server has not
         indicated support through an extended challenge

   Extension set identifiers and extension type identifiers named with
   the prefix "x-" are reserved for private use among mutually
   consenting implementations. Implementations that do not recognise a
   particular "x-" extension MUST ignore that extension. This means that
   all "x-" extensions are likely to be non-interoperable with other
   extensions. Careful consideration should be given to the possibility
   of a server inter