TOTP (Time-based one-time password)

https://www.codemadness.org/totp.html
https://oath-toolkit.codeberg.page/man-oathtool.html

https://www.masteringemacs.org/article/securely-generating-totp-tokens-emacs

https://github.com/susam/mintotp

https://shkspr.mobi/blog/2023/07/an-eink-wrist-mounted-totp-generator/