Time-based one-time password authenticator (2FA)
Etopa is a two-factor-authentication app, which runs as a web server and can be accessed using a web browser or using an Android app. I would consider it stable to use, but there are still many functions yet to be implemented.
Download Android app
F-Droid: Add repository and search for Etopa
Google Play Store: Etopa 2FA
Amazon Appstore: Etopa
Samsung Galaxy Store: Etopa
git clone https://ltheinrich.de/etopa && cd etopa