diff --git a/docs/api.md b/docs/api.md
index 705aa7e..9adefce 100644
--- a/docs/api.md
+++ b/docs/api.md
@@ -50,39 +50,45 @@ theirdata = r.get_data(mydata)
 print("Their data: %s" % theirdata.decode("ascii"))
 ```
 
-## Twisted (TODO)
+## Twisted
 
-The Twisted-friendly flow, which is not yet implemented, may look like this:
+The Twisted-friendly flow looks like this:
 
 ```python
 from twisted.internet import reactor
-from wormhole.transcribe import TwistedInitiator
-data = b"initiator's data"
-ti = TwistedInitiator("appid", data, reactor)
-ti.startService()
-d1 = ti.when_get_code()
-d1.addCallback(lambda code: print("Invitation Code: %s" % code))
-d2 = ti.when_get_data()
-d2.addCallback(lambda theirdata:
-               print("Their data: %s" % theirdata.decode("ascii")))
-d2.addCallback(labmda _: reactor.stop())
+from wormhole.public_relay import RENDEZVOUS_RELAY
+from wormhole.twisted.transcribe import SymmetricWormhole
+outbound_message = b"outbound data"
+w1 = SymmetricWormhole("appid", RENDEZVOUS_RELAY)
+d = w1.get_code()
+def _got_code(code):
+    print "Invitation Code:", code
+    return w1.get_data(outbound_message)
+d.addCallback(_got_code)
+def _got_data(inbound_message):
+    print "Inbound message:", inbound_message
+d.addCallback(_got_data)
+d.addBoth(lambda _: reactor.stop())
 reactor.run()
 ```
 
+On the other side, you call `set_code()` instead of waiting for `get_code()`:
+
 ```python
-from twisted.internet import reactor
-from wormhole.transcribe import TwistedReceiver
-data = b"receiver's data"
-code = sys.argv[1]
-tr = TwistedReceiver("appid", code, data, reactor)
-tr.startService()
-d = tr.when_get_data()
-d.addCallback(lambda theirdata:
-              print("Their data: %s" % theirdata.decode("ascii")))
-d.addCallback(lambda _: reactor.stop())
-reactor.run()
+w2 = SymmetricWormhole("appid", RENDEZVOUS_RELAY)
+w2.set_code(code)
+d = w2.get_data(my_message)
 ```
 
+You can call `d=w.get_verifier()` before `get_data()`: this will perform the
+first half of the PAKE negotiation, then fire the Deferred with a verifier
+object (bytes) which can be converted into a printable representation and
+manually compared. When the users are convinced that `get_verifier()` from
+both sides are the same, call `d=get_data()` to continue the transfer. If you
+call `get_data()` first, it will perform the complete transfer without
+pausing.
+
+
 ## Application Identifier
 
 Applications using this library must provide an "application identifier", a
@@ -135,17 +141,27 @@ sync will be abandoned, and all callbacks will errback with a TimeoutError.
 
 Both have defaults suitable for face-to-face realtime setup environments.
 
-## Serialization (TODO)
+## Serialization
+
+TODO: only the Twisted form supports serialization so far
 
 You may not be able to hold the Initiator/Receiver object in memory for the
 whole sync process: maybe you allow it to wait for several days, but the
 program will be restarted during that time. To support this, you can persist
-the state of the object by calling `data = i.serialize()`, which will return
+the state of the object by calling `data = w.serialize()`, which will return
 a printable bytestring (the JSON-encoding of a small dictionary). To restore,
-call `Initiator.from_serialized(data)`.
+use the `from_serialized(data)` classmethod (e.g. `w =
+SymmetricWormhole.from_serialized(data)`).
 
-Note that callbacks are not serialized: they must be restored after
-deserialization.
+There is exactly one point at which you can serialize the wormhole: *after*
+establishing the invitation code, but before waiting for `get_verifier()` or
+`get_data()`. If you are creating a new code, the correct time is during the
+callback fired by `get_code()`. If you are accepting a pre-generated code,
+the time is just after calling `set_code()`.
+
+To properly checkpoint the process, you should store the first message
+(returned by `start()`) next to the serialized wormhole instance, so you can
+re-send it if necessary.
