If you’ve created OpenSSL certificates with private/public key pairs and want to add the public key to SSH’s authorized_keys file for authentication, do the following:

# /tmp/ssl-to-ssh/a.out tmp.key user@host.com
# chmod 600 CA/certs/First\ Last/First_Last-key.pem
# openssl rsa -aes256 -in  CA/certs/First\ Last/First_Last-key.pem -out  CA/certs/First\ Last/First_Last-key-with-passphrase.pem