Fix invalid parent hash with empty root left subtree

src/treekem.cpp

@@ -903,18 +903,19 @@ TreeKEMPublicKey::parent_hashes(
   }
 
   // The list of nodes for whom parent hashes are computed, namely: Direct path
-  // excluding root, including leaf
+  // excluding the last entry, including leaf
   auto from_node = NodeIndex(from);
   auto dp = fdp;
+  auto [last, _res_last] = dp.back();
   dp.pop_back();
   dp.insert(dp.begin(), { from_node, {} });
 
   if (dp.size() != path_nodes.size()) {
     throw ProtocolError("Malformed UpdatePath");
   }
 
-  // Parent hash for all the parents, starting from the root
-  auto last = NodeIndex::root(size);
+  // Parent hash for all the parents, starting from the last entry of the
+  // filtered direct path
   auto last_hash = bytes{};
   auto ph = std::vector<bytes>(dp.size());
   for (int i = static_cast<int>(dp.size()) - 1; i >= 0; i--) {

test/state.cpp

@@ -1069,3 +1069,48 @@ TEST_CASE_METHOD(RelatedGroupTest, "Reinitialize the group")
     REQUIRE(state == new_states[0]);
   }
 }
+
+TEST_CASE_METHOD(StateTest, "Parent Hash with Empty Left Subtree")
+{
+  // Create a group with 4 members
+  auto state_0 = State(group_id,
+                       suite,
+                       leaf_privs[0],
+                       identity_privs[0],
+                       key_packages[0].leaf_node,
+                       ExtensionList{});
+
+  const auto adds = std::vector{
+    state_0.add_proposal(key_packages[1]),
+    state_0.add_proposal(key_packages[2]),
+    state_0.add_proposal(key_packages[3]),
+  };
+
+  auto [_commit0, welcome0, new_state_0] =
+    state_0.commit(fresh_secret(), CommitOpts{ adds, true, false, {} }, {});
+  silence_unused(_commit0);
+  state_0 = new_state_0;
+
+  auto state_2 = State(init_privs[2],
+                       leaf_privs[2],
+                       identity_privs[2],
+                       key_packages[2],
+                       welcome0,
+                       std::nullopt,
+                       {});
+  // Member @2 removes the members on the left side of the tree
+  const auto removes = std::vector{
+    state_2.remove_proposal(LeafIndex{ 0 }),
+    state_2.remove_proposal(LeafIndex{ 1 }),
+  };
+
+  auto [commit2, welcome2, new_state_2] =
+    state_2.commit(fresh_secret(), CommitOpts{ removes, true, false, {} }, {});
+  silence_unused(commit2);
+  silence_unused(welcome2);
+  state_2 = new_state_2;
+
+  // Member @2 should have a valid tree, even though its filtered direct path no
+  // longer goes to the root.
+  REQUIRE(state_2.tree().parent_hash_valid());
+}