Turning Grille

The turning grille cipher, also known as the Fleissner grille, is a transposition encryption technique which consists of hiding a message by arranging its letters in a grid on which a removable and rotating cover allows the letters to be read/written to be selected.

To encode a message with a rotating grid, the user places a perforated grid or sheet with holes on top of a blank grid.

The locations visible through the holes are then filled with the letters of the plain message.

Once filled, the perforated grid is then turned a quarter turn, revealing new free spaces, which are in turn filled with the letters of the plain message.

After 4 rotations, the grid is supposed to be full.

Reading the letter grid online constitutes the encrypted message.

If the message contains more letters, then repeat the steps with the following letters and a new blank grid (but keeping the same perforated grid).

If the grid is not completely filled, insert random/neutral letters to fill in the holes.

To decode a message encrypted with a spinning grid, the player places the perforated grid over the encoded text, aligning the holes with the characters.

The characters visible through the holes reveal the letters of the original message.

After 4 hourly rotations, the original message is completely reconstituted.

If the message contains more letters, then repeat the steps with the following letters.

The grid must have a quarter hole for every 3/4 non-hole. Since the holes successively occupy 4 positions, only one of these positions can contain a hole.

An easy way to avoid having duplicate holes is to separate the grid into 4 quadrants, then number the boxes as follows:

Make holes for each value, but limiting yourself to 1 hole per value (at any random location among the 4).

The rotating grid method was detailed by the Austrian Colonel Edouard Fleissner von Wostrowitz, who detailed it in his work entitled Handbuch der Kryptographie. Although it cannot be said with certainty that he was the inventor, because encryption techniques using grids had existed for a long time, Fleissner's name remained associated with this method.

Jules Verne, in 1885, incorporated this cryptographic technique into his novel Mathias Sandorf, crediting it to Fleissner.

This type of grilles was used during the First World War by the German army. Each grid size had a code name: Anna 5×5, Berta 6×6, Clara 7×7, Dora 8×8, Emil 9×9, Franz 10×10.

The encrypted message is a transposition of the letters, the coincidence index is similar to that of the plain text.

The presence of a punched card (or any other similar object) should bring to mind this method or the Cardan cipher.

Any reference to Mathias Sandorf or Jules Verne is a clue.

Deciphering a message without having the corresponding grid is difficult.

Potential attacks include searching for recurring patterns in the ciphertext, statistical analysis of letters, but especially brute force methods. The number of permutations of the grid is relatively limited if its size is small.

There are several variations of the rotating grille, including different sized racks or methods of rotating the grille clockwise or counterclockwise.