 
 ## Detailed Example
 
diff --git a/src/wormhole/test/test_twisted.py b/src/wormhole/test/test_twisted.py
new file mode 100644
index 0000000..51be63d
--- /dev/null
+++ b/src/wormhole/test/test_twisted.py
@@ -0,0 +1,132 @@
+import json
+from twisted.trial import unittest
+from twisted.internet import defer, protocol, endpoints, reactor
+from twisted.application import service
+from ..servers.relay import RelayServer
+from ..twisted.transcribe import SymmetricWormhole, UsageError
+from .. import __version__
+#from twisted.python import log
+#import sys
+#log.startLogging(sys.stdout)
+
+def allocate_port():
+    ep = endpoints.serverFromString(reactor, "tcp:0:interface=127.0.0.1")
+    d = ep.listen(protocol.Factory())
+    def _listening(lp):
+        port = lp.getHost().port
+        d2 = lp.stopListening()
+        d2.addCallback(lambda _: port)
+        return d2
+    d.addCallback(_listening)
+    return d
+
+def allocate_ports():
+    d = defer.DeferredList([allocate_port(), allocate_port()])
+    def _done(results):
+        port1 = results[0][1]
+        port2 = results[1][1]
+        return (port1, port2)
+    d.addCallback(_done)
+    return d
+
+class Basic(unittest.TestCase):
+    def setUp(self):
+        self.sp = service.MultiService()
+        self.sp.startService()
+        d = allocate_ports()
+        def _got_ports(ports):
+            relayport, transitport = ports
+            s = RelayServer("tcp:%d:interface=127.0.0.1" % relayport,
+                            "tcp:%s:interface=127.0.0.1" % transitport,
+                            __version__)
+            s.setServiceParent(self.sp)
+            self.relayurl = "http://127.0.0.1:%d/wormhole-relay/" % relayport
+            self.transit = "tcp:127.0.0.1:%d" % transitport
+        d.addCallback(_got_ports)
+        return d
+
+    def tearDown(self):
+        return self.sp.stopService()
+
+    def test_basic(self):
+        appid = "appid"
+        w1 = SymmetricWormhole(appid, self.relayurl)
+        w2 = SymmetricWormhole(appid, self.relayurl)
+        d = w1.get_code()
+        def _got_code(code):
+            w2.set_code(code)
+            d1 = w1.get_data("data1")
+            d2 = w2.get_data("data2")
+            return defer.DeferredList([d1,d2], fireOnOneErrback=False)
+        d.addCallback(_got_code)
+        def _done(dl):
+            ((success1, dataX), (success2, dataY)) = dl
+            r1,r2 = dl
+            self.assertTrue(success1)
+            self.assertTrue(success2)
+            self.assertEqual(dataX, "data2")
+            self.assertEqual(dataY, "data1")
+        d.addCallback(_done)
+        return d
+
+    def test_fixed_code(self):
+        appid = "appid"
+        w1 = SymmetricWormhole(appid, self.relayurl)
+        w2 = SymmetricWormhole(appid, self.relayurl)
+        w1.set_code("123-purple-elephant")
+        w2.set_code("123-purple-elephant")
+        d1 = w1.get_data("data1")
+        d2 = w2.get_data("data2")
+        d = defer.DeferredList([d1,d2], fireOnOneErrback=False)
+        def _done(dl):
+            ((success1, dataX), (success2, dataY)) = dl
+            r1,r2 = dl
+            self.assertTrue(success1)
+            self.assertTrue(success2)
+            self.assertEqual(dataX, "data2")
+            self.assertEqual(dataY, "data1")
+        d.addCallback(_done)
+        return d
+
+    def test_errors(self):
+        appid = "appid"
+        w1 = SymmetricWormhole(appid, self.relayurl)
+        self.assertRaises(UsageError, w1.get_verifier)
+        self.assertRaises(UsageError, w1.get_data, "data")
+        w1.set_code("123-purple-elephant")
+        self.assertRaises(UsageError, w1.set_code, "123-nope")
+        self.assertRaises(UsageError, w1.get_code)
