Fleissner Turning Grille
Four turns, one complete grid: a stencil that scrambles the message instead of hiding it
A transposition cipher: quarter-turn holes permute a complete block, preserving letter frequencies.
Click a cell to move its orbit’s hole there. The other three cells in that orbit remain covered in the starting mask. The demo always keeps a valid grille.
Starting holes (zero-based row-major):
Even-sized grids only. Encrypt pads the last block with X; Decrypt preserves all padding. The rotation button changes the view, while encryption always begins at 0°. The demo uppercases letters, removes accents, and discards spaces, punctuation, and digits.
Why This Matters
A turning grille uses a stencil as a permutation. Its holes visit different cells at each quarter-turn until every cell holds a message letter. Unlike Cardano’s fixed grille, it produces a visibly scrambled block, with no innocent cover letter to hide the secret’s existence.
Fleissner described the turning grille in his 1881 Handbuch der Kryptographie, although rotating masks have earlier precedents. Germany used grille transposition for field traffic during WWI. William Friedman’s 1935 Advanced Military Cryptography records German use in 1915 and named grids of different sizes; other historical accounts describe German turning-grille traffic in late 1916–early 1917.
The distinction matters: this exhibit does not claim Fleissner invented all stencil systems, or that all wartime grilles shared one size and schedule. The demo uses an even-sided square and four clockwise turns.
Cut one hole from each group of four cells that rotate into one another. Place the mask at its agreed starting orientation and write plaintext through the holes in row order. Turn it clockwise 90° and continue writing. Repeat at 180° and 270°, then lift the mask and read the filled grid in ordinary row order.
A valid even-sided grille has N²/4 holes. Four rotations must cover all N² cells exactly once: an overlap means a lost letter and an uncovered cell. The recipient writes ciphertext into the grid by rows and reads through the holes in the same four orientations.
4×4 starting holes (zero-based): 0, 1, 2, 5
Input: ABCDEFGHIJKLMNOP
Grid: A B C E
M D F G
N O I H
P J K L
Output: ABCEMDFGNOIHPJKLThe interactive pads incomplete blocks with X and retains those Xs when decrypting. Because X can also be a real letter, the recipient needs the original length to remove padding unambiguously. Odd-sided historical grids need a separate rule for the central cell; this demo omits them.
| Cardano’s fixed grille | Fleissner’s turning grille |
|---|---|
| Secret letters occupy selected positions in cover text. | Every grid cell is filled by the secret message or padding. |
| Steganography: conceal that a message exists. | Transposition: conceal the order of an evident ciphertext. |
| Read one agreed stencil placement. | Read four successive orientations. |
Visit Cardano Grille to compare the fixed mask. Sharing a physical stencil does not make their security goals identical.
Captain C.J.H. van der Harst solved a Dutch Army turning-grille message during a 1913 exercise. He used likely military words and repeated-letter patterns to reconstruct hole positions, then checked how those positions behaved under rotation. The surviving archival case is analysed in Karl de Leeuw’s HistoCrypt 2018 paper.
More generally, the cipher preserves letter frequencies and repeats the same block permutation when a grille is reused. Cribs, multiple messages under one mask, and language scoring help recover the ordering. The small 4×4 demo has only 4⁴ = 256 starting hole patterns, before varying its fixed direction or readout convention.
| Exhibit | 163 of 167 |
| Associated with | Edouard Fleissner von Wostrowitz |
| Publication | Handbuch der Kryptographie · 1881 |
| Military use | German Army · World War I |
| Family | Block transposition |
| Key | Hole pattern, starting orientation, and rotation direction |