public function getCacheKey()
  {
    if (!$this->useCache) {
      return false;
    }
    $h = $this->sub;
    $h .= $this->assertionType;
    $h .= $this->privateKey;
    $h .= $this->scopes;
    $h .= $this->serviceAccountName;
    return md5($h);
  }