Grille (cryptography)

[1] His proposal was for a rectangular stencil allowing single letters, syllables, or words to be written, then later read, through its various apertures.

There is a modern distinction between cryptography and steganography Sir Francis Bacon gave three fundamental conditions for ciphers.

[2] The Elizabethan spymaster Sir Francis Walsingham (1530–1590) is reported to have used a "trellis" to conceal the letters of a plaintext in communication with his agents.

It appears to have been a transposition tool that produced something much like the Rail fence cipher and resembled a chess board.

Cardano is not known to have proposed this variation, but he was a chess player who wrote a book on gaming, so the pattern would have been familiar to him.

Whereas the ordinary Cardan grille has arbitrary perforations, if his method of cutting holes is applied to the white squares of a chess board a regular pattern results.

Or some other path can be agreed upon, such as a reverse spiral, together with a specific number of nulls to pad the start and end of a message.

Baron Edouard Fleissner von Wostrowitz, a retired Austrian cavalry colonel, described a variation on the chess board cipher in 1880 and his grilles were adopted by the German army during World War I.

[3] Bauer notes that grilles were used in the 18th century, for example in 1745 in the administration of the Dutch Stadthouder William IV.

There is no standard pattern of apertures: they are created by the user, in accordance with the above description, with the intention of producing a good mix.

The method gained wide recognition when Jules Verne used a turning grille as a plot device in his novel Mathias Sandorf, published in 1885.

Another method of indicating the size of the grille in use was to insert a key code at the start of the cipher text: E = 5; F = 6 and so on.

In 1925 Luigi Sacco of the Italian Signals Corps began writing a book on ciphers which included reflections on the codes of the Great War, Nozzioni di crittografia.

He observed that Fleissner's method could be applied to a fractionating cipher, such as a Delastelle Bifid or Four-Square, with considerable increase in security.

The distribution of grilles, an example of the difficult problem of key exchange, can be eased by taking a readily-available third-party grid in the form of a newspaper crossword puzzle.

Although this is not strictly a grille cipher, it resembles the chessboard with the black squares shifted and it can be used in the Cardan manner.

The message text can be written horizontally in the white squares and the ciphertext taken off vertically, or vice versa.

Frequency analysis will show a normal distribution of letters, and will suggest the language in which the plaintext was written.