        self._simple_insert_txn(
+            txn,
+            "user_signature_stream",
+            values={
+                "stream_id": stream_id,
+                "from_user_id": from_user_id,
+                "user_ids": json.dumps(user_ids),
+            },
+        )
+
     def get_device_stream_token(self):
         return self._device_list_id_gen.get_current_token()
 
@@ -469,6 +508,28 @@ class DeviceWorkerStore(SQLBaseStore):
             "get_users_whose_devices_changed", _get_users_whose_devices_changed_txn
         )
 
+    @defer.inlineCallbacks
+    def get_users_whose_signatures_changed(self, user_id, from_key):
+        """Get the users who have new cross-signing signatures made by `user_id` since
+        `from_key`.
+
+        Args:
+            user_id (str): the user who made the signatures
+            from_key (str): The device lists stream token
+        """
+        from_key = int(from_key)
+        if self._user_signature_stream_cache.has_entity_changed(user_id, from_key):
+            sql = """
+                SELECT DISTINCT user_ids FROM user_signature_stream
+                WHERE from_user_id = ? AND stream_id > ?
+            """
+            rows = yield self._execute(
+                "get_users_whose_signatures_changed", None, sql, user_id, from_key
+            )
+            return set(user for row in rows for user in json.loads(row[0]))
+        else:
+            return set()
+
     def get_all_device_list_changes_for_remotes(self, from_key, to_key):
         """Return a list of `(stream_id, user_id, destination)` which is the
         combined list of changes to devices, and which destinations need to be
@@ -592,6 +653,8 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
         Returns:
             defer.Deferred: boolean whether the device was inserted or an
                 existing device existed with that ID.
+        Raises:
+            StoreError: if the device is already in use
         """
         key = (user_id, device_id)
         if self.device_id_exists_cache.get(key, None):
@@ -604,12 +667,25 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
                     "user_id": user_id,
                     "device_id": device_id,
                     "display_name": initial_device_display_name,
+                    "hidden": False,
                 },
                 desc="store_device",
                 or_ignore=True,
             )
+            if not inserted:
+                # if the device already exists, check if it's a real device, or
+                # if the device ID is reserved by something else
+                hidden = yield self._simple_select_one_onecol(
+                    "devices",
+                    keyvalues={"user_id": user_id, "device_id": device_id},
+                    retcol="hidden",
+                )
+                if hidden:
+                    raise StoreError(400, "The device ID is in use", Codes.FORBIDDEN)
             self.device_id_exists_cache.prefill(key, True)
             return inserted
+        except StoreError:
+            raise
         except Exception as e:
             logger.error(
                 "store_device with device_id=%s(%r) user_id=%s(%r)"
@@ -636,7 +712,7 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
         """
         yield self._simple_delete_one(
             table="devices",
-            keyvalues={"user_id": user_id, "device_id": device_id},
+            keyvalues={"user_id": user_id, "device_id": device_id, "hidden": False},
             desc="delete_device",
         )
 
@@ -656,14 +732,15 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
             table="devices",
             column="device_id",
             iterable=device_ids,
-            keyvalues={"user_id": user_id},
+            keyvalues={"user_id": user_id, "hidden": False},
             desc="delete_devices",
         )
         for device_id in device_ids:
             self.device_id_exists_cache.invalidate((user_id, device_id))
 
     def update_device(self, user_id, device_id, new_display_name=None):
-        """Update a device.
+        """Update a device. Only updates the device if it is not marked as
+        hidden.
 
         Args:
             user_id (str): The ID of the user which owns the device
@@ -682,7 +759,7 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
             return defer.succeed(None)
         return self._simple_update_one(
             table="devices",
-            keyvalues={"user_id": user_id, "device_id": device_id},
+            keyvalues={"user_id": user_id, "device_id": device_id, "hidden": False},
             updatevalues=updates,
             desc="update_device",
         )
diff --git a/synapse/storage/end_to_end_keys.py b/synapse/storage/end_to_end_keys.py
index 872bc75490..ebcd1c9ea2 100644
--- a/synapse/storage/end_to_end_keys.py
+++ b/synapse/storage/end_to_end_keys.py
@@ -1,5 +1,7 @@
 # -*- coding: utf-8 -*-
 # Copyright 2015, 2016 OpenMarket Ltd
+# Copyright 2019 New Vector Ltd
+# Copyright 2019 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -14,7 +16,7 @@
 # limitations under the License.
 from six import iteritems
 
