diff --git a/src/wormhole/blocking/transcribe.py b/src/wormhole/blocking/transcribe.py
index 63ef206..167fd9f 100644
--- a/src/wormhole/blocking/transcribe.py
+++ b/src/wormhole/blocking/transcribe.py
@@ -16,6 +16,11 @@ from ..channel_monitor import monitor
 SECOND = 1
 MINUTE = 60*SECOND
 
+CONFMSG_NONCE_LENGTH = 128//8
+CONFMSG_MAC_LENGTH = 256//8
+def make_confmsg(confkey, nonce):
+    return nonce+HKDF(confkey, CONFMSG_MAC_LENGTH, nonce)
+
 def to_bytes(u):
     return unicodedata.normalize("NFC", u).encode("utf-8")
 
@@ -194,6 +199,7 @@ def close_on_error(f): # method decorator
 class Wormhole:
     motd_displayed = False
     version_warning_displayed = False
+    _send_confirm = True
 
     def __init__(self, appid, relay_url, wait=0.5*SECOND, timeout=3*MINUTE):
         if not isinstance(appid, type(u"")): raise TypeError(type(appid))
@@ -214,6 +220,7 @@ class Wormhole:
         self.verifier = None
         self._sent_data = set() # phases
         self._got_data = set()
+        self._got_confirmation = False
         self._closed = False
 
     def __enter__(self):
@@ -314,8 +321,12 @@ class Wormhole:
             pake_msg = self._channel.get(u"pake")
             self.key = self.sp.finish(pake_msg)
             self.verifier = self.derive_key(u"wormhole:verifier")
-            conf = self.derive_key(u"wormhole:confirmation")
-            self._channel.send(u"_confirm", conf)
+            if not self._send_confirm:
+                return
+            confkey = self.derive_key(u"wormhole:confirmation")
+            nonce = os.urandom(CONFMSG_NONCE_LENGTH)
+            confmsg = make_confmsg(confkey, nonce)
+            self._channel.send(u"_confirm", confmsg)
 
     @close_on_error
     def get_verifier(self):
@@ -355,10 +366,22 @@ class Wormhole:
         if self._channel is None: raise UsageError
         self._got_data.add(phase)
         self._get_key()
-        data_key = self.derive_key(u"wormhole:phase:%s" % phase)
-        inbound_encrypted = self._channel.get(phase)
+        phases = []
+        if not self._got_confirmation:
+            phases.append(u"_confirm")
+        phases.append(phase)
+        (got_phase, body) = self._channel.get_first_of(phases)
+        if got_phase == u"_confirm":
+            confkey = self.derive_key(u"wormhole:confirmation")
+            nonce = body[:CONFMSG_NONCE_LENGTH]
+            if body != make_confmsg(confkey, nonce):
+                raise WrongPasswordError
+            self._got_confirmation = True
+            (got_phase, body) = self._channel.get_first_of([phase])
+        assert got_phase == phase
         try:
-            inbound_data = self._decrypt_data(data_key, inbound_encrypted)
+            data_key = self.derive_key(u"wormhole:phase:%s" % phase)
+            inbound_data = self._decrypt_data(data_key, body)
             return inbound_data
         except CryptoError:
             raise WrongPasswordError
diff --git a/src/wormhole/test/test_blocking.py b/src/wormhole/test/test_blocking.py
index 5801e99..44c5a0e 100644
--- a/src/wormhole/test/test_blocking.py
+++ b/src/wormhole/test/test_blocking.py
@@ -3,7 +3,8 @@ import json
 from twisted.trial import unittest
 from twisted.internet.defer import gatherResults, succeed
 from twisted.internet.threads import deferToThread
-from ..blocking.transcribe import Wormhole, UsageError, ChannelManager
+from ..blocking.transcribe import (Wormhole, UsageError, ChannelManager,
+                                   WrongPasswordError)
 from ..blocking.eventsource import EventSourceFollower
 from .common import ServerBase
 
@@ -246,6 +247,53 @@ class Blocking(ServerBase, unittest.TestCase):
         d.addCallback(_got_2)
         return d
 
