/**
  * 获取rsa实例
  * @return null|RSA
  */
 private function _getRsa()
 {
     if (!self::$_rsa instanceof RSA) {
         self::$_rsa = new RSA($this->_envConfig['ownPrivateKey'], $this->_envConfig['partnerPublicKey']);
     }
     return self::$_rsa;
 }