[Zope] how to??

hello I want to do it this: a plugin for PAS to validate users, i put user and password and if user is a correct user, log in plone. class ValidaPlugin(BasePlugin, Cacheable): meta_type = 'ValidaPlugin' security = ClassSecurityInfo() def __init__(self, id, title=None): self._id = self.id = id self.title = title security.declarePrivate('authenticateCredentials') def authenticateCredentials(self, credentials): """ Autentificacion """ login = credentials.get( 'login' ) password = credentials.get( 'password' ) if login is None or password is None: return None return login, login is correct?? Thanks