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diff --git a/synapse/handlers/acme_issuing_service.py b/synapse/handlers/acme_issuing_service.py
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+++ b/synapse/handlers/acme_issuing_service.py
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+# -*- coding: utf-8 -*-
+# 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.
+# 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.
+
+"""
+Utility function to create an ACME issuing service.
+
+This file contains the unconditional imports on the acme and cryptography bits that we
+only need (and may only have available) if we are doing ACME, so is designed to be
+imported conditionally.
+"""
+import logging
+
+import attr
+from cryptography.hazmat.backends import default_backend
+from cryptography.hazmat.primitives import serialization
+from josepy import JWKRSA
+from josepy.jwa import RS256
+from txacme.challenges import HTTP01Responder
+from txacme.client import Client
+from txacme.interfaces import ICertificateStore
+from txacme.service import AcmeIssuingService
+from txacme.util import generate_private_key
+from zope.interface import implementer
+
+from twisted.internet import defer
+from twisted.python.filepath import FilePath
+from twisted.python.url import URL
+
+logger = logging.getLogger(__name__)
+
+
+def create_issuing_service(reactor, acme_url, account_key_file, well_known_resource):
+    """Create an ACME issuing service, and attach it to a web Resource
+
+    Args:
+        reactor: twisted reactor
+        acme_url (str): URL to use to request certificates
+        account_key_file (str): where to store the account key
+        well_known_resource (twisted.web.IResource): web resource for .well-known.
+            we will attach a child resource for "acme-challenge".
+
+    Returns:
+        AcmeIssuingService
+    """
+    responder = HTTP01Responder()
+
+    well_known_resource.putChild(b"acme-challenge", responder.resource)
+
+    store = ErsatzStore()
+
+    return AcmeIssuingService(
+        cert_store=store,
+        client_creator=(
+            lambda: Client.from_url(
+                reactor=reactor,
+                url=URL.from_text(acme_url),
+                key=load_or_create_client_key(account_key_file),
+                alg=RS256,
+            )
+        ),
+        clock=reactor,
+        responders=[responder],
+    )
+
+
+@attr.s
+@implementer(ICertificateStore)
+class ErsatzStore(object):
+    """
+    A store that only stores in memory.
+    """
+
+    certs = attr.ib(default=attr.Factory(dict))
+
+    def store(self, server_name, pem_objects):
+        self.certs[server_name] = [o.as_bytes() for o in pem_objects]
+        return defer.succeed(None)
+
+
+def load_or_create_client_key(key_file):
+    """Load the ACME account key from a file, creating it if it does not exist.
+
+    Args:
+        key_file (str): name of the file to use as the account key
+    """
+    # this is based on txacme.endpoint.load_or_create_client_key, but doesn't
+    # hardcode the 'client.key' filename
+    acme_key_file = FilePath(key_file)
+    if acme_key_file.exists():
+        logger.info("Loading ACME account key from '%s'", acme_key_file)
+        key = serialization.load_pem_private_key(
+            acme_key_file.getContent(), password=None, backend=default_backend()
+        )
+    else:
+        logger.info("Saving new ACME account key to '%s'", acme_key_file)
+        key = generate_private_key("rsa")
+        acme_key_file.setContent(
+            key.private_bytes(
+                encoding=serialization.Encoding.PEM,
+                format=serialization.PrivateFormat.TraditionalOpenSSL,
+                encryption_algorithm=serialization.NoEncryption(),
+            )
+        )
+    return JWKRSA(key=key)