MatrixSynapse/tests/crypto/test_keyring.py

# Copyright 2017-2021 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.
# 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 time
from typing import Dict, List
from unittest.mock import Mock

import attr
import canonicaljson
import signedjson.key
import signedjson.sign
from nacl.signing import SigningKey
from signedjson.key import encode_verify_key_base64, get_verify_key

from twisted.internet import defer
from twisted.internet.defer import Deferred, ensureDeferred

from synapse.api.errors import SynapseError
from synapse.crypto import keyring
from synapse.crypto.keyring import (
    PerspectivesKeyFetcher,
    ServerKeyFetcher,
    StoreKeyFetcher,
)
from synapse.logging.context import (
    LoggingContext,
    current_context,
    make_deferred_yieldable,
)
from synapse.storage.keys import FetchKeyResult

from tests import unittest
from tests.test_utils import make_awaitable
from tests.unittest import logcontext_clean, override_config


class MockPerspectiveServer:
    def __init__(self):
        self.server_name = "mock_server"
        self.key = signedjson.key.generate_signing_key(0)

    def get_verify_keys(self):
        vk = signedjson.key.get_verify_key(self.key)
        return {"%s:%s" % (vk.alg, vk.version): encode_verify_key_base64(vk)}

    def get_signed_key(self, server_name, verify_key):
        key_id = "%s:%s" % (verify_key.alg, verify_key.version)
        res = {
            "server_name": server_name,
            "old_verify_keys": {},
            "valid_until_ts": time.time() * 1000 + 3600,
            "verify_keys": {key_id: {"key": encode_verify_key_base64(verify_key)}},
        }
        self.sign_response(res)
        return res

    def sign_response(self, res):
        signedjson.sign.sign_json(res, self.server_name, self.key)


@attr.s(slots=True)
class FakeRequest:
    id = attr.ib()


@logcontext_clean
class KeyringTestCase(unittest.HomeserverTestCase):
    def check_context(self, val, expected):
        self.assertEquals(getattr(current_context(), "request", None), expected)
        return val

    def test_verify_json_objects_for_server_awaits_previous_requests(self):
        mock_fetcher = Mock()
        mock_fetcher.get_keys = Mock()
        kr = keyring.Keyring(self.hs, key_fetchers=(mock_fetcher,))

        # a signed object that we are going to try to validate
        key1 = signedjson.key.generate_signing_key(1)
        json1 = {}
        signedjson.sign.sign_json(json1, "server10", key1)

        # start off a first set of lookups. We make the mock fetcher block until this
        # deferred completes.
        first_lookup_deferred = Deferred()

        async def first_lookup_fetch(
            server_name: str, key_ids: List[str], minimum_valid_until_ts: int
        ) -> Dict[str, FetchKeyResult]:
            # self.assertEquals(current_context().request.id, "context_11")
            self.assertEqual(server_name, "server10")
            self.assertEqual(key_ids, [get_key_id(key1)])
            self.assertEqual(minimum_valid_until_ts, 0)

            await make_deferred_yieldable(first_lookup_deferred)
            return {get_key_id(key1): FetchKeyResult(get_verify_key(key1), 100)}

        mock_fetcher.get_keys.side_effect = first_lookup_fetch

        async def first_lookup():
            with LoggingContext("context_11", request=FakeRequest("context_11")):
                res_deferreds = kr.verify_json_objects_for_server(
                    [("server10", json1, 0), ("server11", {}, 0)]
                )

                # the unsigned json should be rejected pretty quickly
                self.assertTrue(res_deferreds[1].called)
                try:
                    await res_deferreds[1]
                    self.assertFalse("unsigned json didn't cause a failure")
                except SynapseError:
                    pass

                self.assertFalse(res_deferreds[0].called)
                res_deferreds[0].addBoth(self.check_context, None)

                await make_deferred_yieldable(res_deferreds[0])

        d0 = ensureDeferred(first_lookup())

        self.pump()

        mock_fetcher.get_keys.assert_called_once()

