SudoZKPu

Someone says they’ve solved this Sudoku, but they won’t show you the solution. You can still test whether they’re telling the truth, using a zero-knowledge proof.

1. You choose a row, column or box. They show its nine cells as letters, in a secret code.
2. Nine different letters mean nine different digits, and that’s all you learn.
3. They can’t make up letters to suit your choice: the hidden answers are locked in before you choose.
4. The code changes with every check, so the answers stay hidden however many you run.
5. A cheater’s grid has a mistake, so sooner or later you’ll open a repeated letter.
6. A cheater could also hand in the solution to a different Sudoku. Open the printed numbers to check the grid fits this one.