【google authenticator】使用基于时间的一次性密码（Time-based One-time Password，简称TOTP）实现。
离线实现即不需要联网。
https://blog.seetee.me/post/2011/google-two-step-verification/
https://segmentfault.com/a/1190000016100672
https://www.iteye.com/blog/awtqty-zhang-1986275
https://github.com/wstrange/GoogleAuth
https://notes.mengxin.science/2017/05/30/hotp-totp-algorithm-analysis/