        # a second request for a server with outstanding requests
        # should block rather than start a second call

        async def second_lookup_fetch(
            server_name: str, key_ids: List[str], minimum_valid_until_ts: int
        ) -> Dict[str, FetchKeyResult]:
            # self.assertEquals(current_context().request.id, "context_12")
            return {get_key_id(key1): FetchKeyResult(get_verify_key(key1), 100)}

        mock_fetcher.get_keys.reset_mock()
        mock_fetcher.get_keys.side_effect = second_lookup_fetch
        second_lookup_state = [0]

        async def second_lookup():
            with LoggingContext("context_12", request=FakeRequest("context_12")):
                res_deferreds_2 = kr.verify_json_objects_for_server(
                    [
                        (
                            "server10",
                            json1,
                            0,
                        )
                    ]
                )
                res_deferreds_2[0].addBoth(self.check_context, None)
                second_lookup_state[0] = 1
                await make_deferred_yieldable(res_deferreds_2[0])
                second_lookup_state[0] = 2

        d2 = ensureDeferred(second_lookup())

        self.pump()
        # the second request should be pending, but the fetcher should not yet have been
        # called
        self.assertEqual(second_lookup_state[0], 1)
        mock_fetcher.get_keys.assert_not_called()

        # complete the first request
        first_lookup_deferred.callback(None)

        # and now both verifications should succeed.
        self.get_success(d0)
        self.get_success(d2)

    def test_verify_json_for_server(self):
        kr = keyring.Keyring(self.hs)

        key1 = signedjson.key.generate_signing_key(1)
        r = self.hs.get_datastores().main.store_server_verify_keys(
            "server9",
            time.time() * 1000,
            [("server9", get_key_id(key1), FetchKeyResult(get_verify_key(key1), 1000))],
        )
        self.get_success(r)

        json1 = {}
        signedjson.sign.sign_json(json1, "server9", key1)

        # should fail immediately on an unsigned object
        d = kr.verify_json_for_server("server9", {}, 0)
        self.get_failure(d, SynapseError)

        # should succeed on a signed object
        d = kr.verify_json_for_server("server9", json1, 500)
        # self.assertFalse(d.called)
        self.get_success(d)

    def test_verify_for_local_server(self):
        """Ensure that locally signed JSON can be verified without fetching keys
        over federation
        """
        kr = keyring.Keyring(self.hs)
        json1 = {}
        signedjson.sign.sign_json(json1, self.hs.hostname, self.hs.signing_key)

        # Test that verify_json_for_server succeeds on a object signed by ourselves
        d = kr.verify_json_for_server(self.hs.hostname, json1, 0)
        self.get_success(d)

    OLD_KEY = signedjson.key.generate_signing_key("old")

    @override_config(
        {
            "old_signing_keys": {
                f"{OLD_KEY.alg}:{OLD_KEY.version}": {
                    "key": encode_verify_key_base64(OLD_KEY.verify_key),
                    "expired_ts": 1000,
                }
            }
        }
    )
    def test_verify_for_local_server_old_key(self):
        """Can also use keys in old_signing_keys for verification"""
        json1 = {}
        signedjson.sign.sign_json(json1, self.hs.hostname, self.OLD_KEY)

        kr = keyring.Keyring(self.hs)
        d = kr.verify_json_for_server(self.hs.hostname, json1, 0)
        self.get_success(d)

    def test_verify_for_local_server_unknown_key(self):
        """Local keys that we no longer have should be fetched via the fetcher"""

        # the key we'll sign things with (nb, not known to the Keyring)
        key2 = signedjson.key.generate_signing_key("2")

        # set up a mock fetcher which will return the key
        async def get_keys(
            server_name: str, key_ids: List[str], minimum_valid_until_ts: int
        ) -> Dict[str, FetchKeyResult]:
            self.assertEqual(server_name, self.hs.hostname)
            self.assertEqual(key_ids, [get_key_id(key2)])

            return {get_key_id(key2): FetchKeyResult(get_verify_key(key2), 1200)}

        mock_fetcher = Mock()
        mock_fetcher.get_keys = Mock(side_effect=get_keys)
        kr = keyring.Keyring(
            self.hs, key_fetchers=(StoreKeyFetcher(self.hs), mock_fetcher)
        )

        # sign the json
        json1 = {}
        signedjson.sign.sign_json(json1, self.hs.hostname, key2)

        # ... and check we can verify it.
        d = kr.verify_json_for_server(self.hs.hostname, json1, 0)
        self.get_success(d)

    def test_verify_json_for_server_with_null_valid_until_ms(self):
        """Tests that we correctly handle key requests for keys we've stored
        with a null `ts_valid_until_ms`
        """
        mock_fetcher = Mock()
        mock_fetcher.get_keys = Mock(return_value=make_awaitable({}))

        kr = keyring.Keyring(
            self.hs, key_fetchers=(StoreKeyFetcher(self.hs), mock_fetcher)
        )

        key1 = signedjson.key.generate_signing_key(1)
        r = self.hs.get_datastores().main.store_server_verify_keys(
            "server9",
            time.time() * 1000,
            [("server9", get_key_id(key1), FetchKeyResult(get_verify_key(key1), None))],
        )
        self.get_success(r)

        json1 = {}
        signedjson.sign.sign_json(json1, "server9", key1)

