fix: tests pass

lib/spec/runners/ssz_generic.ex

@@ -78,15 +78,17 @@ defmodule SszGenericTestRunner do
     assert serialized == real_serialized
   end
 
-  defp assert_ssz("valid", schema, real_serialized, real_deserialized, hash_tree_root) do
+  defp assert_ssz("valid", schema, real_serialized, real_deserialized, expected_hash_tree_root) do
     {:ok, deserialized} = SszEx.decode(real_serialized, schema)
     assert deserialized == real_deserialized
 
     {:ok, serialized} = SszEx.encode(real_deserialized, schema)
 
     assert serialized == real_serialized
 
-    assert hash_tree_root == SszEx.hash_tree_root!(real_deserialized, schema)
+    actual_hash_tree_root = SszEx.hash_tree_root!(real_deserialized, schema)
+
+    assert actual_hash_tree_root == expected_hash_tree_root
   end
 
   defp assert_ssz("invalid", schema, real_serialized) do

lib/ssz_ex.ex

@@ -392,7 +392,8 @@ defmodule LambdaEthereumConsensus.SszEx do
   defp variable_size?({:bytes, _}), do: false
 
   defp pack(value, size) when is_integer(value) and value >= 0 do
-    <<value::size(size)-little>> |> String.pad_trailing(32, <<0>>)
+    pad = 256 - size
+    <<value::size(size)-little, 0::size(pad)>>
   end
 
   defp pack(value) when is_boolean(value) do