From HOSTA to HOSTB. Create key without passphrase:
HOSTA% ssh-keygen Generating public/private rsa key pair. Enter file in which to save the key (/home/cbo/.ssh/id_rsa): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/cbo/.ssh/id_rsa. Your public key has been saved in /home/cbo/.ssh/id_rsa.pub.
And copy over to HOSTB
HOSTA% cat .ssh/id_rsa.pub | ssh HOSTB 'cat >> .ssh/authorized_keys'
In /etc/ssh/ssh_config:
HashKnownHosts no