Specific authentiation schemes can extend this base implementation providing
specific logic in the authenticate method.
author: Sebastian Gonzalez Oyuela version: $Id: AbstractAuthenticationScheme.java 508 2008-02-18 13:32:29Z sgonzalez $
Utility to load credentials from the store.
the array of konw credentials associated with the authenticated Principal. throws: SSOAuthenticationException - if an error occures while accessing the store.