ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
"$ ssh-keygen -s ca_key -I key_id -h -n host.domain user_key.pub"
# ssh-keygen -T moduli-2048 -f moduli-2048.candidates
ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
# ssh-keygen -G moduli-2048.candidates -b 2048