diffdiff.net is designed to work without the text leaving your computer. That makes it generally safe to use for confidential text.
The only time your text is sent to the server at all is when it is absolutely required. For example:
When you click “Publish” (the text is sent to the server and stored in a database).
When you send feedback and check the “Include diffed text” option (the text is stored in a database and sent over e-mail to me).
You’re welcome to verify all this using tools like the Chrome network panel or Wireshark. If you find a bug, please let me know by sending feedback, and I’ll fix it as soon as I can.
This app uses an analytics tool called Amplitude to log various events on the page. It doesn’t send the text you type, just information like what browser you’re using (so I can know what browsers to test) and metrics like the length of the diffed text.