diff --git a/yubikey-ssh-pkcs11.md b/yubikey-ssh-pkcs11.md
index c5240a9..95fcc5f 100644
--- a/yubikey-ssh-pkcs11.md
+++ b/yubikey-ssh-pkcs11.md
@@ -1,3 +1,5 @@
+OpenSC: https://github.com/OpenSC/OpenSC
+
 
 ```shell
 ssh-keygen -D /Library/OpenSC/lib/opensc-pkcs11.so