'The name of the subject can be in either of
BaseID, NameID or EncryptedID
:return: The identifier if there is one'

def subject_id(self):

    if ('subject' in self.message.keys()):
        _subj = self.message.subject
        if (('base_id' in _subj.keys()) and _subj.base_id):
            return _subj.base_id
        elif _subj.name_id:
            return _subj.name_id
    elif (('base_id' in self.message.keys()) and self.message.base_id):
        return self.message.base_id
    elif self.message.name_id:
        return self.message.name_id
    else:
        pass