+        w2 = SymmetricWormhole(appid, self.relayurl)
+        d = w2.get_code()
+        self.assertRaises(UsageError, w2.get_code)
+        return d
+
+    def test_serialize(self):
+        appid = "appid"
+        w1 = SymmetricWormhole(appid, self.relayurl)
+        self.assertRaises(UsageError, w1.serialize) # too early
+        w2 = SymmetricWormhole(appid, self.relayurl)
+        d = w1.get_code()
+        def _got_code(code):
+            self.assertRaises(UsageError, w2.serialize) # too early
+            w2.set_code(code)
+            w2.serialize() # ok
+            s = w1.serialize()
+            self.assertEqual(type(s), type(""))
+            unpacked = json.loads(s) # this is supposed to be JSON
+            self.assertEqual(type(unpacked), dict)
+            new_w1 = SymmetricWormhole.from_serialized(s)
+            d1 = new_w1.get_data("data1")
+            d2 = w2.get_data("data2")
+            return defer.DeferredList([d1,d2], fireOnOneErrback=False)
+        d.addCallback(_got_code)
+        def _done(dl):
+            ((success1, dataX), (success2, dataY)) = dl
+            r1,r2 = dl
+            self.assertTrue(success1)
+            self.assertTrue(success2)
+            self.assertEqual(dataX, "data2")
+            self.assertEqual(dataY, "data1")
+            self.assertRaises(UsageError, w2.serialize) # too late
+        d.addCallback(_done)
+        return d
diff --git a/src/wormhole/twisted/__init__.py b/src/wormhole/twisted/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/src/wormhole/twisted/demo.py b/src/wormhole/twisted/demo.py
new file mode 100644
index 0000000..adc3b74
--- /dev/null
+++ b/src/wormhole/twisted/demo.py
@@ -0,0 +1,30 @@
+import sys
+from twisted.internet import reactor
+from .transcribe import SymmetricWormhole
+from .. import public_relay
+
+APPID = "lothar.com/wormhole/text-xfer"
+
+w = SymmetricWormhole(APPID, public_relay.RENDEZVOUS_RELAY)
+
+if sys.argv[1] == "send-text":
+    message = sys.argv[2]
+    d = w.get_code()
+    def _got_code(code):
+        print "code is:", code
+        return w.get_data(message)
+    d.addCallback(_got_code)
+    def _got_data(their_data):
+        print "ack:", their_data
+    d.addCallback(_got_data)
+elif sys.argv[1] == "receive-text":
+    code = sys.argv[2]
+    w.set_code(code)
+    d = w.get_data("ok")
+    def _got_data(their_data):
+        print their_data
+    d.addCallback(_got_data)
+else:
+    raise ValueError("bad command")
+d.addCallback(lambda _: reactor.stop())
+reactor.run()
diff --git a/src/wormhole/twisted/eventsource.py b/src/wormhole/twisted/eventsource.py
new file mode 100644
index 0000000..521ac76
--- /dev/null
+++ b/src/wormhole/twisted/eventsource.py
@@ -0,0 +1,219 @@
+from twisted.python import log, failure
+from twisted.internet import reactor, defer, protocol
+from twisted.application import service
+from twisted.protocols import basic
+from twisted.web.client import Agent, ResponseDone
+from twisted.web.http_headers import Headers
+from ..util.eventual import eventually
+
+class EventSourceParser(basic.LineOnlyReceiver):
+    delimiter = "\n"
+
+    def __init__(self, handler):
+        self.current_field = None
+        self.current_lines = []
+        self.handler = handler
+        self.done_deferred = defer.Deferred()
+        self.eventtype = "message"
+
+    def connectionLost(self, why):
+        if why.check(ResponseDone):
+            why = None
+        self.done_deferred.callback(why)
+
+    def dataReceived(self, data):
+        # exceptions here aren't being logged properly, and tests will hang
+        # rather than halt. I suspect twisted.web._newclient's
+        # HTTP11ClientProtocol.dataReceived(), which catches everything and
+        # responds with self._giveUp() but doesn't log.err.
