I need to ssh to localhost using root account, by ssh root@localhost. When it prompts for passwords, I can not login with all possible passwords. On setting of localhost machine, regular user xxx and root user share the same password (the password that works for sudo -s), but it does not works for ssh root@localhost. So where to look at the password for ssh root@localhost

PS: with the password, I can login to regular account on ssh xxx@localhost.

PPS: to answer further questions on motivation of doing so, localhost is just a computer in a private network and setting up ssh root@localhost is just to relieve some manual management in a prototype system.