Turning grille cipher

The turning grille cipher, also called a Fleissner grille, is a classical transposition system that hides order rather than letter values. You lay a square cardboard mask with cut-out holes over a blank grid, write the next plaintext letters through the openings, rotate the mask ninety degrees, and repeat until the grid is full; reading the filled square row by row yields the ciphertext. This capability implements that process deterministically: supply the text, an even grid size, the base hole pattern, and mode encrypt or decrypt. The engine validates that four clockwise turns cover every cell exactly once and reports an invalid grille when holes overlap, leave gaps, sit outside the board, or have the wrong count. Spaces and punctuation travel with the letters; short plaintext is padded with a single pad character so the message fills whole squares.

How the turning grille works

A turning grille is not a substitution cipher: every character keeps its identity and only its position changes. Fix an even side length n so the board has n squared cells. A valid mask for four quarter-turns has exactly one quarter of those cells open—n squared over four holes—chosen so that rotating the openings ninety degrees clockwise four times visits every cell once and never twice.

That combinatorial condition is the heart of the system: each rotation orbit under the square’s cyclic group contributes exactly one hole in the base mask. Encryption walks the four orientations in order. At each orientation the visible holes are sorted row-major, the next plaintext characters are written through them, and the mask turns.

When all four turns finish, the grid is complete and the ciphertext is the concatenation of rows from top to bottom, left to right. Decryption fills that same grid from the ciphertext in row-major order, then reads letters back out through the identical sequence of rotated holes, restoring the original order including any padding that was appended so the last block would fill. An odd side length leaves a centre cell fixed under rotation and is rejected as an invalid grille for this four-turn model.

Overlapping orbits, duplicate base holes, out-of-range coordinates, or the wrong hole count likewise fail validation before any character is moved, so callers always know whether the mask is a legal key.

How to read the scrambled result

Send text, size, holes, and mode. Text is the plaintext or ciphertext string; nothing is stripped or case-folded, so mixed case, spaces, and punctuation participate in the transposition like any other character. Size is an even integer from two through sixteen inclusive—the side of the square.

Holes is an array of objects each with integer row and col indices starting at zero; the length of that array must equal size squared divided by four. Round-trip by encrypting with a given size and hole pattern, then decrypting with the same pair; a different mask yields a different permutation and will not restore the original. Empty text is rejected.

Classroom, CTF, product use, and cost

History and cryptography courses use the turning grille to introduce geometric keys and orbit arguments before modern ciphers: students design a legal four-hole pattern on a four-by-four board, encrypt a short slogan, pass the ciphertext and mask description to a partner, and recover the slogan only when both sides share the same openings. Capture-the-flag authors hide flags under a published or partial grille and verify solver submissions against this same deterministic routine. Documentation writers embed encrypt and decrypt pairs in tutorials without hand-maintaining permutation tables for every rotation.

Do not treat a turning grille as confidentiality for real secrets—it is a teaching and puzzle tool—but do treat the hole pattern and size as the secret parameters of the toy system.