diff --git a/library/ecdsa.c b/library/ecdsa.c
index 42a65dc..1f0b37d 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -20,7 +20,7 @@
 /*
  * References:
  *
- * SEC1 http://www.secg.org/index.php?action=secg,docs_secg
+ * SEC1 https://www.secg.org/sec1-v2.pdf
  */
 
 #include "common.h"