Store your authentication keys securily on your Yubikey. Use gpg-agent to ssh to your servers.
Configuring multiple keys
You have lost your GPG keys and you need to restore your backups. I list the ways to do this.
You GPG keys are precious, this posts shows how you can back them up securely.
GnuPG is a complete and free implementation of the OpenPGP standard which lets you send and receive encrypted emails, amongst other things.