/**
  * @return \XMLSecurityKey
  */
 public function getPrivateKey()
 {
     if (!$this->_key) {
         $filename = $this->keyFile;
         if ($filename[0] == '@') {
             $filename = $this->kernel->locateResource($filename);
         }
         $this->_key = KeyHelper::createPrivateKey($filename, $this->keyPass, true, false);
     }
     return $this->_key;
 }