core/cardano: simplify keychain implementation

core/src/apps/cardano/seed.py

@@ -16,21 +16,14 @@ class Keychain:
     """Cardano keychain hard-coded to Byron and Shelley seed namespaces."""
 
     def __init__(self, root: bip32.HDNode) -> None:
-        self.root = root
-        self.byron_root = self._create_namespace_root(seed_namespaces.BYRON)
-        self.shelley_root = self._create_namespace_root(seed_namespaces.SHELLEY)
+        self.byron_root = derive_path_cardano(root, seed_namespaces.BYRON)
+        self.shelley_root = derive_path_cardano(root, seed_namespaces.SHELLEY)
+        root.__del__()
 
     def verify_path(self, path: Bip32Path) -> None:
         if not is_byron_path(path) and not is_shelley_path(path):
             raise wire.DataError("Forbidden key path")
 
-    def _create_namespace_root(self, namespace: list):
-        new_root = self.root.clone()
-        for i in namespace:
-            new_root.derive_cardano(i)
-
-        return new_root
-
     def _get_path_root(self, path: list):
         if is_byron_path(path):
             return self.byron_root
@@ -48,10 +41,7 @@ class Keychain:
         suffix = node_path[len(seed_namespaces.SHELLEY) :]
 
         # derive child node from the root
-        node = path_root.clone()
-        for i in suffix:
-            node.derive_cardano(i)
-        return node
+        return derive_path_cardano(path_root, suffix)
 
     # XXX the root node remains in session cache so we should not delete it
     # def __del__(self) -> None:
@@ -66,6 +56,13 @@ def is_shelley_path(path: Bip32Path):
     return path[: len(seed_namespaces.SHELLEY)] == seed_namespaces.SHELLEY
 
 
+def derive_path_cardano(root: bip32.HDNode, path: Bip32Path) -> bip32.HDNode:
+    node = root.clone()
+    for i in path:
+        node.derive_cardano(i)
+    return node
+
+
 @cache.stored_async(cache.APP_CARDANO_ROOT)
 async def get_keychain(ctx: wire.Context) -> Keychain:
     if not device.is_initialized():