        # should fail immediately on an unsigned object
        d = kr.verify_json_for_server("server9", {}, 0)
        self.get_failure(d, SynapseError)

        # should fail on a signed object with a non-zero minimum_valid_until_ms,
        # as it tries to refetch the keys and fails.
        d = kr.verify_json_for_server("server9", json1, 500)
        self.get_failure(d, SynapseError)

        # We expect the keyring tried to refetch the key once.
        mock_fetcher.get_keys.assert_called_once_with(
            "server9", [get_key_id(key1)], 500
        )

        # should succeed on a signed object with a 0 minimum_valid_until_ms
        d = kr.verify_json_for_server(
            "server9",
            json1,
            0,
        )
        self.get_success(d)

    def test_verify_json_dedupes_key_requests(self):
        """Two requests for the same key should be deduped."""
        key1 = signedjson.key.generate_signing_key(1)

        async def get_keys(
            server_name: str, key_ids: List[str], minimum_valid_until_ts: int
        ) -> Dict[str, FetchKeyResult]:
            # there should only be one request object (with the max validity)
            self.assertEqual(server_name, "server1")
            self.assertEqual(key_ids, [get_key_id(key1)])
            self.assertEqual(minimum_valid_until_ts, 1500)

            return {get_key_id(key1): FetchKeyResult(get_verify_key(key1), 1200)}

        mock_fetcher = Mock()
        mock_fetcher.get_keys = Mock(side_effect=get_keys)
        kr = keyring.Keyring(self.hs, key_fetchers=(mock_fetcher,))

        json1 = {}
        signedjson.sign.sign_json(json1, "server1", key1)

        # the first request should succeed; the second should fail because the key
        # has expired
        results = kr.verify_json_objects_for_server(
            [
                (
                    "server1",
                    json1,
                    500,
                ),
                ("server1", json1, 1500),
            ]
        )
        self.assertEqual(len(results), 2)
        self.get_success(results[0])
        e = self.get_failure(results[1], SynapseError).value
        self.assertEqual(e.errcode, "M_UNAUTHORIZED")
        self.assertEqual(e.code, 401)

        # there should have been a single call to the fetcher
        mock_fetcher.get_keys.assert_called_once()

    def test_verify_json_falls_back_to_other_fetchers(self):
        """If the first fetcher cannot provide a recent enough key, we fall back"""
        key1 = signedjson.key.generate_signing_key(1)

        async def get_keys1(
            server_name: str, key_ids: List[str], minimum_valid_until_ts: int
        ) -> Dict[str, FetchKeyResult]:
            self.assertEqual(server_name, "server1")
            self.assertEqual(key_ids, [get_key_id(key1)])
            self.assertEqual(minimum_valid_until_ts, 1500)
            return {get_key_id(key1): FetchKeyResult(get_verify_key(key1), 800)}

        async def get_keys2(
            server_name: str, key_ids: List[str], minimum_valid_until_ts: int
        ) -> Dict[str, FetchKeyResult]:
            self.assertEqual(server_name, "server1")
            self.assertEqual(key_ids, [get_key_id(key1)])
            self.assertEqual(minimum_valid_until_ts, 1500)
            return {get_key_id(key1): FetchKeyResult(get_verify_key(key1), 1200)}

        mock_fetcher1 = Mock()
        mock_fetcher1.get_keys = Mock(side_effect=get_keys1)
        mock_fetcher2 = Mock()
        mock_fetcher2.get_keys = Mock(side_effect=get_keys2)
        kr = keyring.Keyring(self.hs, key_fetchers=(mock_fetcher1, mock_fetcher2))

        json1 = {}
        signedjson.sign.sign_json(json1, "server1", key1)

        results = kr.verify_json_objects_for_server(
            [
                (
                    "server1",
                    json1,
                    1200,
                ),
                (
                    "server1",
                    json1,
                    1500,
                ),
            ]
        )
        self.assertEqual(len(results), 2)
        self.get_success(results[0])
        e = self.get_failure(results[1], SynapseError).value
        self.assertEqual(e.errcode, "M_UNAUTHORIZED")
        self.assertEqual(e.code, 401)

