diff --git a/lib/pbkdf2.c b/lib/pbkdf2.c
index f2d3ca97f..083446ab9 100644
--- a/lib/pbkdf2.c
+++ b/lib/pbkdf2.c
@@ -16,6 +16,7 @@
    Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
 
 /* Written by Simon Josefsson.  */
+/* Imported from gnulib.  */
 
 #include <grub/crypto.h>
 #include <grub/mm.h>