Cookie “sid”
A random string, nothing else.
- What for
- So the server knows which puzzle is yours and – if you're signed in – which account you belong to.
- How long
- As a guest until you close the browser. Signed in for 30 days or until you sign out.
Privacy.
This is a translation for convenience. The German version is legally binding.
In short: this site only stores what it needs for playing. No ads, no tracking, no analytics, no social media buttons, no fonts or scripts from other servers. Nothing is sold or passed on to third parties. An account is optional and works for all games; its name and password are kept on the selection page, here only your solved puzzles, Groschen and badges. You only appear in the public best times if you turn that on yourself. Below is every single piece of data that comes up here.
Who is responsible
Müller & Wulff GmbH
Hans-Fallada-Str. 20
D – 17489 Greifswald
E-Mail: info@mueller-wulff.de
What is stored – the complete list
This is everything. Whatever isn't listed here isn't stored.
In your browser
Cookie “kb”
A random string, only while you're signed in. It is set by the selection page, for all games at once – that's why you're signed in everywhere.
- What for
- So this game knows which account you're signed in with. It asks the selection page on the same server; nothing leaves the server for that.
- How long
- 30 days, or until you sign out – in a game or on the selection page.
Cookie “lang”
“de” or “en” – only if you switch the language at the bottom of the page.
- What for
- So the site stays in the language you picked. Without it, the site follows your browser's language setting.
- How long
- One year, or until you clear your browser data.
Game state
Per game (Sudoku, Yukendo) in localStorage: under yukendo:game:… the ID of the current puzzle, the digits and notes you entered, the history of your moves (for “Undo”), which cells are marked wrong, the time, whether the game is paused, size, level, mode and counters for notes and erasing; under yukendo:prefs:… the last chosen size, level and mode; under yukendo:eingabe, whether “Keep number” is on.
- What for
- So your game continues after a reload or a visit to other pages, and your settings are preselected for the next puzzle. To check and solve, the browser sends the digits in question, the time and the two counters to the server; only what's listed under “Progress” is stored.
- How long
- The game state until the puzzle is solved or the server no longer knows it; the settings until you delete them (button below) or clear your browser data.
On the server, in memory only
Current puzzles
At most one per game (Sudoku, Yukendo): the puzzle, its solution, start time, mode, which cells you checked or revealed, how often you checked the full grid and, in “Relaxed” mode, which wrong digits you entered where.
- What for
- So the server can check your solution and give hints. The solution never leaves the server – only the digit you reveal with a hint in “Like on paper” mode, which cells are wrong, and in “Relaxed” mode, whether an entered digit is right.
- How long
- Until you solve it or start a new one, at most until the session ends.
Name for the best times as a guest
Only if you add yourself to the best times as a guest: the name you picked for it and – until you decide – your last puzzle that would make the top 20 (table, time, date, mode, in “Relaxed” mode the number of mistakes).
- What for
- So your next fast puzzles go in under the same name without asking, and adding one works at the press of a button.
- How long
- Until the session ends. “Stop adding me” stops adding right away; the name then stays yours until the session ends, so you can still delete your entries.
Progress as a guest
The same as “Progress with an account” and “Groschen and badges” (see below) – in memory only.
- What for
- For the “Progress” page, Groschen and badges.
- How long
- Until the session ends – at the latest after 24 hours without a visit or when the server restarts.
On the server, permanently – only with an account
Account
Your account name, when the account was created and since when it has results here. The name and password hash themselves are kept with the shared account on the selection page.
- What for
- So your progress here belongs to your account. When you sign in, your progress from the guest session is moved into the account.
- How long
- Until you delete your account; the selection page then deletes the data here too.
Progress with an account
Per solved puzzle: game, size and level, mode, time taken, start and end time, your clock's offset from UTC, how many hints you took, how often you checked the full grid, whether exactly one cell was ever wrong, the number of mistakes in “Relaxed” mode, and how often you set notes, erased or undid.
- What for
- For best times, streaks and badges. The offset is needed so “Night owl” and friends count by your clock; the counters for a few secret badges.
- How long
- Until you delete your account.
Groschen and badges
Balance, total earned and spent, the last 100 entries (amount, kind, time) and when you got which badge.
- What for
- So you can buy hints and badges are only paid out once. Groschen are play money without any real value.
- How long
- Until you delete your account.
Sign-in
A hash of the ID in the “sid” cookie, which account it belongs to, and an expiry date.
- What for
- So your session here belongs to your account, even after the server restarts.
- How long
- 30 days, deleted immediately when you sign out.
