Save link_key in CTKD over BR/EDR

Since keystore.update() overwrites all existing keys, the existing link
key will be wiped out. To avoid this, SMP also need to keep the key.

bumble/smp.py

@@ -1134,8 +1134,10 @@ class Session:
 
     async def get_link_key_and_derive_ltk(self) -> None:
         '''Retrieves BR/EDR Link Key from storage and derive it to LE LTK.'''
-        link_key = await self.manager.device.get_link_key(self.connection.peer_address)
-        if link_key is None:
+        self.link_key = await self.manager.device.get_link_key(
+            self.connection.peer_address
+        )
+        if self.link_key is None:
             logging.warning(
                 'Try to derive LTK but host does not have the LK. Send a SMP_PAIRING_FAILED but the procedure will not be paused!'
             )
@@ -1143,7 +1145,7 @@ class Session:
                 SMP_CROSS_TRANSPORT_KEY_DERIVATION_NOT_ALLOWED_ERROR
             )
         else:
-            self.ltk = self.derive_ltk(link_key, self.ct2)
+            self.ltk = self.derive_ltk(self.link_key, self.ct2)
 
     def distribute_keys(self) -> None:
         # Distribute the keys as required

tests/self_test.py

@@ -547,6 +547,13 @@ async def test_self_smp_over_classic():
         MockSmpSession.send_public_key_command.assert_not_called()
         MockSmpSession.send_pairing_random_command.assert_not_called()
 
+    for i in range(2):
+        assert (
+            await two_devices.devices[i].keystore.get(
+                str(two_devices.connections[i].peer_address)
+            )
+        ).link_key
+
 
 # -----------------------------------------------------------------------------
 @pytest.mark.asyncio