        # there should have been a single call to each fetcher
        mock_fetcher1.get_keys.assert_called_once()
        mock_fetcher2.get_keys.assert_called_once()


@logcontext_clean
class ServerKeyFetcherTestCase(unittest.HomeserverTestCase):
    def make_homeserver(self, reactor, clock):
        self.http_client = Mock()
        hs = self.setup_test_homeserver(federation_http_client=self.http_client)
        return hs

    def test_get_keys_from_server(self):
        # arbitrarily advance the clock a bit
        self.reactor.advance(100)

        SERVER_NAME = "server2"
        fetcher = ServerKeyFetcher(self.hs)
        testkey = signedjson.key.generate_signing_key("ver1")
        testverifykey = signedjson.key.get_verify_key(testkey)
        testverifykey_id = "ed25519:ver1"
        VALID_UNTIL_TS = 200 * 1000

        # valid response
        response = {
            "server_name": SERVER_NAME,
            "old_verify_keys": {},
            "valid_until_ts": VALID_UNTIL_TS,
            "verify_keys": {
                testverifykey_id: {
                    "key": signedjson.key.encode_verify_key_base64(testverifykey)
                }
            },
        }
        signedjson.sign.sign_json(response, SERVER_NAME, testkey)

        async def get_json(destination, path, **kwargs):
            self.assertEqual(destination, SERVER_NAME)
            self.assertEqual(path, "/_matrix/key/v2/server/key1")
            return response

        self.http_client.get_json.side_effect = get_json

        keys = self.get_success(fetcher.get_keys(SERVER_NAME, ["key1"], 0))
        k = keys[testverifykey_id]
        self.assertEqual(k.valid_until_ts, VALID_UNTIL_TS)
        self.assertEqual(k.verify_key, testverifykey)
        self.assertEqual(k.verify_key.alg, "ed25519")
        self.assertEqual(k.verify_key.version, "ver1")

        # check that the perspectives store is correctly updated
        lookup_triplet = (SERVER_NAME, testverifykey_id, None)
        key_json = self.get_success(
            self.hs.get_datastores().main.get_server_keys_json([lookup_triplet])
        )
        res = key_json[lookup_triplet]
        self.assertEqual(len(res), 1)
        res = res[0]
        self.assertEqual(res["key_id"], testverifykey_id)
        self.assertEqual(res["from_server"], SERVER_NAME)
        self.assertEqual(res["ts_added_ms"], self.reactor.seconds() * 1000)
        self.assertEqual(res["ts_valid_until_ms"], VALID_UNTIL_TS)

        # we expect it to be encoded as canonical json *before* it hits the db
        self.assertEqual(
            bytes(res["key_json"]), canonicaljson.encode_canonical_json(response)
        )

        # change the server name: the result should be ignored
        response["server_name"] = "OTHER_SERVER"

        keys = self.get_success(fetcher.get_keys(SERVER_NAME, ["key1"], 0))
        self.assertEqual(keys, {})


class PerspectivesKeyFetcherTestCase(unittest.HomeserverTestCase):
    def make_homeserver(self, reactor, clock):
        self.mock_perspective_server = MockPerspectiveServer()
        self.http_client = Mock()

        config = self.default_config()
        config["trusted_key_servers"] = [
            {
                "server_name": self.mock_perspective_server.server_name,
                "verify_keys": self.mock_perspective_server.get_verify_keys(),
            }
        ]

        return self.setup_test_homeserver(
            federation_http_client=self.http_client, config=config
        )

    def build_perspectives_response(
        self,
        server_name: str,
        signing_key: SigningKey,
        valid_until_ts: int,
    ) -> dict:
        """
        Build a valid perspectives server response to a request for the given key
        """
        verify_key = signedjson.key.get_verify_key(signing_key)
        verifykey_id = "%s:%s" % (verify_key.alg, verify_key.version)

        response = {
            "server_name": server_name,
            "old_verify_keys": {},
            "valid_until_ts": valid_until_ts,
            "verify_keys": {
                verifykey_id: {
                    "key": signedjson.key.encode_verify_key_base64(verify_key)
                }
            },
        }
        # the response must be signed by both the origin server and the perspectives
        # server.
        signedjson.sign.sign_json(response, server_name, signing_key)
        self.mock_perspective_server.sign_response(response)
        return response

    def expect_outgoing_key_query(
        self, expected_server_name: str, expected_key_id: str, response: dict
    ) -> None:
        """
        Tell the mock http client to expect a perspectives-server key query
        """

        async def post_json(destination, path, data, **kwargs):
            self.assertEqual(destination, self.mock_perspective_server.server_name)
            self.assertEqual(path, "/_matrix/key/v2/query")

