diff --git a/tool/util.h b/tool/util.h
index 7a76466..d96d879 100644
--- a/tool/util.h
+++ b/tool/util.h
@@ -32,6 +32,8 @@
 
 #include "cmdline.h"
 
+#include <openssl/x509.h>
+
 #define INPUT 1
 #define OUTPUT 2