challenge-response is an authentication method that does not need the
plain text password to be emitted over the (encrypted) connection.
This is nice if you don't fully trust the server.
It is enabled (upstream) by default.
To the end user, it still looks like normal password authentication,
but instead of sending it, it is used to hash some challenge.
This means that if you don't want passwords to be used ever at all,
and just stick to public key authentication, you probably want to
disable this option too.
svn path=/nixos/trunk/; revision=33513
a1e86494d0