code: public static String getPrivateKey(Map<String, Object> keyMap) throws Exception {
          Key key = (Key) keyMap.get(PRIVATE_KEY);
          return String.valueOf(Base64Utils.encode(key.getEncoded()));
      }
docs: <p>
  获取私钥
  </p>
  
  @param keyMap 密钥对
  @return
  @throws Exception