+        try:
+            basic.LineOnlyReceiver.dataReceived(self, data)
+        except:
+            log.err()
+            raise
+
+    def lineReceived(self, line):
+        if not line:
+            # blank line ends the field
+            self.fieldReceived(self.current_field,
+                               "\n".join(self.current_lines))
+            self.current_field = None
+            self.current_lines[:] = []
+            return
+        if self.current_field is None:
+            self.current_field, data = line.split(": ", 1)
+            self.current_lines.append(data)
+        else:
+            self.current_lines.append(line)
+
+    def fieldReceived(self, fieldname, data):
+        if fieldname == "event":
+            self.eventtype = data
+        elif fieldname == "data":
+            self.eventReceived(self.eventtype, data)
+            self.eventtype = "message"
+        else:
+            log.msg("weird fieldname", fieldname, data)
+
+    def eventReceived(self, eventtype, data):
+        self.handler(eventtype, data)
+
+class EventSourceError(Exception):
+    pass
+
+# es = EventSource(url, handler)
+# d = es.start()
+# es.cancel()
+
+class EventSource: # TODO: service.Service
+    def __init__(self, url, handler, when_connected=None, agent=None):
+        self.url = url
+        self.handler = handler
+        self.when_connected = when_connected
+        self.started = False
+        self.cancelled = False
+        self.proto = EventSourceParser(self.handler)
+        if not agent:
+            agent = Agent(reactor)
+        self.agent = agent
+
+    def start(self):
+        assert not self.started, "single-use"
+        self.started = True
+        d = self.agent.request("GET", self.url,
+                               Headers({"accept": ["text/event-stream"]}))
+        d.addCallback(self._connected)
+        return d
+
+    def _connected(self, resp):
+        if resp.code != 200:
+            raise EventSourceError("%d: %s" % (resp.code, resp.phrase))
+        if self.when_connected:
+            self.when_connected()
+        #if resp.headers.getRawHeaders("content-type") == ["text/event-stream"]:
+        resp.deliverBody(self.proto)
+        if self.cancelled:
+            self.kill_connection()
+        return self.proto.done_deferred
+
+    def cancel(self):
+        self.cancelled = True
+        if not self.proto.transport:
+            # _connected hasn't been called yet, but that self.cancelled
+            # should take care of it when the connection is established
+            def kill(data):
+                # this should kill it as soon as any data is delivered
+                raise ValueError("dead")
+            self.proto.dataReceived = kill # just in case
+            return
+        self.kill_connection()
+
+    def kill_connection(self):
+        if (hasattr(self.proto.transport, "_producer")
+            and self.proto.transport._producer):
+            # This is gross and fragile. We need a clean way to stop the
+            # client connection. p.transport is a
+            # twisted.web._newclient.TransportProxyProducer , and its
+            # ._producer is the tcp.Port.
+            self.proto.transport._producer.loseConnection()
+        else:
+            log.err("get_events: unable to stop connection")
+            # oh well
+            #err = EventSourceError("unable to cancel")
+            try:
+                self.proto.done_deferred.callback(None)
+            except defer.AlreadyCalledError:
+                pass
+
+
+class Connector:
+    # behave enough like an IConnector to appease ReconnectingClientFactory
+    def __init__(self, res):
+        self.res = res
+    def connect(self):
+        self.res._maybeStart()
+    def stopConnecting(self):
+        self.res._stop_eventsource()
+
+class ReconnectingEventSource(service.MultiService,
+                              protocol.ReconnectingClientFactory):
+    def __init__(self, baseurl, connection_starting, handler, agent=None):
+        service.MultiService.__init__(self)
+        # we don't use any of the basic Factory/ClientFactory methods of
+        # this, just the ReconnectingClientFactory.retry, stopTrying, and
+        # resetDelay methods.
+
+        self.baseurl = baseurl
+        self.connection_starting = connection_starting
+        self.handler = handler
+        self.agent = agent
+        # IService provides self.running, toggled by {start,stop}Service.
+        # self.active is toggled by {,de}activate. If both .running and
+        # .active are True, then we want to have an outstanding EventSource
+        # and will start one if necessary. If either is False, then we don't
+        # want one to be outstanding, and will initiate shutdown.