            # check that the request is for the expected key
            q = data["server_keys"]
            self.assertEqual(list(q[expected_server_name].keys()), [expected_key_id])
            return {"server_keys": [response]}

        self.http_client.post_json.side_effect = post_json

    def test_get_keys_from_perspectives(self):
        # arbitrarily advance the clock a bit
        self.reactor.advance(100)

        fetcher = PerspectivesKeyFetcher(self.hs)

        SERVER_NAME = "server2"
        testkey = signedjson.key.generate_signing_key("ver1")
        testverifykey = signedjson.key.get_verify_key(testkey)
        testverifykey_id = "ed25519:ver1"
        VALID_UNTIL_TS = 200 * 1000

        response = self.build_perspectives_response(
            SERVER_NAME,
            testkey,
            VALID_UNTIL_TS,
        )

        self.expect_outgoing_key_query(SERVER_NAME, "key1", response)

        keys = self.get_success(fetcher.get_keys(SERVER_NAME, ["key1"], 0))
        self.assertIn(testverifykey_id, keys)
        k = keys[testverifykey_id]
        self.assertEqual(k.valid_until_ts, VALID_UNTIL_TS)
        self.assertEqual(k.verify_key, testverifykey)
        self.assertEqual(k.verify_key.alg, "ed25519")
        self.assertEqual(k.verify_key.version, "ver1")

        # check that the perspectives store is correctly updated
        lookup_triplet = (SERVER_NAME, testverifykey_id, None)
        key_json = self.get_success(
            self.hs.get_datastores().main.get_server_keys_json([lookup_triplet])
        )
        res = key_json[lookup_triplet]
        self.assertEqual(len(res), 1)
        res = res[0]
        self.assertEqual(res["key_id"], testverifykey_id)
        self.assertEqual(res["from_server"], self.mock_perspective_server.server_name)
        self.assertEqual(res["ts_added_ms"], self.reactor.seconds() * 1000)
        self.assertEqual(res["ts_valid_until_ms"], VALID_UNTIL_TS)

        self.assertEqual(
            bytes(res["key_json"]), canonicaljson.encode_canonical_json(response)
        )

    def test_get_multiple_keys_from_perspectives(self):
        """Check that we can correctly request multiple keys for the same server"""

        fetcher = PerspectivesKeyFetcher(self.hs)

        SERVER_NAME = "server2"

        testkey1 = signedjson.key.generate_signing_key("ver1")
        testverifykey1 = signedjson.key.get_verify_key(testkey1)
        testverifykey1_id = "ed25519:ver1"

        testkey2 = signedjson.key.generate_signing_key("ver2")
        testverifykey2 = signedjson.key.get_verify_key(testkey2)
        testverifykey2_id = "ed25519:ver2"

        VALID_UNTIL_TS = 200 * 1000

        response1 = self.build_perspectives_response(
            SERVER_NAME,
            testkey1,
            VALID_UNTIL_TS,
        )
        response2 = self.build_perspectives_response(
            SERVER_NAME,
            testkey2,
            VALID_UNTIL_TS,
        )

        async def post_json(destination, path, data, **kwargs):
            self.assertEqual(destination, self.mock_perspective_server.server_name)
            self.assertEqual(path, "/_matrix/key/v2/query")

            # check that the request is for the expected keys
            q = data["server_keys"]

            self.assertEqual(
                list(q[SERVER_NAME].keys()), [testverifykey1_id, testverifykey2_id]
            )
            return {"server_keys": [response1, response2]}

        self.http_client.post_json.side_effect = post_json

        # fire off two separate requests; they should get merged together into a
        # single HTTP hit.
        request1_d = defer.ensureDeferred(
            fetcher.get_keys(SERVER_NAME, [testverifykey1_id], 0)
        )
        request2_d = defer.ensureDeferred(
            fetcher.get_keys(SERVER_NAME, [testverifykey2_id], 0)
        )

        keys1 = self.get_success(request1_d)
        self.assertIn(testverifykey1_id, keys1)
        k = keys1[testverifykey1_id]
        self.assertEqual(k.valid_until_ts, VALID_UNTIL_TS)
        self.assertEqual(k.verify_key, testverifykey1)
        self.assertEqual(k.verify_key.alg, "ed25519")
        self.assertEqual(k.verify_key.version, "ver1")

        keys2 = self.get_success(request2_d)
        self.assertIn(testverifykey2_id, keys2)
        k = keys2[testverifykey2_id]
        self.assertEqual(k.valid_until_ts, VALID_UNTIL_TS)
        self.assertEqual(k.verify_key, testverifykey2)
        self.assertEqual(k.verify_key.alg, "ed25519")
        self.assertEqual(k.verify_key.version, "ver2")

