Use a smart card or HSM to securely store your SSH CA keys

Depending on the use case, SSH CA keys can be extremely sensitive. Possession of the private key gives you the ability to sign new certificate for arbitrary usernames that will grant access to all machines where this CA is trusted. This is why you should keep your CA keys very safe, preferably offline and of course encrypted (password protected).

One big problem with digital encryption keys is that it is extremely difficult to find out if you lost control over they keys. If they where copied you have no way of finding out. This is where hardware tokens comes in. If you can be sure that the key will never leave the hardware token, you have a better chance of knowing when they keys are lost or stolen.

OpenSSH have support for storing keys onto hardware tokens that talk PKCS#11 such as smart cards and hardware security modules.

If you want to store your personal SSH key on a smart card you can read more about that here: Using Smart Card enabled yubikey for ssh authentication in FreeBSD

Since a SSH CA is just a regular SSH key they can also be stored on smart cards. But there is a few subtle differences in how you use it. First of all you need a token with a RSA key present. There is no option in OpenSSH to ask for a specific key stored on the token, instead we specifiy the public key that matches the private key we want to use. Lets download the public key from the token:

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

We put this into a file, let say ssh_ca.pub. Now we are ready to start using the token for certificate signing.

$ ssh-keygen -s ssh_ca.pub -D /usr/local/lib/opensc-pkcs11.so -I key_id wack.pub 
Enter PIN for 'Framkant HSM Test (UserPIN)': 
Signed user key wack-cert.pub: id "key_id" serial 0 valid forever

To look at the certificate we can use ssh-keygen.

% ssh-keygen -L -f wack-cert.pub 
wack-cert.pub:
        Type: ssh-rsa-cert-v01@openssh.com user certificate
        Public key: RSA-CERT SHA256:AtPyAu1DL5cFruTo9XnsVz7tdec7xF9SbpX8DzsQrbs
        Signing CA: RSA SHA256:Pbdx6TAvXvwZTKQVMRYWsWYPomw6AxBVoqbtXgy9pfs
        Key ID: "key_id"
        Serial: 0
        Valid: forever
        Principals: (none)
        Critical Options: (none)
        Extensions: 
                permit-X11-forwarding
                permit-agent-forwarding
                permit-port-forwarding
                permit-pty
                permit-user-rc

In a real life scenario I would recommend against this kind of certificate. Give it some validity and specify principals.

Two nice affordable options for hardware tokens are the Yubikey and the Smart Card HSM