feat: allow one failed decryption before reauth (#76)

src/xiaomi_ble/parser.py

@@ -1486,6 +1486,10 @@ def __init__(self, bindkey: bytes | None = None) -> None:
         # or encryption is not in use
         self.bindkey_verified = False
 
+        # If True then the decryption has failed or has not been verified yet.
+        # If False then the decryption has succeeded.
+        self.decryption_failed = True
+
         # If this is True, then we have not seen an advertisement with a payload
         # Until we see a payload, we can't tell if this device is encrypted or not
         self.pending = True
@@ -1739,10 +1743,15 @@ def _parse_xiaomi(
                 if payload_length < next_start:
                     # The payload segments are corrupted - if this is legacy encryption
                     # then the key is probably just wrong
-                    # V4 encryption has an authentication tag, so we don't apply the
+                    # V4/V5 encryption has an authentication tag, so we don't apply the
                     # same restriction there.
                     if self.encryption_scheme == EncryptionScheme.MIBEACON_LEGACY:
-                        self.bindkey_verified = False
+                        if self.decryption_failed is True:
+                            # we only ask for reautentification
+                            # till the decryption has failed twice.
+                            self.bindkey_verified = False
+                        else:
+                            self.decryption_failed = True
                     _LOGGER.debug(
                         "Invalid payload data length, payload: %s", payload.hex()
                     )
@@ -1893,7 +1902,12 @@ def _decrypt_mibeacon_v4_v5(
                 nonce, encrypted_payload + mic, associated_data
             )
         except InvalidTag as error:
-            self.bindkey_verified = False
+            if self.decryption_failed is True:
+                # we only ask for reautentification till
+                # the decryption has failed twice.
+                self.bindkey_verified = False
+            else:
+                self.decryption_failed = True
             _LOGGER.warning("Decryption failed: %s", error)
             _LOGGER.debug("mic: %s", mic.hex())
             _LOGGER.debug("nonce: %s", nonce.hex())
@@ -1906,6 +1920,7 @@ def _decrypt_mibeacon_v4_v5(
                 to_mac(xiaomi_mac),
             )
             return None
+        self.decryption_failed = False
         self.bindkey_verified = True
         return decrypted_payload
 
@@ -1938,6 +1953,11 @@ def _decrypt_mibeacon_legacy(
 
         assert cipher is not None  # nosec
         # decrypt the data
+        # note that V2/V3 encryption will often pass the decryption process with a
+        # wrong encryption key, resulting in useless data, and we won't be able
+        # to verify this, as V2/V3 encryption does not use a tag to verify
+        # the decrypted data. This will be filtered as wrong data length
+        # during the conversion of the payload to sensor data.
         try:
             decrypted_payload = cipher.decrypt(encrypted_payload)
         except ValueError as error:

tests/test_parser.py

@@ -214,6 +214,7 @@ def test_bindkey_wrong():
     device = XiaomiBluetoothDeviceData(bindkey=bytes.fromhex(bindkey))
     assert device.supported(advertisement)
     assert not device.bindkey_verified
+    assert device.decryption_failed
     assert device.update(advertisement) == SensorUpdate(
         title="Motion Sensor C40F (RTCGQ02LM)",
         devices={
@@ -250,8 +251,18 @@ def test_bindkey_verified_can_be_unset_v4():
 
     device = XiaomiBluetoothDeviceData(bindkey=bytes.fromhex(bindkey))
     device.bindkey_verified = True
+    device.decryption_failed = False
 
     assert device.supported(advertisement)
+    # the first advertisement will fail decryption, but we don't ask to reauth yet
+    assert device.bindkey_verified
+    assert device.decryption_failed
+
+    data_string = b"XY\x8d\n\x18\x0f\xc4\xe0D\xefT|\xc2z\\\x03\xa1\x00\x00\x00y"
+    advertisement = bytes_to_service_info(data_string, address="54:EF:44:E0:C4:0F")
+    assert device.supported(advertisement)
+    # the second advertisement will fail decryption again, but now we ask to reauth
+    assert device.decryption_failed
     assert not device.bindkey_verified
 
 
@@ -264,6 +275,7 @@ def test_bindkey_wrong_legacy():
     device = XiaomiBluetoothDeviceData(bindkey=bytes.fromhex(bindkey))
     assert device.supported(advertisement)
     assert not device.bindkey_verified
+    assert device.decryption_failed
     assert device.update(advertisement) == SensorUpdate(
         title="Dimmer Switch 988B (YLKG07YL/YLKG08YL)",
         devices={
@@ -288,7 +300,6 @@ def test_bindkey_wrong_legacy():
             ),
         },
     )
-
     assert device.unhandled == {}
 
 
@@ -300,8 +311,18 @@ def test_bindkey_verified_can_be_unset_legacy():
 
     device = XiaomiBluetoothDeviceData(bindkey=bytes.fromhex(bindkey))
     device.bindkey_verified = True
+    device.decryption_failed = False
 
     assert device.supported(advertisement)
+    # the first advertisement will fail decryption, but we don't ask to reauth yet
+    assert device.bindkey_verified
+    assert device.decryption_failed
+
+    data_string = b"X0\xb6\x03\xd3\x8b\x98\xc5A$\xf8\xc3I\x14vu~\x00\x00\x00\x99"
+    advertisement = bytes_to_service_info(data_string, address="F8:24:41:C5:98:8B")
+    assert device.supported(advertisement)
+    # the second advertisement will fail decryption again, but now we ask to reauth
+    assert device.decryption_failed
     assert not device.bindkey_verified