Grille (cryptography) - Cryptanalysis

Cryptanalysis

The original Cardano Grille was a literary device for gentlemen’s private correspondence. Any suspicion of its use can lead to discoveries of hidden messages where no hidden messages exist at all, thus confusing the cryptanalyst. Letters and numbers in a random grid can take shape without substance. Obtaining the grille itself is a chief goal of the attacker.

But all is not lost if a grille copy can't be obtained. The later variants of the Cardano grille present problems which are common to all transposition ciphers. Frequency analysis will show a normal distribution of letters, and will suggest the language in which the plaintext was written. The problem, easily stated though less easily accomplished, is to identify the transposition pattern and so decrypt the ciphertext. Possession of several messages written using the same grille is a considerable aid.

Gaines, in her standard work on hand ciphers and their cryptanalysis, gave a lengthy account of transposition ciphers, and devoted a chapter to the turning grille.