Issue macaroons as opaque auth tokens

This just replaces random bytes with macaroons. The macaroons are not
inspected by the client or server.

In particular, they claim to have an expiry time, but nothing verifies
that they have not expired.

Follow-up commits will actually enforce the expiration, and allow for
token refresh.

See https://bit.ly/matrix-auth for more information

synapse/handlers/register.py

@@ -25,9 +25,9 @@ import synapse.util.stringutils as stringutils
 from synapse.util.async import run_on_reactor
 from synapse.http.client import CaptchaServerHttpClient
 
-import base64
 import bcrypt
 import logging
+import pymacaroons
 import urllib
 
 logger = logging.getLogger(__name__)
@@ -274,11 +274,18 @@ class RegistrationHandler(BaseHandler):
                 )
 
     def generate_token(self, user_id):
-        # urlsafe variant uses _ and - so use . as the separator and replace
-        # all =s with .s so http clients don't quote =s when it is used as
-        # query params.
-        return (base64.urlsafe_b64encode(user_id).replace('=', '.') + '.' +
-                stringutils.random_string(18))
+        macaroon = pymacaroons.Macaroon(
+            location = self.hs.config.server_name,
+            identifier = "key",
+            key = self.hs.config.macaroon_secret_key)
+        macaroon.add_first_party_caveat("gen = 1")
+        macaroon.add_first_party_caveat("user_id = %s" % user_id)
+        macaroon.add_first_party_caveat("type = access")
+        now = self.hs.get_clock().time()
+        expiry = now + 60 * 60
+        macaroon.add_first_party_caveat("time < %s" % expiry)
+
+        return macaroon.serialize()
 
     def _generate_user_id(self):
         return "-" + stringutils.random_string(18)