A config setting containing a list of challengers available to the system when it needs to ask for a user. If there is more than one challenger the user is presented with a list of those available. If there is only one, the user is automatically directed to just that one. A challenger needs to work with the extractors system so that the challenger puts something in future requests that the extractor can extract.
The default challenger is called cookie_form and is configured as follows:
Plugins, such as tiddlywebplugins.openid2, extend
auth_systems to add new authentication systems.