On the server, permanently – only if you want to be in the best times
The rankings page shows the fastest puzzles per table (Sudoku per level, Yukendo per size and level, each separately per mode). Only people who explicitly want to are listed. Every visitor sees per entry: name, time, date, table and mode, in “Relaxed” mode the number of mistakes – for guests also the tag “guest”. The server measures the time itself (from the start of the puzzle to the solution).
Best times with an account
Whether the switch “Show me in the best times” is on (it starts off). Your best times themselves are worked out from your solved puzzles (see “Progress with an account”) – no extra copy is made for them.
- What for
- So others see your account name and your best time per table, for as long as you want. With the switch off, only you see your best time and the place you would have.
- How long
- Visible until you turn the switch off under Progress – then immediately not any more. Stored until you delete your account; then all your best times go too.
Best times as a guest
Per table your best time with the name you picked (or the suggested made-up name), date, mode and, in “Relaxed” mode, the number of mistakes – nothing else: no session, no cookie, nothing that leads back to you. The name can't be an account name known here.
- What for
- So guests can be in the best times too, if they want.
- How long
- Until the entry drops out of the best 100 of its table. While your session lasts, you can delete your entries yourself on the best times page (“Stop and delete my entries”). After that the entry is no longer linked to you; email us the name at info@mueller-wulff.de and we'll remove it.
What is not stored
No email address, no real name (unless you choose it as your user name or as your name for the best times), no IP addresses – neither by the app nor by the web server –, no device fingerprint, no clicks, no referrers, no statistics about your behaviour. There is no profiling and no automated decision-making about you (Art. 22 GDPR).
Why there is no cookie banner
By law a cookie banner is only needed when a site stores things on your device that it doesn't need for the actual service – for ads, tracking or statistics, say. This site doesn't do that. The cookies and the game state in your browser are strictly necessary for the game you opened, or to keep the language you explicitly chose (§ 25 (2) no. 2 TDDDG). That needs no consent – just this explanation.
Hosting and server
The site is hosted by: Hetzner Online GmbH, Industriestr. 25, 91710 Gunzenhausen, Deutschland (eigener Root-Server im Rechenzentrum Nürnberg).
To deliver a web page at all, the server briefly has to process your IP address. Neither the app nor the web server writes access logs. On technical errors they log an error message with the requested address, but without IP address and without details about your browser; these messages are kept only in limited amounts and then overwritten automatically. The legal basis is our legitimate interest in secure and working operation (Art. 6 (1) (f) GDPR).
Recipients
Data is not passed on to anyone. The hosting provider only runs the data centre and the hardware the server runs on; in doing so it may technically come into contact with connection data, for example to fend off attacks. All data stays in Germany; nothing is transferred to countries outside the EU.
Legal bases
- Playing, account and progress – as a guest or with an account: Art. 6 (1) (b) GDPR – you use the service, and it needs this data.
- Public best times: Art. 6 (1) (a) GDPR – your consent, which you can withdraw at any time (switch off, delete your entries or send an email).
- Cookies and game state in the browser: storing and reading them under § 25 (2) no. 2 TDDDG – strictly necessary for the service you open; processing the cookie ID itself under Art. 6 (1) (b) GDPR.
- Delivering the pages and the server's error messages: Art. 6 (1) (f) GDPR – legitimate interest in secure, working operation.
You don't have to provide any data: you can play without an account. Without the “sid” cookie the game doesn't work, though, because the server couldn't tell which puzzle is yours.
Your rights
You have the right of access (Art. 15 GDPR), rectification (Art. 16), erasure (Art. 17), restriction of processing (Art. 18) and data portability (Art. 20). Most of it you can do yourself:
- Access and portability: download all your data as a file.
- Erasure: with an account on the selection page under Account – that deletes it in all games, here with all your progress and best times. As a guest with the button below; your entries in the best times on the best times page or by email.
- Anything else: send an email to info@mueller-wulff.de.
Right to object (Art. 21 GDPR): where we rely on legitimate interests (delivering the pages, the server's error messages), you can object at any time on grounds relating to your particular situation – an email is enough.
You can also lodge a complaint with a data protection supervisory authority (Art. 77 GDPR), for example: Der Landesbeauftragte für Datenschutz und Informationsfreiheit Mecklenburg-Vorpommern, Werderstraße 74a, 19055 Schwerin.
Clear everything in this browser
Ends your session and deletes the session cookie and the game state in this browser. As a guest that also deletes your progress. With an account it also signs you out on the selection page and in the other games (cookie “kb”); the account itself stays – to delete it, see above.
Last updated: Oktober 2026