How to generate password automatically using authlogic

I have problem , i want to generate password automatically
where can i manipulate :password and :confirm_password in action create

def create
@user = User.new(params[:user])
if @user.save
flash[:notice] = “Registration successful.”
redirect_to root_url
else
render :action => ‘new’
end
end

Sorry, i have posted on the wrong group.