xmss secret key: simplify some fields (leanEthereum#203)

Author: tcoratger

src/lean_spec/subspecs/xmss/containers.py

@@ -374,7 +374,7 @@ class SecretKey(StrictBaseModel):
     `sqrt(LIFETIME)`, with a minimum of `2 * sqrt(LIFETIME)`.
     """
 
-    top_tree: HashSubTree | None = None
+    top_tree: HashSubTree
     """
     The top tree containing the root and top `LOG_LIFETIME/2` layers.
 
@@ -394,15 +394,15 @@ class SecretKey(StrictBaseModel):
 
     """
 
-    left_bottom_tree: HashSubTree | None = None
+    left_bottom_tree: HashSubTree
     """
     The left bottom tree in the sliding window.
 
     This covers epochs:
     [left_bottom_tree_index * sqrt(LIFETIME), (left_bottom_tree_index + 1) * sqrt(LIFETIME))
     """
 
-    right_bottom_tree: HashSubTree | None = None
+    right_bottom_tree: HashSubTree
     """
     The right bottom tree in the sliding window.
 

src/lean_spec/subspecs/xmss/interface.py

@@ -359,13 +359,6 @@ def sign(self, sk: SecretKey, epoch: Uint64, message: bytes) -> Signature:
         # With top-bottom tree traversal, we use combined_path to merge paths from
         # the bottom tree and top tree.
 
-        # Ensure we have the required top-bottom tree structures.
-        if sk.top_tree is None:
-            raise ValueError(
-                "Secret key is missing top-bottom tree structures. "
-                "This may be a legacy key format that is no longer supported."
-            )
-
         # Determine which bottom tree contains this epoch.
         leafs_per_bottom_tree = 1 << (config.LOG_LIFETIME // 2)
         boundary = (sk.left_bottom_tree_index + Uint64(1)) * Uint64(leafs_per_bottom_tree)
@@ -548,12 +541,8 @@ def advance_preparation(self, sk: SecretKey) -> SecretKey:
             A new SecretKey with the advanced preparation window.
 
         Raises:
-            ValueError: If the secret key is missing top-bottom tree structures
-            or if advancing would exceed the activation interval.
+            ValueError: If advancing would exceed the activation interval.
         """
-        if sk.left_bottom_tree is None or sk.right_bottom_tree is None:
-            raise ValueError("Secret key missing bottom tree data")
-
         leafs_per_bottom_tree = 1 << (self.config.LOG_LIFETIME // 2)
 
         # Check if advancing would exceed the activation interval