+    def test_wrong_password(self):
+        w1 = Wormhole(APPID, self.relayurl)
+        w2 = Wormhole(APPID, self.relayurl)
+
+        # make sure we can detect WrongPasswordError even if one side only
+        # does get_data() and not send_data(), like "wormhole receive" does
+        d = deferToThread(w1.get_code)
+        d.addCallback(lambda code: w2.set_code(code+"not"))
+
+        # w2 can't throw WrongPasswordError until it sees a CONFIRM message,
+        # and w1 won't send CONFIRM until it sees a PAKE message, which w2
+        # won't send until we call get_data. So we need both sides to be
+        # running at the same time for this test.
+        def _w1_sends():
+            w1.send_data(b"data1")
+        def _w2_gets():
+            self.assertRaises(WrongPasswordError, w2.get_data)
+        d.addCallback(lambda _: self.doBoth([_w1_sends], [_w2_gets]))
+
+        # and now w1 should have enough information to throw too
+        d.addCallback(lambda _: deferToThread(self.assertRaises,
+                                              WrongPasswordError, w1.get_data))
+        def _done(_):
+            # both sides are closed automatically upon error, but it's still
+            # legal to call .close(), and should be idempotent
+            return self.doBoth([w1.close], [w2.close])
+        d.addCallback(_done)
+        return d
+
+    def test_no_confirm(self):
+        # newer versions (which check confirmations) should will work with
+        # older versions (that don't send confirmations)
+        w1 = Wormhole(APPID, self.relayurl)
+        w1._send_confirm = False
+        w2 = Wormhole(APPID, self.relayurl)
+
+        d = deferToThread(w1.get_code)
+        d.addCallback(lambda code: w2.set_code(code))
+        d.addCallback(lambda _: self.doBoth([w1.send_data, b"data1"],
+                                            [w2.get_data]))
+        d.addCallback(lambda dl: self.assertEqual(dl[1], b"data1"))
+        d.addCallback(lambda _: self.doBoth([w1.get_data],
+                                            [w2.send_data, b"data2"]))
+        d.addCallback(lambda dl: self.assertEqual(dl[0], b"data2"))
+        d.addCallback(lambda _: self.doBoth([w1.close], [w2.close]))
+        return d
+
     def test_verifier(self):
         w1 = Wormhole(APPID, self.relayurl)
         w2 = Wormhole(APPID, self.relayurl)
diff --git a/src/wormhole/test/test_interop.py b/src/wormhole/test/test_interop.py
new file mode 100644
index 0000000..7a8bab6
--- /dev/null
+++ b/src/wormhole/test/test_interop.py
@@ -0,0 +1,64 @@
+from __future__ import print_function
+import sys
+from twisted.trial import unittest
+from twisted.internet.defer import gatherResults
+from twisted.internet.threads import deferToThread
+from ..twisted.transcribe import Wormhole as twisted_Wormhole
+from ..blocking.transcribe import Wormhole as blocking_Wormhole
+from .common import ServerBase
+
+# make sure the two implementations (Twisted-style and blocking-style) can
+# interoperate
+
+APPID = u"appid"
+
+class Basic(ServerBase, unittest.TestCase):
+
+    def doBoth(self, call1, d2):
+        f1 = call1[0]
+        f1args = call1[1:]
+        return gatherResults([deferToThread(f1, *f1args), d2], True)
+
+    def test_twisted_to_blocking(self):
+        tw = twisted_Wormhole(APPID, self.relayurl)
+        bw = blocking_Wormhole(APPID, self.relayurl)
+        d = tw.get_code()
+        def _got_code(code):
+            bw.set_code(code)
+            return self.doBoth([bw.send_data, b"data2"], tw.send_data(b"data1"))
+        d.addCallback(_got_code)
+        def _sent(res):
+            return self.doBoth([bw.get_data], tw.get_data())
+        d.addCallback(_sent)
+        def _done(dl):
+            (dataX, dataY) = dl
+            self.assertEqual(dataX, b"data1")
+            self.assertEqual(dataY, b"data2")
+            return self.doBoth([bw.close], tw.close())
+        d.addCallback(_done)
+        return d
+
+    def test_blocking_to_twisted(self):
+        bw = blocking_Wormhole(APPID, self.relayurl)
+        tw = twisted_Wormhole(APPID, self.relayurl)
+        d = deferToThread(bw.get_code)
+        def _got_code(code):
+            tw.set_code(code)
+            return self.doBoth([bw.send_data, b"data1"], tw.send_data(b"data2"))
+        d.addCallback(_got_code)
+        def _sent(res):
+            return self.doBoth([bw.get_data], tw.get_data())
+        d.addCallback(_sent)
+        def _done(dl):
+            (dataX, dataY) = dl
+            self.assertEqual(dataX, b"data2")
+            self.assertEqual(dataY, b"data1")
+            return self.doBoth([bw.close], tw.close())
+        d.addCallback(_done)
+        return d
+
+if sys.version_info[0] >= 3:
+    Basic.skip = "twisted is not yet sufficiently ported to py3"
+    # as of 15.4.0, Twisted is still missing:
+    # * web.client.Agent (for all non-EventSource POSTs in transcribe.py)
+    # * python.logfile (to allow daemonization of 'wormhole server')
diff --git a/src/wormhole/test/test_twisted.py b/src/wormhole/test/test_twisted.py
index fd20498..341a44a 100644
--- a/src/wormhole/test/test_twisted.py
+++ b/src/wormhole/test/test_twisted.py
@@ -2,7 +2,8 @@ from __future__ import print_function
 import sys, json
 from twisted.trial import unittest
 from twisted.internet.defer import gatherResults, succeed
-from ..twisted.transcribe import Wormhole, UsageError, ChannelManager
+from ..twisted.transcribe import (Wormhole, UsageError, ChannelManager,
+                                  WrongPasswordError)
 from ..twisted.eventsource_twisted import EventSourceParser
 from .common import ServerBase
 
