What is stored and how

Data	Form
USOS access token and its secret	AES-256-GCM with `TOKEN_ENC_KEY`, bound to the user id and the field
Calendar address token	32 random bytes; a SHA-256 hash to look the feed up, and an encrypted copy to show the address on the page
Browser session	32 random bytes in an `HttpOnly`, `SameSite=Lax` cookie for seven days; only its SHA-256 hash is stored
Timetable	Plain: course, class type, group, room, building, address, lecturers, times
Profile	USOS user id and name

Only the studies and offline_access scopes are requested. No e-mail, no personal data, no grades.

Revoking or reissuing the calendar address replaces the stored hash in one statement; the next request to the old address is answered with 404.
Sign-out revokes the token in USOS, then deletes the tokens, the calendar address and the sessions. With the checkbox, the user and the timetable are deleted as well.
When USOS rejects a token, it is deleted; the timetable stays until the user signs out with deletion.
Nothing is logged per request. Error logs carry the user id and the USOS error message, never a token, a signature or a calendar address. The reverse proxy in front must not log request paths of /f/.
Forms are protected against cross-site requests by the Sec-Fetch-Site and Origin check of Go's http.CrossOriginProtection. A sign-in is bound to the browser that started it by a one-time cookie.
The page sends Content-Security-Policy: default-src 'none' with inline styles only and has no scripts.