Spiral Cipher

Spiral writing is a method of arranging text within a grid (square or rectangular) in a spiral pattern, with a predetermined starting point and direction of rotation.

When used to encrypt a message, it constitutes a transposition cipher: the letters are not changed, but simply rearranged according to a geometric rule.

The plaintext is written following the spiral pattern, and then the ciphertext is obtained by reading it in a different order (by rows, by columns, or according to another agreed-upon sequence).

To encrypt a message by spiral writing, imagine a quadrangular spiral (written in a square). Then write the message by turning like a spiral/snail.

If the spiral doesn't form an exact square (or rectangle), filler characters like X can be added. Avoid spaces, as they are visually invisible and can introduce ambiguities.

Then read the grid according to another agreed-upon rule (by rows, by columns, etc.) to obtain the encrypted text.

Decryption is writing the message in rows or in columns and reading the text in a spiral. It is necessary to know the mode of writing (inward or outward, trigonometric direction or clockwise, etc.)

a spiral reading gives the plain message ROTATIONS.

Since spiral encryption is a transposition cipher, the frequencies of the letters are preserved, the coincidence index of the text is the same of the original plain message language.

Sometimes a word or text is formed on one or more created lines or columns (the word may be written backwards).

Ideally the encrypted message has a number of square characters (whose square root is an integer: 9, 16, 25, 36 etc.)

Any reference to a whirlwind, cyclone, vortex, or anything rotating around something is a clue.

Variations of the spiral cipher involve several parameters:

— The starting point (corner, center, edge)

— The type of spiral: inner spiral (towards the center), outer spiral (from the center)

— The direction of rotation: clockwise, counterclockwise

— The final reading method: by rows, by columns, boustrophedon, etc.

More generally, this method belongs to the family of route ciphers, in which the message follows a predefined geometric path within a grid.