I have got the following scenario with reference to Portal Authentication.
I need to enable users to log on to the Portal using <b>email</b> and <b>password</b> and these credentials ought to be checked against a datasource (CUA or LDAP directory).
Maybe I have to tweak some parameters of the UME data source configuration file, especially those belonging to <i>privateSection/check_credentials.login_attributes</i> but I'm not much sure.
I think you were very close to the option.. i think it is 'Attribute Mapping' and not 'Private Section' that you should look into.
Hope that Helps.