        # finally, ensure that only one request was sent
        self.assertEqual(self.http_client.post_json.call_count, 1)

    def test_get_perspectives_own_key(self):
        """Check that we can get the perspectives server's own keys

        This is slightly complicated by the fact that the perspectives server may
        use different keys for signing notary responses.
        """

        # arbitrarily advance the clock a bit
        self.reactor.advance(100)

        fetcher = PerspectivesKeyFetcher(self.hs)

        SERVER_NAME = self.mock_perspective_server.server_name
        testkey = signedjson.key.generate_signing_key("ver1")
        testverifykey = signedjson.key.get_verify_key(testkey)
        testverifykey_id = "ed25519:ver1"
        VALID_UNTIL_TS = 200 * 1000

        response = self.build_perspectives_response(
            SERVER_NAME, testkey, VALID_UNTIL_TS
        )

        self.expect_outgoing_key_query(SERVER_NAME, "key1", response)

        keys = self.get_success(fetcher.get_keys(SERVER_NAME, ["key1"], 0))
        self.assertIn(testverifykey_id, keys)
        k = keys[testverifykey_id]
        self.assertEqual(k.valid_until_ts, VALID_UNTIL_TS)
        self.assertEqual(k.verify_key, testverifykey)
        self.assertEqual(k.verify_key.alg, "ed25519")
        self.assertEqual(k.verify_key.version, "ver1")

        # check that the perspectives store is correctly updated
        lookup_triplet = (SERVER_NAME, testverifykey_id, None)
        key_json = self.get_success(
            self.hs.get_datastores().main.get_server_keys_json([lookup_triplet])
        )
        res = key_json[lookup_triplet]
        self.assertEqual(len(res), 1)
        res = res[0]
        self.assertEqual(res["key_id"], testverifykey_id)
        self.assertEqual(res["from_server"], self.mock_perspective_server.server_name)
        self.assertEqual(res["ts_added_ms"], self.reactor.seconds() * 1000)
        self.assertEqual(res["ts_valid_until_ms"], VALID_UNTIL_TS)

        self.assertEqual(
            bytes(res["key_json"]), canonicaljson.encode_canonical_json(response)
        )

    def test_invalid_perspectives_responses(self):
        """Check that invalid responses from the perspectives server are rejected"""
        # arbitrarily advance the clock a bit
        self.reactor.advance(100)

        SERVER_NAME = "server2"
        testkey = signedjson.key.generate_signing_key("ver1")
        testverifykey = signedjson.key.get_verify_key(testkey)
        testverifykey_id = "ed25519:ver1"
        VALID_UNTIL_TS = 200 * 1000

        def build_response():
            return self.build_perspectives_response(
                SERVER_NAME, testkey, VALID_UNTIL_TS
            )

        def get_key_from_perspectives(response):
            fetcher = PerspectivesKeyFetcher(self.hs)
            self.expect_outgoing_key_query(SERVER_NAME, "key1", response)
            return self.get_success(fetcher.get_keys(SERVER_NAME, ["key1"], 0))

        # start with a valid response so we can check we are testing the right thing
        response = build_response()
        keys = get_key_from_perspectives(response)
        k = keys[testverifykey_id]
        self.assertEqual(k.verify_key, testverifykey)

        # remove the perspectives server's signature
        response = build_response()
        del response["signatures"][self.mock_perspective_server.server_name]
        keys = get_key_from_perspectives(response)
        self.assertEqual(keys, {}, "Expected empty dict with missing persp server sig")

        # remove the origin server's signature
        response = build_response()
        del response["signatures"][SERVER_NAME]
        keys = get_key_from_perspectives(response)
        self.assertEqual(keys, {}, "Expected empty dict with missing origin server sig")


def get_key_id(key):
    """Get the matrix ID tag for a given SigningKey or VerifyKey"""
    return "%s:%s" % (key.alg, key.version)