diff --git a/auth_algo.hpp b/auth_algo.hpp
index 5bf1a1e..894a853 100644
--- a/auth_algo.hpp
+++ b/auth_algo.hpp
@@ -4,6 +4,7 @@
 #include "integrity_algo.hpp"
 
 #include <array>
+#include <cstddef>
 #include <string>
 #include <vector>