Thanks,
I can ssh in as root with password authentication. That is,
I can ssh root@N900_ip and it asks for password and it works. Then as root, I can, as you suggest, su - user, and become "user". But I was trying to login using public keys just because I think it would be more secure in addition to having changed the default root password when I installed openssh server. The next step would be to disable root login.