diff --git a/src/apk_crypto.h b/src/apk_crypto.h
index ac6e828..4697901 100644
--- a/src/apk_crypto.h
+++ b/src/apk_crypto.h
@@ -11,6 +11,7 @@
 
 #include <assert.h>
 #include <errno.h>
+#include <string.h>
 #include <openssl/evp.h>
 #include "apk_openssl.h"