-from canonicaljson import encode_canonical_json
+from canonicaljson import encode_canonical_json, json
 
 from twisted.internet import defer
 
@@ -102,7 +104,7 @@ class EndToEndKeyWorkerStore(SQLBaseStore):
             "    k.key_json"
             " FROM devices d"
             "    %s JOIN e2e_device_keys_json k USING (user_id, device_id)"
-            " WHERE %s"
+            " WHERE %s AND NOT d.hidden"
         ) % (
             "LEFT" if include_all_devices else "INNER",
             " OR ".join("(" + q + ")" for q in query_clauses),
@@ -321,3 +323,169 @@ class EndToEndKeyStore(EndToEndKeyWorkerStore, SQLBaseStore):
         return self.runInteraction(
             "delete_e2e_keys_by_device", delete_e2e_keys_by_device_txn
         )
+
+    def _set_e2e_cross_signing_key_txn(self, txn, user_id, key_type, key):
+        """Set a user's cross-signing key.
+
+        Args:
+            txn (twisted.enterprise.adbapi.Connection): db connection
+            user_id (str): the user to set the signing key for
+            key_type (str): the type of key that is being set: either 'master'
+                for a master key, 'self_signing' for a self-signing key, or
+                'user_signing' for a user-signing key
+            key (dict): the key data
+        """
+        # the cross-signing keys need to occupy the same namespace as devices,
+        # since signatures are identified by device ID.  So add an entry to the
+        # device table to make sure that we don't have a collision with device
+        # IDs
+
+        # the 'key' dict will look something like:
+        # {
+        #   "user_id": "@alice:example.com",
+        #   "usage": ["self_signing"],
+        #   "keys": {
+        #     "ed25519:base64+self+signing+public+key": "base64+self+signing+public+key",
+        #   },
+        #   "signatures": {
+        #     "@alice:example.com": {
+        #       "ed25519:base64+master+public+key": "base64+signature"
+        #     }
+        #   }
+        # }
+        # The "keys" property must only have one entry, which will be the public
+        # key, so we just grab the first value in there
+        pubkey = next(iter(key["keys"].values()))
+        self._simple_insert_txn(
+            txn,
+            "devices",
+            values={
+                "user_id": user_id,
+                "device_id": pubkey,
+                "display_name": key_type + " signing key",
+                "hidden": True,
+            },
+        )
+
+        # and finally, store the key itself
+        with self._cross_signing_id_gen.get_next() as stream_id:
+            self._simple_insert_txn(
+                txn,
+                "e2e_cross_signing_keys",
+                values={
+                    "user_id": user_id,
+                    "keytype": key_type,
+                    "keydata": json.dumps(key),
+                    "stream_id": stream_id,
+                },
+            )
+
+    def set_e2e_cross_signing_key(self, user_id, key_type, key):
+        """Set a user's cross-signing key.
+
+        Args:
+            user_id (str): the user to set the user-signing key for
+            key_type (str): the type of cross-signing key to set
+            key (dict): the key data
+        """
+        return self.runInteraction(
+            "add_e2e_cross_signing_key",
+            self._set_e2e_cross_signing_key_txn,
+            user_id,
+            key_type,
+            key,
+        )
+
+    def _get_e2e_cross_signing_key_txn(self, txn, user_id, key_type, from_user_id=None):
+        """Returns a user's cross-signing key.
+
+        Args:
+            txn (twisted.enterprise.adbapi.Connection): db connection
+            user_id (str): the user whose key is being requested
+            key_type (str): the type of key that is being set: either 'master'
+                for a master key, 'self_signing' for a self-signing key, or
+                'user_signing' for a user-signing key
+            from_user_id (str): if specified, signatures made by this user on
+                the key will be included in the result
+
+        Returns:
+            dict of the key data or None if not found
+        """
+        sql = (
+            "SELECT keydata "
+            "  FROM e2e_cross_signing_keys "
+            " WHERE user_id = ? AND keytype = ? ORDER BY stream_id DESC LIMIT 1"
+        )
+        txn.execute(sql, (user_id, key_type))
+        row = txn.fetchone()
+        if not row:
+            return None
+        key = json.loads(row[0])
+
+        device_id = None
+        for k in key["keys"].values():
+            device_id = k
+
+        if from_user_id is not None:
+            sql = (
+                "SELECT key_id, signature "
+                "  FROM e2e_cross_signing_signatures "
+                " WHERE user_id = ? "
+                "   AND target_user_id = ? "
