offer a wrong-password error message

bin/receive_file.py

@@ -1,7 +1,7 @@
 from __future__ import print_function
 import sys, os, json
 from nacl.secret import SecretBox
-from wormhole.blocking.transcribe import Receiver
+from wormhole.blocking.transcribe import Receiver, WrongPasswordError
 from wormhole.blocking.transit import TransitReceiver
 
 APPID = "lothar.com/wormhole/file-xfer"
@@ -18,7 +18,11 @@ mydata = json.dumps({
 r = Receiver(APPID, mydata)
 r.set_code(r.input_code("Enter receive-file wormhole code: "))
 
-data = json.loads(r.get_data().decode("utf-8"))
+try:
+    data = json.loads(r.get_data().decode("utf-8"))
+except WrongPasswordError as e:
+    print("ERROR: " + e.explain(), file=sys.stderr)
+    sys.exit(1)
 #print("their data: %r" % (data,))
 
 file_data = data["file"]

bin/receive_text.py

@@ -1,6 +1,6 @@
 from __future__ import print_function
-import time, json
-from wormhole.blocking.transcribe import Receiver
+import sys, time, json
+from wormhole.blocking.transcribe import Receiver, WrongPasswordError
 
 APPID = "lothar.com/wormhole/text-xfer"
 
@@ -9,7 +9,11 @@ data = json.dumps({"message": "ok"}).encode("utf-8")
 r = Receiver(APPID, data)
 r.set_code(r.input_code("Enter receive-text wormhole code: "))
 start = time.time()
-them_bytes = r.get_data()
+try:
+    them_bytes = r.get_data()
+except WrongPasswordError as e:
+    print("ERROR: " + e.explain(), file=sys.stderr)
+    sys.exit(1)
 them_d = json.loads(them_bytes.decode("utf-8"))
 print(them_d["message"])
 print("elapsed time: %.2f" % (time.time() - start))

bin/send_file.py

@@ -1,7 +1,7 @@
 from __future__ import print_function
 import os, sys, json
 from nacl.secret import SecretBox
-from wormhole.blocking.transcribe import Initiator
+from wormhole.blocking.transcribe import Initiator, WrongPasswordError
 from wormhole.blocking.transit import TransitSender
 
 APPID = "lothar.com/wormhole/file-xfer"
@@ -28,7 +28,11 @@ code = i.get_code()
 print("On the other computer, please run: receive_file")
 print("Wormhole code is '%s'" % code)
 print("")
-them_bytes = i.get_data()
+try:
+    them_bytes = i.get_data()
+except WrongPasswordError as e:
+    print("ERROR: " + e.explain(), file=sys.stderr)
+    sys.exit(1)
 them_d = json.loads(them_bytes.decode("utf-8"))
 #print("them: %r" % (them_d,))
 xfer_key = i.derive_key(APPID+"/xfer-key", SecretBox.KEY_SIZE)

bin/send_text.py

@@ -1,6 +1,6 @@
 from __future__ import print_function
 import sys, json
-from wormhole.blocking.transcribe import Initiator
+from wormhole.blocking.transcribe import Initiator, WrongPasswordError
 
 APPID = "lothar.com/wormhole/text-xfer"
 
@@ -13,6 +13,10 @@ code = i.get_code()
 print("On the other computer, please run: receive_text")
 print("Wormhole code is: %s" % code)
 print("")
-them_bytes = i.get_data()
+try:
+    them_bytes = i.get_data()
+except WrongPasswordError as e:
+    print("ERROR: " + e.explain(), file=sys.stderr)
+    sys.exit(1)
 them_d = json.loads(them_bytes.decode("utf-8"))
 print("them: %r" % (them_d,))

src/wormhole/blocking/transcribe.py

@@ -1,7 +1,8 @@
-import time, requests, json
+import time, requests, json, textwrap
 from binascii import hexlify, unhexlify
 from spake2 import SPAKE2_A, SPAKE2_B
 from nacl.secret import SecretBox
+from nacl.exceptions import CryptoError
 from nacl import utils
 from .. import codes
 from ..util.hkdf import HKDF
@@ -13,6 +14,29 @@ MINUTE = 60*SECOND
 class Timeout(Exception):
     pass
 
+class WrongPasswordError(Exception):
+    """
+    Key confirmation failed.
+    """
+    # or the data blob was corrupted, and that's why decrypt failed
+    def explain(self):
+        return textwrap.dedent(self.__doc__)
+
+class InitiatorWrongPasswordError(WrongPasswordError):
+    """
+    Key confirmation failed. Either your correspondent typed the code wrong,
+    or a would-be man-in-the-middle attacker guessed incorrectly. You could
+    try again, giving both your correspondent and the attacker another
+    chance.
+    """
+
+class ReceiverWrongPasswordError(WrongPasswordError):
+    """
+    Key confirmation failed. Either you typed the code wrong, or a would-be
+    man-in-the-middle attacker guessed incorrectly. You could try again,
+    giving both you and the attacker another chance.
+    """
+
 # POST /allocate                                  -> {channel-id: INT}
 # POST /CHANNEL-ID/SIDE/pake/post  {message: STR} -> {messages: [STR..]}
 # POST /CHANNEL-ID/SIDE/pake/poll                 -> {messages: [STR..]}
@@ -116,7 +140,11 @@ class Initiator(Common):
 
             inbound_encrypted = self._poll_data(other_msgs)
             inbound_key = self.derive_key(b"receiver")
-            inbound_data = self._decrypt_data(inbound_key, inbound_encrypted)
+            try:
+                inbound_data = self._decrypt_data(inbound_key,
+                                                  inbound_encrypted)
+            except CryptoError:
+                raise InitiatorWrongPasswordError
         finally:
             self._deallocate()
         return inbound_data
@@ -169,7 +197,11 @@ class Receiver(Common):
 
             inbound_encrypted = self._poll_data(other_msgs)
             inbound_key = self.derive_key(b"sender")
-            inbound_data = self._decrypt_data(inbound_key, inbound_encrypted)
+            try:
+                inbound_data = self._decrypt_data(inbound_key,
+                                                  inbound_encrypted)
+            except CryptoError:
+                raise ReceiverWrongPasswordError
         finally:
             self._deallocate()
         return inbound_data