code: public OpenPgpMetadata addOxMessage(Message message, OpenPgpContact contact, List<ExtensionElement> payload)
              throws SmackException.NotLoggedInException, PGPException, IOException {
          return addOxMessage(message, Collections.singleton(contact), payload);
      }
docs: Add an OX-IM message element to a message.
  
  @param message message
  @param contact recipient of the message
  @param payload payload which will be encrypted and signed
  
  @return {@link OpenPgpMetadata} about the messages encryption + metadata.
  
  @throws SmackException.NotLoggedInException in case we are not logged in
  @throws PGPException in case something goes wrong during encryption
  @throws IOException IO is dangerous (we need to read keys)