Ssh – How to setup sshd on Mac OS X to only allow key-based authentication

There are many levels of logging available through the sshd_config file. See the man page and look for LogLevel. The default level is INFO but it's trivially easy to bump it up to VERBOSE or even one of the DEBUG# levels.

Additionally, you should explore sudo as an alternative to su. A full discussion of the benefits of sudo would be a question of its own. But I can say that with sudo you can tailor how often you have to enter your password, which commands may be run, etc., all controllable through the sudoers config file.

AFAIK using PasswordAuthentication the client prompts for your username and password and then sends both to the server. In keyboard-interactive the client asks for your username, sends it to the server and then the server responds back with a password prompt which is in turn relayed back via the client to you.

The whole thing has been done to be able to implement further security measures to make sure you (the user) actually interactively types the password and it is not stored in some way and then only entered.