@@ -229,6 +230,50 @@ class Basic(ServerBase, unittest.TestCase):
         d.addCallback(_got_2)
         return d
 
+    def test_wrong_password(self):
+        w1 = Wormhole(APPID, self.relayurl)
+        w2 = Wormhole(APPID, self.relayurl)
+        d = w1.get_code()
+        d.addCallback(lambda code: w2.set_code(code+"not"))
+
+        # w2 can't throw WrongPasswordError until it sees a CONFIRM message,
+        # and w1 won't send CONFIRM until it sees a PAKE message, which w2
+        # won't send until we call get_data. So we need both sides to be
+        # running at the same time for this test.
+        def _w1_sends():
+            return w1.send_data(b"data1")
+        def _w2_gets():
+            return self.assertFailure(w2.get_data(), WrongPasswordError)
+        d.addCallback(lambda _: self.doBoth(_w1_sends(), _w2_gets()))
+
+        # and now w1 should have enough information to throw too
+        d.addCallback(lambda _: self.assertFailure(w1.get_data(),
+                                                   WrongPasswordError))
+        def _done(_):
+            # both sides are closed automatically upon error, but it's still
+            # legal to call .close(), and should be idempotent
+            return self.doBoth(w1.close(), w2.close())
+        d.addCallback(_done)
+        return d
+
+    def test_no_confirm(self):
+        # newer versions (which check confirmations) should will work with
+        # older versions (that don't send confirmations)
+        w1 = Wormhole(APPID, self.relayurl)
+        w1._send_confirm = False
+        w2 = Wormhole(APPID, self.relayurl)
+
+        d = w1.get_code()
+        d.addCallback(lambda code: w2.set_code(code))
+        d.addCallback(lambda _: self.doBoth(w1.send_data(b"data1"),
+                                            w2.get_data()))
+        d.addCallback(lambda dl: self.assertEqual(dl[1], b"data1"))
+        d.addCallback(lambda _: self.doBoth(w1.get_data(),
+                                            w2.send_data(b"data2")))
+        d.addCallback(lambda dl: self.assertEqual(dl[0], b"data2"))
+        d.addCallback(lambda _: self.doBoth(w1.close(), w2.close()))
+        return d
+
     def test_verifier(self):
         w1 = Wormhole(APPID, self.relayurl)
         w2 = Wormhole(APPID, self.relayurl)
diff --git a/src/wormhole/twisted/transcribe.py b/src/wormhole/twisted/transcribe.py
index 5a36d90..c38f702 100644
--- a/src/wormhole/twisted/transcribe.py
+++ b/src/wormhole/twisted/transcribe.py
@@ -18,6 +18,11 @@ from ..errors import ServerError, WrongPasswordError, UsageError
 from ..util.hkdf import HKDF
 from ..channel_monitor import monitor
 