+        self.active = False
+        self.connector = Connector(self)
+        self.es = None # set we have an outstanding EventSource
+        self.when_stopped = [] # list of Deferreds
+
+    def isStopped(self):
+        return not self.es
+
+    def startService(self):
+        service.MultiService.startService(self) # sets self.running
+        self._maybeStart()
+
+    def stopService(self):
+        # clears self.running
+        d = defer.maybeDeferred(service.MultiService.stopService, self)
+        d.addCallback(self._maybeStop)
+        return d
+
+    def activate(self):
+        assert not self.active
+        self.active = True
+        self._maybeStart()
+
+    def deactivate(self):
+        assert self.active # XXX
+        self.active = False
+        return self._maybeStop()
+
+    def _maybeStart(self):
+        if not (self.active and self.running):
+            return
+        self.continueTrying = True
+        url = self.connection_starting()
+        self.es = EventSource(url, self.handler, self.resetDelay,
+                              agent=self.agent)
+        d = self.es.start()
+        d.addBoth(self._stopped)
+
+    def _stopped(self, res):
+        self.es = None
+        # we might have stopped because of a connection error, or because of
+        # an intentional shutdown.
+        if self.active and self.running:
+            # we still want to be connected, so schedule a reconnection
+            if isinstance(res, failure.Failure):
+                log.err(res)
+            self.retry() # will eventually call _maybeStart
+            return
+        # intentional shutdown
+        self.stopTrying()
+        for d in self.when_stopped:
+            eventually(d.callback, None)
+        self.when_stopped = []
+
+    def _stop_eventsource(self):
+        if self.es:
+            eventually(self.es.cancel)
+
+    def _maybeStop(self, _=None):
+        self.stopTrying() # cancels timer, calls _stop_eventsource()
+        if not self.es:
+            return defer.succeed(None)
+        d = defer.Deferred()
+        self.when_stopped.append(d)
+        return d
diff --git a/src/wormhole/twisted/transcribe.py b/src/wormhole/twisted/transcribe.py
index dfdf821..775dd67 100644
--- a/src/wormhole/twisted/transcribe.py
+++ b/src/wormhole/twisted/transcribe.py
@@ -1,27 +1,271 @@
-from twisted.application import service
-from ..const import RELAY
+from __future__ import print_function
+import os, sys, json, re
+from binascii import hexlify, unhexlify
+from zope.interface import implementer
+from twisted.internet import reactor, defer
+from twisted.web import client as web_client
+from twisted.web import error as web_error
+from twisted.web.iweb import IBodyProducer
+from nacl.secret import SecretBox
+from nacl.exceptions import CryptoError
+from nacl import utils
+from spake2 import SPAKE2_Symmetric
+from .eventsource import ReconnectingEventSource
+from .. import __version__
+from .. import codes
+from ..errors import ServerError
+from ..util.hkdf import HKDF
 
-class TwistedInitiator(service.MultiService):
-    def __init__(self, appid, data, reactor, relay=RELAY):
-        self.appid = appid
+class WrongPasswordError(Exception):
+    """
+    Key confirmation failed.
+    """
+
+class ReflectionAttack(Exception):
+    """An attacker (or bug) reflected our outgoing message back to us."""
+
+class UsageError(Exception):
+    """The programmer did something wrong."""