+                "   AND target_device_id = ? "
+            )
+            txn.execute(sql, (from_user_id, user_id, device_id))
+            row = txn.fetchone()
+            if row:
+                key.setdefault("signatures", {}).setdefault(from_user_id, {})[
+                    row[0]
+                ] = row[1]
+
+        return key
+
+    def get_e2e_cross_signing_key(self, user_id, key_type, from_user_id=None):
+        """Returns a user's cross-signing key.
+
+        Args:
+            user_id (str): the user whose self-signing key is being requested
+            key_type (str): the type of cross-signing key to get
+            from_user_id (str): if specified, signatures made by this user on
+                the self-signing key will be included in the result
+
+        Returns:
+            dict of the key data or None if not found
+        """
+        return self.runInteraction(
+            "get_e2e_cross_signing_key",
+            self._get_e2e_cross_signing_key_txn,
+            user_id,
+            key_type,
+            from_user_id,
+        )
+
+    def store_e2e_cross_signing_signatures(self, user_id, signatures):
+        """Stores cross-signing signatures.
+
+        Args:
+            user_id (str): the user who made the signatures
+            signatures (iterable[(str, str, str, str)]): signatures to add - each
+                a tuple of (key_id, target_user_id, target_device_id, signature),
+                where key_id is the ID of the key (including the signature
+                algorithm) that made the signature, target_user_id and
+                target_device_id indicate the device being signed, and signature
+                is the signature of the device
+        """
+        return self._simple_insert_many(
+            "e2e_cross_signing_signatures",
+            [
+                {
+                    "user_id": user_id,
+                    "key_id": key_id,
+                    "target_user_id": target_user_id,
+                    "target_device_id": target_device_id,
+                    "signature": signature,
+                }
+                for (key_id, target_user_id, target_device_id, signature) in signatures
+            ],
+            "add_e2e_signing_key",
+        )
diff --git a/synapse/storage/schema/delta/56/hidden_devices.sql b/synapse/storage/schema/delta/56/hidden_devices.sql
new file mode 100644
index 0000000000..67f8b20297
--- /dev/null
+++ b/synapse/storage/schema/delta/56/hidden_devices.sql
@@ -0,0 +1,18 @@
+/* Copyright 2019 New Vector 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.
+ */
+
+-- device list needs to know which ones are "real" devices, and which ones are
+-- just used to avoid collisions
+ALTER TABLE devices ADD COLUMN hidden BOOLEAN DEFAULT FALSE;
diff --git a/synapse/storage/schema/delta/56/signing_keys.sql b/synapse/storage/schema/delta/56/signing_keys.sql
new file mode 100644
index 0000000000..27a96123e3
--- /dev/null
+++ b/synapse/storage/schema/delta/56/signing_keys.sql
@@ -0,0 +1,55 @@
+/* Copyright 2019 New Vector 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.
+ */
+
+-- cross-signing keys
+CREATE TABLE IF NOT EXISTS e2e_cross_signing_keys (
+    user_id TEXT NOT NULL,
+    -- the type of cross-signing key (master, user_signing, or self_signing)
+    keytype TEXT NOT NULL,
+    -- the full key information, as a json-encoded dict
+    keydata TEXT NOT NULL,
+    -- for keeping the keys in order, so that we can fetch the latest one
+    stream_id BIGINT NOT NULL
+);
+
+CREATE UNIQUE INDEX e2e_cross_signing_keys_idx ON e2e_cross_signing_keys(user_id, keytype, stream_id);
+
+-- cross-signing signatures
+CREATE TABLE IF NOT EXISTS e2e_cross_signing_signatures (
+    -- user who did the signing
+    user_id TEXT NOT NULL,
+    -- key used to sign
+    key_id TEXT NOT NULL,
+    -- user who was signed
+    target_user_id TEXT NOT NULL,
+    -- device/key that was signed
+    target_device_id TEXT NOT NULL,
+    -- the actual signature
+    signature TEXT NOT NULL
+);
+
+CREATE UNIQUE INDEX e2e_cross_signing_signatures_idx ON e2e_cross_signing_signatures(user_id, target_user_id, target_device_id);
+
+-- stream of user signature updates
+CREATE TABLE IF NOT EXISTS user_signature_stream (
+    -- uses the same stream ID as device list stream
+    stream_id BIGINT NOT NULL,
+    -- user who did the signing
+    from_user_id TEXT NOT NULL,
+    -- list of users who were signed, as a JSON array
+    user_ids TEXT NOT NULL
+);
+
+CREATE UNIQUE INDEX user_signature_stream_idx ON user_signature_stream(stream_id);
diff --git a/synapse/types.py b/synapse/types.py
index 8f79797f17..aafc3ffe74 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2019 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -17,6 +18,8 @@ import string
 from collections import namedtuple
 
