diff --git a/sm4/asm_amd64.s b/sm4/asm_amd64.s
index 475565b..5e20765 100644
--- a/sm4/asm_amd64.s
+++ b/sm4/asm_amd64.s
@@ -1,3 +1,4 @@
+// This SM4 implementation referenced https://github.com/mjosaarinen/sm4ni/blob/master/sm4ni.c
 #include "textflag.h"
 
 #define x X0