+
+@implementer(IBodyProducer)
+class DataProducer:
+    def __init__(self, data):
         self.data = data
-        self.reactor = reactor
+        self.length = len(data)
+    def startProducing(self, consumer):
+        consumer.write(self.data)
+        return defer.succeed(None)
+    def stopProducing(self):
+        pass
+    def pauseProducing(self):
+        pass
+    def resumeProducing(self):
+        pass
+
+
+class SymmetricWormhole:
+    def __init__(self, appid, relay):
+        self.appid = appid
         self.relay = relay
+        self.agent = web_client.Agent(reactor)
+        self.side = None
+        self.code = None
+        self.key = None
+        self._started_get_code = False
 
-    def when_get_code(self):
-        pass # return Deferred
+    def get_code(self, code_length=2):
+        if self.code is not None: raise UsageError
+        if self._started_get_code: raise UsageError
+        self._started_get_code = True
+        self.side = hexlify(os.urandom(5))
+        d = self._allocate_channel()
+        def _got_channel_id(channel_id):
+            code = codes.make_code(channel_id, code_length)
+            self._set_code_and_channel_id(code)
+            self._start()
+            return code
+        d.addCallback(_got_channel_id)
+        return d
 
-    def when_get_data(self):
-        pass # return Deferred
+    def _allocate_channel(self):
+        url = self.relay + "allocate/%s" % self.side
+        d = self.post(url)
+        def _got_channel(data):
+            if "welcome" in data:
+                self.handle_welcome(data["welcome"])
+            return data["channel-id"]
+        d.addCallback(_got_channel)
+        return d
 
-class TwistedReceiver(service.MultiService):
-    def __init__(self, appid, data, code, reactor, relay=RELAY):
-        self.appid = appid
-        self.data = data
+    def set_code(self, code):
+        if self.code is not None: raise UsageError
+        if self.side is not None: raise UsageError
+        self._set_code_and_channel_id(code)
+        self.side = hexlify(os.urandom(5))
+        self._start()
+
+    def _set_code_and_channel_id(self, code):
+        if self.code is not None: raise UsageError
+        mo = re.search(r'^(\d+)-', code)
+        if not mo:
+            raise ValueError("code (%s) must start with NN-" % code)
+        self.channel_id = int(mo.group(1))
         self.code = code
-        self.reactor = reactor
-        self.relay = relay
 
-    def when_get_data(self):
-        pass # return Deferred
+    def _start(self):
+        # allocate the rest now too, so it can be serialized
+        self.sp = SPAKE2_Symmetric(self.code.encode("ascii"),
+                                   idA=self.appid+":SymmetricA",
+                                   idB=self.appid+":SymmetricB")
+        self.msg1 = self.sp.start()
 
+    def serialize(self):
+        # I can only be serialized after get_code/set_code and before
+        # get_verifier/get_data
+        if self.code is None: raise UsageError
+        if self.key is not None: raise UsageError
+        data = {
+            "appid": self.appid,
+            "relay": self.relay,
+            "code": self.code,
+            "side": self.side,
+            "spake2": json.loads(self.sp.serialize()),
+            "msg1": self.msg1.encode("hex"),
+        }
+        return json.dumps(data)
+
+    @classmethod
+    def from_serialized(klass, data):
+        d = json.loads(data)
+        self = klass(d["appid"].encode("ascii"), d["relay"].encode("ascii"))
+        self._set_code_and_channel_id(d["code"].encode("ascii"))
+        self.side = d["side"].encode("ascii")
+        self.sp = SPAKE2_Symmetric.from_serialized(json.dumps(d["spake2"]))
+        self.msg1 = d["msg1"].decode("hex")
+        return self
+
+    motd_displayed = False
+    version_warning_displayed = False
+
+    def handle_welcome(self, welcome):
+        if ("motd" in welcome and
+            not self.motd_displayed):
+            motd_lines = welcome["motd"].splitlines()
+            motd_formatted = "\n ".join(motd_lines)
+            print("Server (at %s) says:\n %s" % (self.relay, motd_formatted),
+                  file=sys.stderr)
+            self.motd_displayed = True
+
+        # Only warn if we're running a release version (e.g. 0.0.6, not
+        # 0.0.6-DISTANCE-gHASH). Only warn once.
+        if ("-" not in __version__ and
+            not self.version_warning_displayed and
+            welcome["current_version"] != __version__):
+            print("Warning: errors may occur unless both sides are running the same version", file=sys.stderr)
+            print("Server claims %s is current, but ours is %s"
+                  % (welcome["current_version"], __version__), file=sys.stderr)
+            self.version_warning_displayed = True
+
+        if "error" in welcome:
+            raise ServerError(welcome["error"], self.relay)
+
+    def url(self, verb, msgnum=None):
+        url = "%s%d/%s/%s" % (self.relay, self.channel_id, self.side, verb)
+        if msgnum is not None:
+            url += "/" + msgnum
+        return url
+
+    def post(self, url, post_json=None):
+        # TODO: retry on failure, with exponential backoff. We're guarding
+        # against the rendezvous server being temporarily offline.
