get_key (OpenID::Message)

    # File lib/openid/message.rb, line 357
        def get_key(namespace, ns_key)
          namespace = _fix_ns(namespace)
          return ns_key if namespace == BARE_NS

          ns_alias = @namespaces.get_alias(namespace)

          # no alias is defined, so no key can exist
          return nil if ns_alias.nil?

          if ns_alias == NULL_NAMESPACE
            tail = ns_key
          else
            tail = "#{ns_alias}.#{ns_key}"
          end

          return 'openid.' + tail
        end