diff --git a/README.md b/README.md
index a2d9776..e52dfa6 100644
--- a/README.md
+++ b/README.md
@@ -13,7 +13,18 @@ OpenSSH_8.6p1, OpenSSL 1.1.1k  25 Mar 2021
 ssh-keygen -t ecdsa-sk [-C fido-u2f]
 ```
 
+# Usage
 
-# dependencies
+Edit `allowed_keys` file:
+```
+sk-ecdsa-sha2-nistp256@openssh.com AAAAInNrL...... fido-u2f
+```
+
+
+# Dependencies
 * https://github.com/gliderlabs/ssh
 
+
+# References
+* https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.u2f
+