Yukendo.

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. 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 “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.

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.

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.

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

Name, password as a hash, date created.

What for
So you can sign in. The password is hashed with PBKDF2-SHA256 and a random salt and can't be reversed. When you create an account or sign in, your progress from the guest session is moved into the account.
How long
Until you delete your account.

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 cookie ID, which account it belongs to, and an expiry date.

What for
So you stay signed in after the server restarts.
How long
30 days, deleted immediately when you sign out.

What is not stored

No email address, no real name (unless you choose it as your user name), 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

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:

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. An account stays – to delete it, see above.

Last updated: Oktober 2026

How to play.

Open as a page

Sudoku

The goal

Every row, every column and each of the nine 3×3 boxes holds each digit from 1 to 9 exactly once. Some digits are given; you work out the rest – no maths involved.

53?678912

Only the 4 is missing in this row – so it goes in the empty cell.

How to start

  1. Nearly full lines. Look for a row, column or box with only a few empty cells. Which digits are missing?
  2. Only one digit fits. Pick an empty cell and rule out every digit already in its row, column or box. If only one is left, that's it.
  3. Only one place. Pick a digit, say 7, and a box where it's still missing. Often there's just one cell left where it's allowed.
  4. Notes. If several digits could fit, write them into the cell as small notes. When you place a digit, it disappears from the notes around it.

You never have to guess: every puzzle here has exactly one solution, and Easy and Medium can be solved with these steps alone.

Yukendo

The goal

The grid is 4×4 to 9×9. Every row and every column holds each number from 1 up to the grid size exactly once – on a 6×6, that's 1 to 6. There are no boxes; instead the grid is split into cages with a thick outline.

The cages

The corner of each cage shows a target and an operation. The numbers in the cage have to make the target with it:

  • 7+34added up: 3 + 4 = 7
  • 12×34multiplied: 3 × 4 = 12
  • 1−12difference: 2 − 1 = 1
  • 2÷42divided: 4 ÷ 2 = 2

With − and ÷ a cage always has two cells, and you always take the larger number first – which one sits where doesn't matter. A number may appear twice in a cage, as long as not in the same row or column. A cell without an operation is given.

An example

The puzzle
Its only solution

How to start

  1. Cages with few options. “3+” in two cells can only be 1 and 2; “12×” on a 4×4 only 3 and 4. Row and column tell you which goes where.
  2. Rule things out. A number that's already in a row can't appear there again – just like in Sudoku.
  3. Notes. Write the possible numbers into the cells as small notes and cross them off step by step.
  4. The row total. Every row adds up to the same: 1 + 2 + … up to the size, so 21 on a 6×6. Handy for long plus cages.

Easy uses only +, Medium + and −, from Hard on all four operations. Start with 4×4 or 5×5.

Playing here

Playing here

  • Fill in: tap a cell (or move with the arrow keys), then pick a number on the pad or keyboard. The same number again clears it.
  • Notes (N or ✎) switches to small pencil marks; Undo (Z) takes back the last step.
  • Three modes: Relaxed checks every number right away; Show mistakes marks duplicate numbers (and wrong cages) red; Like on paper marks nothing – you check at the end. The less help, the more Groschen.
  • Stuck? Check (1 Groschen) tells you if a number is right, Reveal (3 Groschen) shows the right one.

Keyboard

1–9
Put a number in the selected cell
← → ↑ ↓
Move between cells
N
Notes
Z
Undo
⌫
Erase
? · g h
This guide
g s · g y · g e
Go to Sudoku, Yukendo, Progress