+        p = None
+        if post_json:
+            data = json.dumps(post_json).encode("utf-8")
+            p = DataProducer(data)
+        d = self.agent.request("POST", url, bodyProducer=p)
+        def _check_error(resp):
+            if resp.code != 200:
+                raise web_error.Error(resp.code, resp.phrase)
+            return resp
+        d.addCallback(_check_error)
+        d.addCallback(web_client.readBody)
+        d.addCallback(lambda data: json.loads(data))
+        return d
+
+    def _get_msgs(self, old_msgs, verb, msgnum):
+        # fire with a list of messages that match verb/msgnum, which either
+        # came from old_msgs, or from an EventSource that we attached to the
+        # corresponding URL
+        if old_msgs:
+            return defer.succeed(old_msgs)
+        d = defer.Deferred()
+        msgs = []
+        def _handle(name, data):
+            if name == "welcome":
+                self.handle_welcome(json.loads(data))
+            if name == "message":
+                msgs.append(json.loads(data)["message"])
+                d.callback(None)
+        es = ReconnectingEventSource(None, lambda: self.url(verb, msgnum),
+                                     _handle)#, agent=self.agent)
+        es.startService() # TODO: .setServiceParent(self)
+        es.activate()
+        d.addCallback(lambda _: es.deactivate())
+        d.addCallback(lambda _: es.stopService())
+        d.addCallback(lambda _: msgs)
+        return d
+
+    def derive_key(self, purpose, length=SecretBox.KEY_SIZE):
+        assert self.key is not None # call after get_verifier() or get_data()
+        assert type(purpose) == type(b"")
+        return HKDF(self.key, length, CTXinfo=purpose)
+
+    def _encrypt_data(self, key, data):
+        assert len(key) == SecretBox.KEY_SIZE
+        box = SecretBox(key)
+        nonce = utils.random(SecretBox.NONCE_SIZE)
+        return box.encrypt(data, nonce)
+
+    def _decrypt_data(self, key, encrypted):
+        assert len(key) == SecretBox.KEY_SIZE
+        box = SecretBox(key)
+        data = box.decrypt(encrypted)
+        return data
+
+
+    def _get_key(self):
+        # TODO: prevent multiple invocation
+        if self.key:
+            return defer.succeed(self.key)
+        data = {"message": hexlify(self.msg1).decode("ascii")}
+        d = self.post(self.url("post", "pake"), data)
+        d.addCallback(lambda j: self._get_msgs(j["messages"], "poll", "pake"))
+        def _got_pake(msgs):
+            pake_msg = unhexlify(msgs[0].encode("ascii"))
+            key = self.sp.finish(pake_msg)
+            self.key = key
+            self.verifier = self.derive_key(self.appid+b":Verifier")
+            return key
+        d.addCallback(_got_pake)
+        return d
+
+    def get_verifier(self):
+        if self.code is None: raise UsageError
+        d = self._get_key()
+        d.addCallback(lambda _: self.verifier)
+        return d
+
+    def get_data(self, outbound_data):
+        # only call this once
+        if self.code is None: raise UsageError
+        d = self._get_key()
+        d.addCallback(self._get_data2, outbound_data)
+        return d
+
+    def _get_data2(self, key, outbound_data):
+        # Without predefined roles, we can't derive predictably unique keys
+        # for each side, so we use the same key for both. We use random
+        # nonces to keep the messages distinct, and check for reflection.
+        data_key = self.derive_key(b"data-key")
+        outbound_encrypted = self._encrypt_data(data_key, outbound_data)
+        data = {"message": hexlify(outbound_encrypted).decode("ascii")}
+        d = self.post(self.url("post", "data"), data)
+        d.addCallback(lambda j: self._get_msgs(j["messages"], "poll", "data"))
+        def _got_data(msgs):
+            inbound_encrypted = unhexlify(msgs[0].encode("ascii"))
+            if inbound_encrypted == outbound_encrypted:
+                raise ReflectionAttack
+            try:
+                inbound_data = self._decrypt_data(data_key, inbound_encrypted)
+                return inbound_data
+            except CryptoError:
+                raise WrongPasswordError
+        d.addCallback(_got_data)
+        d.addBoth(self._deallocate)
+        return d
+
+    def _deallocate(self, res):
+        # only try once, no retries
+        d = self.agent.request("POST", self.url("deallocate"))
+        d.addBoth(lambda _: res) # ignore POST failure, pass-through result
+        return d