diff --git a/tests/crypto/test_event_signing.py b/tests/crypto/test_event_signing.py
new file mode 100644
index 0000000000..0b560e9317
--- /dev/null
+++ b/tests/crypto/test_event_signing.py
@@ -0,0 +1,98 @@
+# -*- 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 tests import unittest
+from tests.utils import MockClock
+
+from synapse.events.builder import EventBuilderFactory
+from synapse.crypto.event_signing import add_hashes_and_signatures
+from synapse.types import EventID
+
+from unpaddedbase64 import decode_base64
+
+import nacl.signing
+
+
+# Perform these tests using given secret key so we get entirely deterministic
+# signatures output that we can test against.
+SIGNING_KEY_SEED = decode_base64(
+    "YJDBA9Xnr2sVqXD9Vj7XVUnmFZcZrlw8Md7kMW+3XA1"
+)
+
+KEY_ALG = "ed25519"
+KEY_VER = 1
+KEY_NAME = "%s:%d" % (KEY_ALG, KEY_VER)
+
+HOSTNAME = "domain"
+
+
+class EventBuilderFactoryWithPredicableIDs(EventBuilderFactory):
+    """ A subclass of EventBuilderFactory that generates entirely predicatable
+    event IDs, so we can assert on them. """
+    def create_event_id(self):
+        i = str(self.event_id_count)
+        self.event_id_count += 1
+
+        return EventID.create(i, self.hostname).to_string()
+
+
+class EventSigningTestCase(unittest.TestCase):
+
+    def setUp(self):
+        self.event_builder_factory = EventBuilderFactoryWithPredicableIDs(
+            clock=MockClock(),
+            hostname=HOSTNAME,
+        )
+
+        self.signing_key = nacl.signing.SigningKey(SIGNING_KEY_SEED)
+        self.signing_key.alg = KEY_ALG
+        self.signing_key.version = KEY_VER
+
+    def test_sign(self):
+        builder = self.event_builder_factory.new(
+            {'type': "X"}
+        )
+        self.assertEquals(
+            builder.build().get_dict(),
+            {
+                'event_id': "$0:domain",
+                'origin': "domain",
+                'origin_server_ts': 1000000,
+                'signatures': {},
+                'type': "X",
+                'unsigned': {'age_ts': 1000000},
+            },
+        )
+
+        add_hashes_and_signatures(builder, HOSTNAME, self.signing_key)
+
+        event = builder.build()
+
+        self.assertTrue(hasattr(event, 'hashes'))
+        self.assertTrue('sha256' in event.hashes)
+        self.assertEquals(
+            event.hashes['sha256'],
+            "6tJjLpXtggfke8UxFhAKg82QVkJzvKOVOOSjUDK4ZSI",
+        )
+
+        self.assertTrue(hasattr(event, 'signatures'))
+        self.assertTrue(HOSTNAME in event.signatures)
+        self.assertTrue(KEY_NAME in event.signatures["domain"])
+        self.assertEquals(
+            event.signatures[HOSTNAME][KEY_NAME],
+            "2Wptgo4CwmLo/Y8B8qinxApKaCkBG2fjTWB7AbP5Uy+"
+            "aIbygsSdLOFzvdDjww8zUVKCmI02eP9xtyJxc/cLiBA",
+        )