diff --git a/README.md b/README.md
index bd3725a..ddfc9a1 100644
--- a/README.md
+++ b/README.md
@@ -247,6 +247,9 @@ SSH to server:
 ssh -i id_user root@example.com
 ```
 
+<br>
+> `external_*` subcommands follow <<Cryptography external command specification>>
+> Specification: https://openwebstandard.org/rfc1
 
 <br><br>