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{{Short description|Method for writing secret messages}}
The '''Cardan grille''' is a method of writing secret messages using a grid.

==History==
This technique was used in [[ancient China]].<ref>Fabien A. P. Petitcolas and Stefan Katzenbeisser. [https://www.researchgate.net/profile/Manikandan_V_M/post/Can_anyone_provide_to_me_some_books_about_steganography_in_image_cover/attachment/59d638e6c49f478072ea5ab5/AS%3A273706549481503%401442268134726/download/Stefan+Katzenbeisser+Information+Hiding+Techniques+for+Steganography+and+Digital+Watermarking+-Book1.pdf Information Hiding Techniques for Steganography and Digital Watermarking.] 2000.</ref>{{additional citation needed|date=September 2020}}

In 1550, [[Girolamo Cardano]] (1501–1576), known in French as {{lang|fr|Jérôme {{em|Cardan}}}}, 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]], which is a sub-branch of general [[cryptography]].  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]] (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]] were familiar with word games in literature, including [[acrostics]], [[anagram]]s, and [[ciphers]].

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]], for example, claimed to have written Cardan-grille verse – but as a demonstration of verbal skill rather than a serious cipher.{{citation needed|date=December 2016}}

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]] 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 (architecture)|trellis]], based on the [[chess board]], 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==
[[Image:CardanGrille.png|frame|{{center|A Cardan grille has no fixed pattern}}]]
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.

[[Image:4Positions.svg|frame|left|Four positions of the grille.]]

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”''{{cn|date=December 2012}} in the words of [[Francis Bacon]].  But the task was easier for Cardano, because 16th century spelling was not standardised and left much room for [[Poetic contraction|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]] 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.<ref>''Nature'' news article: [http://www.nature.com/nsu/031215/031215-5.html World's most mysterious book may be a hoax] (2003), a summary of Gordon Rugg's paper</ref>

== See also ==
* [[Grille (cryptography)]]

==References==
{{reflist}}
* David Kahn, ''The Codebreakers — The Comprehensive History of Secret Communication from Ancient Times to the Internet'', 1996,  {{ISBN|0-684-83130-9}}.

==External links==
* https://web.archive.org/web/20111005155213/http://www2.si.umich.edu/spies/methods-mask.html

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