diff --git a/crypt_algo.hpp b/crypt_algo.hpp
index 366f2b0..f4d7363 100644
--- a/crypt_algo.hpp
+++ b/crypt_algo.hpp
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <array>
+#include <cstddef>
 #include <cstdint>
 #include <vector>