Grille (cryptography)

Class of cipher

In the history of cryptography, a grille cipher was a technique for encrypting a plaintext by writing it onto a sheet of paper through a pierced sheet (of paper or cardboard or similar). The earliest known description is due to Jacopo Silvestri in 1526. His proposal was for a rectangular stencil allowing single letters, syllables, or words to be written, then later read, through its various apertures. The written fragments of the plaintext could be further disguised by filling the gaps between the fragments with anodyne words or letters. This variant is also an example of steganography, as are many of the grille ciphers.

Not all ciphers are used for communication with others: records and reminders may be kept in cipher for use of the author alone. A grille is easily usable for protection of brief information such as a key word or a key number in such a use. In the case of communication by grille cipher, both sender and recipient must possess an identical copy of the grille. The loss of a grille leads to the probable loss of all secret correspondence encrypted with that grille. Either the messages cannot be read (i. e., decrypted) or someone else (with the lost grille) may be reading them. A further use for such a grille has been suggested: it is a method of generating pseudo random sequences from a pre existing text. This view has been proposed in connection with the Voynich manuscript. It is an area of cryptography that David Kahn termed enigmatology and touches on the works of Dr John Dee and ciphers supposedly embedded in the works of Shakespeare proving that Francis Bacon wrote them, which William F. Friedman examined and discredited.

The unsolved D'Agapeyeff cipher, which was set as a challenge in 1939, contains 14x14 dinomes and might be based on Sacco's idea of transposing a fractionated cipher text by means of a grille.

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.