Abstract

To enable certificate based login on a host, the public rsa key 'ssh_host_rsa_key.pub' needs to be signed. The resulting certificate is called 'ssh_host_rsa_key-cert.pub'. To enable ssh based login two things are required on the host:

• host certificate: 'ssh_host_rsa_key-cert.pub'
• CA public key: 'yubikeyX.pub'

Step 1 - Host Certifiacte

To tell the SSH daemon about the certificate add the following configuration lines to the file '/etc/ssh/sshd_config'. In addition copy the certificate to the specified location. The host sends this certificate to the client to identify itsself as a trusted host.

### Host certificate
HostCertificate /etc/ssh/ssh_host_rsa_key-cert.pub

Step 2 - Trust User CA Certificate

Add the following lines to the file '/etc/ssh/sshd_config' to tell the SSH daemon about the public key to verify client certificates. In addition copy the public key to the specified location. The host trusts all certificates the are signed by our CA.

### User CA certificate
TrustedUserCAKeys /etc/ssh/yubikeyX.pub

Step 3 - Principals

Now, we'll configure one of our hosts to accept only certain principals. To do so, add this line to '/etc/ssh/sshd_config'

### Auth Principals
AuthorizedPrincipalsFile /etc/ssh/auth_principals/%u

Then we need to populate the principals file:

mkdir /etc/ssh/auth_principals
echo -e 'host.netdef.org\nroot-everywhere' > /etc/ssh/auth_principals/root

This allows to all users to login as root that have either host.netdef.org or root-everywhere specified in the list of principals within their certificate.

You can control access to any other local user by creating the corresponding files under '/etc/ssh/auth_principals'.

Step 4 - Restart SSH