How to get to the given solution for this hidden words puzzle?

I got a (German) puzzle advent calendar. Today's puzzle is the following:

We are looking for a full (German) sentence. If you aren't a German speaker, this is the sentence we are looking for:

SCHAU IN DEN LUEFTUNGSSCHACHT UNTER DER PALME FUER ANTWORTEN

The question is, how do you find this answer?

Hint: The "solution" has this to say about the process to find this sentence. However this is not clear to me. Even already knowing the sentence I'm looking for and reading this process I can't figure out any system that would result in the above sentence. The puzzle was supposed to be solved without knowing the sentence beforehand.

German:

Anhand der freien Felder zeigt sich, welche Buchstaben gelesen werden.
Anschließend wird die Schablone jeweils um 90° im Uhrzeigersinn gedreht und die nächsten Buchstaben werden freigelegt, bis sich die Lösung offenbart.

English:

The empty cells show which letters should be read.
Then the mask is rotated 90 degrees clockwise to reveal/expose the next letters, until the solution is found.

• Hello, welcome to Puzzling.SE. We require questions that ask about puzzles from external sources to provide proper attribution. Any web link to the product? Dec 4 '21 at 12:20
• This looks like a Sandorf cipher. You need the Schablone the answer mentions. Dec 4 '21 at 12:32
• Thank you, I added a link where to buy it. I hope that's enough. Regarding the Schablone. The image is all I have. I think the solution is talking about using the empty spaces as the Schablone Dec 4 '21 at 12:53

1 Answer

The answer is hidden ...

... by means of a grille, which is the Schablone mentioned in the solution. It is a square template with holes through which you can read the hidden text.

You need the grille in order to encrypt and decrypt the message. In your case, the grille looks like this:

Then ...

... you can read four parts of the message. Each time, the template is rotated by 90° clockwise:

The small letters at the bottom are just filler after the end of the message. Your advent calendar looks like a room escape game. You've probably come across the grille pattern earlier.

• Thank you, this is the answer. I found the grille you created in a picture for the room for the 6th of December. This puzzle is from the 4th. Dec 4 '21 at 14:24
• How can you know what is in the 6th of december when we are only the 4th? Dec 4 '21 at 19:49
• Ask the designers of the calendar. :> (Advent calendar suggests that every day a numbered "door" is opened, but the pages in the screenshots end with a multiple choice question and symbols that indicate the next page. I don't know whether you can take different paths through the story or whether the wrong answers are just dead ends. Even if the story is linear, the design might make sense if the relevant question is asked on 6 Dec or later.) Dec 4 '21 at 20:46