The Cardan grille is a method of writing secret messages using a grid.

History

This technique was used in

ancient China The earliest known written records of the history of China date from as early as 1250 BC, from the Shang dynasty (c. 1600–1046 BC), during the reign of king Wu Ding. Ancient historical texts such as the '' Book of Documents'' (early chapte ...

. In 1550,

Girolamo Cardano Gerolamo Cardano (; also Girolamo or Geronimo; french: link=no, Jérôme Cardan; la, Hieronymus Cardanus; 24 September 1501– 21 September 1576) was an Italian polymath, whose interests and proficiencies ranged through those of mathematician, ...

(1501–1576), known in French as , proposed a simple grid for writing hidden messages. He intended to cloak his messages inside an ordinary letter so that the whole would not appear to be a cipher at all. Such a disguised message is considered to be an example of

steganography Steganography ( ) is the practice of representing information within another message or physical object, in such a manner that the presence of the information is not evident to human inspection. In computing/electronic contexts, a computer file, ...

, which is a sub-branch of general

cryptography Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adver ...

. But the name Cardan was applied to grilles that may not have been Cardan's invention, and, so, ''Cardan'' is a generic name for cardboard grille ciphers.

Cardinal Richelieu Armand Jean du Plessis, Duke of Richelieu (; 9 September 1585 – 4 December 1642), known as Cardinal Richelieu, was a French clergyman and statesman. He was also known as ''l'Éminence rouge'', or "the Red Eminence", a term derived from the ...

(1585–1642) was reputed to be fond of the Cardan grille and to have used it in both private and diplomatic correspondence. Educated men in 17th century

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

were familiar with word games in literature, including

acrostics An acrostic is a poetry, poem or other word composition in which the ''first'' letter (or syllable, or word) of each new line (or paragraph, or other recurring feature in the text) spells out a word, message or the alphabet. The term comes from ...

anagram An anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once. For example, the word ''anagram'' itself can be rearranged into ''nag a ram'', also the word ...

s, and

ciphers In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode i ...

. Although the original Cardan grilles were little used by the end of the 17th century, they still appeared occasionally in the form of ''masked letters'' and as literary curiosities.

George Gordon Byron George Gordon Byron, 6th Baron Byron (22 January 1788 – 19 April 1824), known simply as Lord Byron, was an English romantic poet and Peerage of the United Kingdom, peer. He was one of the leading figures of the Romantic movement, and h ...

, for example, claimed to have written Cardan-grille verse – but as a demonstration of verbal skill rather than a serious cipher. Alternative grilles possess apertures for single letters only and can be used quickly. Messages are filled out with a jumble of letters or numbers and are clearly

cryptograms A cryptogram is a type of puzzle that consists of a short piece of encrypted text. Generally the cipher used to encrypt the text is simple enough that the cryptogram can be solved by hand. Substitution ciphers where each letter is replaced by ...

whereas Cardano intended to create steganograms. These single-letter grilles may be named after Cardano, but they are also called simply ''cardboard ciphers''. A further variation is a turning grille or trellis, based on the

chess board A chessboard is a used to play chess. It consists of 64 squares, 8 rows by 8 columns, on which the chess pieces are placed. It is square in shape and uses two colours of squares, one light and one dark, in a chequered pattern. During play, the b ...

, which was used in the latter 16th century. The turning grille reappeared in a more sophisticated form at the end of the 19th century; but, by this time, any connection with Cardano was in name alone.

Construction

A Cardan grille is made from a sheet of fairly rigid paper or parchment, or from thin metal. The paper is ruled to represent lines of handwriting and rectangular areas are cut out at arbitrary intervals between these lines. An encipherer places the grille on a sheet of paper and writes his message in the rectangular apertures, some of which might allow a single letter, a syllable, or a whole word. Then, removing the grille, the fragments are filled out to create a note or letter that disguises the true message. Cardano suggested drafting the text three times in order to smooth any irregularities that might indicate the hidden words. The recipient of the message must possess an identical grille. Copies of grilles are cut from an original template, but many different patterns could be made for one-to-one correspondence. The grille can be placed in four positions – face up and face down, upright and reversed – which increases the number of possible cell positions fourfold. In practice it can be difficult to construct an innocent message around a hidden text. Stilted language draws attention to itself and the purpose of the Cardan grille is to create a message ''“without suspicion”'' in the words of

Francis Bacon Francis Bacon, 1st Viscount St Alban (; 22 January 1561 – 9 April 1626), also known as Lord Verulam, was an English philosopher and statesman who served as Attorney General and Lord Chancellor of England. Bacon led the advancement of both ...

. But the task was easier for Cardano, because 16th century spelling was not standardised and left much room for contractions and adornments of penmanship.

Detection of messages

When executed badly, a Cardan message stands out because of stilted language and uneven writing. An analyst may attempt to reconstruct the grille when there are several examples of suspect messages from a correspondent. When executed well, a Cardan message can be difficult to spot. Even when an analyst suspects the presence of a message, an innocent letter can contain apparently hidden text. So, in practice, the only solution is to obtain the grille itself.

Disadvantages

The method is slow and requires literary skill. Above all, any physical cipher device is subject to loss, theft and seizure; so to lose one grille is to lose all secret correspondence constructed with that grille. Possession of a grille may be incriminating. The Cardan grille, in its original form, is of more literary than cryptographic interest. For example, controversy surrounds the

Voynich manuscript The Voynich manuscript is an illustrated codex hand-written in an otherwise unknown writing system, referred to as 'Voynichese'. The vellum on which it is written has been carbon-dated to the early 15th century (1404–1438), and stylistic anal ...

which could be a 16th-century fake cipher text, possibly constructed with a Cardan grille which was used to generate pseudo-random nonsense from a pre-existing text.''Nature'' news article
World's most mysterious book may be a hoax
(2003), a summary of Gordon Rugg's paper

References

* David Kahn, ''The Codebreakers — The Comprehensive History of Secret Communication from Ancient Times to the Internet'', 1996, .

External links