+CONFMSG_NONCE_LENGTH = 128//8
+CONFMSG_MAC_LENGTH = 256//8
+def make_confmsg(confkey, nonce):
+    return nonce+HKDF(confkey, CONFMSG_MAC_LENGTH, nonce)
+
 def to_bytes(u):
     return unicodedata.normalize("NFC", u).encode("utf-8")
 
@@ -192,6 +197,7 @@ class ChannelManager:
 class Wormhole:
     motd_displayed = False
     version_warning_displayed = False
+    _send_confirm = True
 
     def __init__(self, appid, relay_url):
         if not isinstance(appid, type(u"")): raise TypeError(type(appid))
@@ -206,6 +212,7 @@ class Wormhole:
         self._started_get_code = False
         self._sent_data = set() # phases
         self._got_data = set()
+        self._got_confirmation = False
 
     def _set_side(self, side):
         self._side = side
@@ -332,8 +339,12 @@ class Wormhole:
             key = self.sp.finish(pake_msg)
             self.key = key
             self.verifier = self.derive_key(u"wormhole:verifier")
-            conf = self.derive_key(u"wormhole:confirmation")
-            d1 = self._channel.send(u"_confirm", conf)
+            if not self._send_confirm:
+                return key
+            confkey = self.derive_key(u"wormhole:confirmation")
+            nonce = os.urandom(CONFMSG_NONCE_LENGTH)
+            confmsg = make_confmsg(confkey, nonce)
+            d1 = self._channel.send(u"_confirm", confmsg)
             d1.addCallback(lambda _: key)
             return d1
         d.addCallback(_got_pake)
@@ -375,16 +386,32 @@ class Wormhole:
         self._got_data.add(phase)
         d = self._get_key()
         def _get(key):
-            data_key = self.derive_key(u"wormhole:phase:%s" % phase)
-            d1 = self._channel.get(phase)
-            def _decrypt(inbound_encrypted):
+            phases = []
+            if not self._got_confirmation:
+                phases.append(u"_confirm")
+            phases.append(phase)
+            d1 = self._channel.get_first_of(phases)
+            def _maybe_got_confirm(phase_and_body):
+                (got_phase, body) = phase_and_body
+                if got_phase == u"_confirm":
+                    confkey = self.derive_key(u"wormhole:confirmation")
+                    nonce = body[:CONFMSG_NONCE_LENGTH]
+                    if body != make_confmsg(confkey, nonce):
+                        raise WrongPasswordError
+                    self._got_confirmation = True
+                    return self._channel.get_first_of([phase])
+                return phase_and_body
+            d1.addCallback(_maybe_got_confirm)
+            def _got(phase_and_body):
+                (got_phase, body) = phase_and_body
+                assert got_phase == phase
                 try:
-                    inbound_data = self._decrypt_data(data_key,
-                                                      inbound_encrypted)
+                    data_key = self.derive_key(u"wormhole:phase:%s" % phase)
+                    inbound_data = self._decrypt_data(data_key, body)
                     return inbound_data
                 except CryptoError:
                     raise WrongPasswordError
-            d1.addCallback(_decrypt)
+            d1.addCallback(_got)
             return d1
         d.addCallback(_get)
         return d