diff --git a/verifier.cpp b/verifier.cpp
index 6e15812..1d6cf81 100644
--- a/verifier.cpp
+++ b/verifier.cpp
@@ -22,6 +22,7 @@
 #include <algorithm>
 #include <memory>
 
+#include <openssl/bn.h>
 #include <openssl/ecdsa.h>
 #include <openssl/obj_mac.h>