This guide will try to show you how to use yubikey for ssh authentication

I recently found out that the yubikey neo had a built in smart card. Also, a few days ago Yubico released their fourth version of the yubikey introducing built in smart card as a standard feature. Me, having very little knowledge and experience with smart cards bought one just to play around with and I quickly found out that the card could be used to securely store encryption keys and certificates.

I use SSH alot both at home and at work, so this was the natural place to start. There is a short guide on how to use the yubikey with ssh on the yubikey developer site but this only covers the yubikey-part. This article will show you how to get things working in FreeBSD. But it will also skip the steps needed to configure the yubikey to have the smart card enabled, and how to generate keys and so on (yubikey software needed)

First if all you need to install a few packages.

# pkg install pcsc-lite
# pkg install opensc
# pkg install libccid

Then there is alot of stuff you can fiddle with using the pkcs11-tool, pkcs15-tool commands. But to get going you can just start the pcscd daemon manually and insert the yubikey into your computer.

# pcscd -f -d

If you have correctly generated a key (and certificate) on the yubikey in one of the slots you should now be able to run

$ ssh-keygen -D /usr/local/lib/opensc-pkcs11.so
C_GetAttributeValue failed: 18
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCOvhEqFq9Ualp1iYiNs0JFs3MgvGU+By/VvyW4qkymW+w/MmHAaHl+/UFnE+kgXChdYHaGEVxxGi6dQlSq+1ZKAWPJsOEbkysK6cgjgvP21gVNjL62TlQz+QfGF82mv0hfSGXQrZQR7VDs+6xJOe3S/0i1HvnnRTdR2v9QSJzd2EWNLmUcPy7+4x4rEB11G0oPt+Xyx60WaleJctwJHHhJS/jqHdvuf7HO6MS/EQn2NTnwIjChlmm2kUT7obnev/r6uEwz87NubnYJUrYImRDMafjS9taq8l7y33ydT00QHEI76kmrSSi7hTfmxUgStQWuQ2mq10YEVd8kZ2sqmC3N

To show the public part of your keys residing on the smart card. If this command was successful you can also add the keys to your ssh-agent using

$ ssh-add -s /usr/local/lib/opensc-pkcs11.so
Enter passphrase for PKCS#11: 
Card added: /usr/local/lib/opensc-pkcs11.so

and then you can list the keys with

$ssh-add -l
2048 8c:c9:99:1f:1b:98:42:af:9b:9b:93:e6:5b:ff:2a:94 /usr/local/lib/opensc-pkcs11.so (RSA)

To remove the card from the agent use

$ ssh-add -e /usr/local/lib/opensc-pkcs11.so
Card removed: /usr/local/lib/opensc-pkcs11.so

To connect somewhere using the keys on the smart card without the agent you can just use

$ ssh -I /usr/local/lib/opensc-pkcs11.so user@host.tld

There will probably be more guides in the future on how to use this further.

Update:

You can now use ECDSA keys stored on yubikey with OpenSSH.