 import attr
+from signedjson.key import decode_verify_key_bytes
+from unpaddedbase64 import decode_base64
 
 from synapse.api.errors import SynapseError
 
@@ -476,3 +479,24 @@ class ReadReceipt(object):
     user_id = attr.ib()
     event_ids = attr.ib()
     data = attr.ib()
+
+
+def get_verify_key_from_cross_signing_key(key_info):
+    """Get the key ID and signedjson verify key from a cross-signing key dict
+
+    Args:
+        key_info (dict): a cross-signing key dict, which must have a "keys"
+            property that has exactly one item in it
+
+    Returns:
+        (str, VerifyKey): the key ID and verify key for the cross-signing key
+    """
+    # make sure that exactly one key is provided
+    if "keys" not in key_info:
+        raise ValueError("Invalid key")
+    keys = key_info["keys"]
+    if len(keys) != 1:
+        raise ValueError("Invalid key")
+    # and return that one key
+    for key_id, key_data in keys.items():
+        return (key_id, decode_verify_key_bytes(key_id, decode_base64(key_data)))
diff --git a/tests/handlers/test_e2e_keys.py b/tests/handlers/test_e2e_keys.py
index 8dccc6826e..a62c52eefa 100644
--- a/tests/handlers/test_e2e_keys.py
+++ b/tests/handlers/test_e2e_keys.py
@@ -1,5 +1,7 @@
 # -*- coding: utf-8 -*-
 # Copyright 2016 OpenMarket Ltd
+# Copyright 2019 New Vector Ltd
+# Copyright 2019 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -145,3 +147,66 @@ class E2eKeysHandlerTestCase(unittest.TestCase):
                 "one_time_keys": {local_user: {device_id: {"alg1:k1": "key1"}}},
             },
         )
+
+    @defer.inlineCallbacks
+    def test_replace_master_key(self):
+        """uploading a new signing key should make the old signing key unavailable"""
+        local_user = "@boris:" + self.hs.hostname
+        keys1 = {
+            "master_key": {
+                # private key: 2lonYOM6xYKdEsO+6KrC766xBcHnYnim1x/4LFGF8B0
+                "user_id": local_user,
+                "usage": ["master"],
+                "keys": {
+                    "ed25519:nqOvzeuGWT/sRx3h7+MHoInYj3Uk2LD/unI9kDYcHwk": "nqOvzeuGWT/sRx3h7+MHoInYj3Uk2LD/unI9kDYcHwk"
+                },
+            }
+        }
+        yield self.handler.upload_signing_keys_for_user(local_user, keys1)
+
+        keys2 = {
+            "master_key": {
+                # private key: 4TL4AjRYwDVwD3pqQzcor+ez/euOB1/q78aTJ+czDNs
+                "user_id": local_user,
+                "usage": ["master"],
+                "keys": {
+                    "ed25519:Hq6gL+utB4ET+UvD5ci0kgAwsX6qP/zvf8v6OInU5iw": "Hq6gL+utB4ET+UvD5ci0kgAwsX6qP/zvf8v6OInU5iw"
+                },
+            }
+        }
+        yield self.handler.upload_signing_keys_for_user(local_user, keys2)
+
+        devices = yield self.handler.query_devices(
+            {"device_keys": {local_user: []}}, 0, local_user
+        )
+        self.assertDictEqual(devices["master_keys"], {local_user: keys2["master_key"]})
+
+    @defer.inlineCallbacks
+    def test_self_signing_key_doesnt_show_up_as_device(self):
+        """signing keys should be hidden when fetching a user's devices"""
+        local_user = "@boris:" + self.hs.hostname
+        keys1 = {
+            "master_key": {
+                # private key: 2lonYOM6xYKdEsO+6KrC766xBcHnYnim1x/4LFGF8B0
+                "user_id": local_user,
+                "usage": ["master"],
+                "keys": {
+                    "ed25519:nqOvzeuGWT/sRx3h7+MHoInYj3Uk2LD/unI9kDYcHwk": "nqOvzeuGWT/sRx3h7+MHoInYj3Uk2LD/unI9kDYcHwk"
+                },
+            }
+        }
+        yield self.handler.upload_signing_keys_for_user(local_user, keys1)
+
+        res = None
+        try:
+            yield self.hs.get_device_handler().check_device_registered(
+                user_id=local_user,
+                device_id="nqOvzeuGWT/sRx3h7+MHoInYj3Uk2LD/unI9kDYcHwk",
+                initial_device_display_name="new display name",
+            )
+        except errors.SynapseError as e:
+            res = e.code
+        self.assertEqual(res, 400)
+
+        res = yield self.handler.query_local_devices({local_user: None})
+        self.assertDictEqual(res, {local_user: {}})