feat: allow one failed decryption before reauth (#114)

* feat: allow one failed decryption before reauth

* fix: revert change for empty payload after decryption

src/bthome_ble/parser.py

@@ -156,6 +156,10 @@ 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
@@ -701,7 +705,11 @@ def _decrypt_bthome(
                 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 after the decryption has failed twice.
+                self.bindkey_verified = False
+            else:
+                self.decryption_failed = True
             _LOGGER.warning("%s: Decryption failed: %s", self.title, error)
             _LOGGER.debug("%s: mic: %s", self.title, mic.hex())
             _LOGGER.debug("%s: nonce: %s", self.title, nonce.hex())
@@ -717,6 +725,7 @@ def _decrypt_bthome(
                 to_mac(bthome_mac),
             )
             raise ValueError
+        self.decryption_failed = False
         self.bindkey_verified = True
 
         return decrypted_payload

tests/test_parser_v2.py

@@ -246,6 +246,7 @@ def test_bindkey_wrong():
     device = BTHomeBluetoothDeviceData(bindkey=bytes.fromhex(bindkey))
     assert device.supported(advertisement)
     assert not device.bindkey_verified
+    assert device.decryption_failed
     assert device.update(advertisement) == SensorUpdate(
         title="Test Sensor 18B2",
         devices={
@@ -285,6 +286,7 @@ def test_bindkey_correct():
     device = BTHomeBluetoothDeviceData(bindkey=bytes.fromhex(bindkey))
     assert device.supported(advertisement)
     assert device.bindkey_verified
+    assert not device.decryption_failed
     assert device.update(advertisement) == SensorUpdate(
         title="TEST DEVICE 80A5",
         devices={
@@ -342,6 +344,7 @@ def test_incorrect_bindkey_length(caplog):
     device = BTHomeBluetoothDeviceData(bindkey=bytes.fromhex(bindkey))
     assert device.supported(advertisement)
     assert not device.bindkey_verified
+    assert device.decryption_failed
     assert (
         "TEST DEVICE 80A5: Encryption key should be 16 bytes (32 characters) long"
         in caplog.text
@@ -416,8 +419,22 @@ def test_bindkey_verified_can_be_unset():
 
     device = BTHomeBluetoothDeviceData(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"\x41\xa4\x72\x66\xc9\x5f\x73\x01\x11\x22\x33\x78\x23\x72\x14"
+    advertisement = bytes_to_service_info(
+        data_string,
+        local_name="ATC_8D18B2",
+        address="A4:C1:38:8D:18:B2",
+    )
+    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