page_title: Grille (cryptography)
section_title: Trellis ciphers
hierarchical_section_title: Grille (cryptography) / Cardan grille and variations / Trellis ciphers
caption_attribution_description: English: Two steps that create a trans position cipher on a chessboard, created specifically for an article on grille cryptography Italiano: I due passaggi che creano un cifrario a trasposizione su una scacchiera
context_page_description: 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. The earliest known description is due to the polymath Girolamo Cardano in 1550. 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.
context_section_description: 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. However, he generally preferred the combined code-cipher method known as a nomenclator, which was the practical state-of-the-art in his day. The trellis was described as a device with spaces that was reversible. 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.
  The encipherer begins with the board in the wrong position for chess.  Each successive letter of the message is written in a single square.  If the message is written vertically, it is taken off horizontally and vice versa.
  After filling in 32 letters, the board is turned through 90 degrees and another 32 letters written (note that flipping the board horizontally or vertically is the equivalent).  Shorter messages are filled with null letters (i.e., padding).  Messages longer than 64 letters require another turn of the board and another sheet of paper.  If the plaintext is too short, each square must be filled up entirely with nulls.
  J M T H H D L I S I Y P S L U I A O W A E T I E E N W A P D E N E N E L G O O N N A I T E E F N K E R L O O N D D N T T E N R X
  This transposition method produces an invariant pattern and is not satisfactorily secure for anything other than cursory notes.
  33, 5, 41, 13, 49, 21, 57, 29, 1, 37, 9, 45, 17, 53, 25, 61, 34, 6, 42, 14, 50, 22, 58, 30, 2, 38, 10, 46, 18, 54, 26, 62, 35, 7, 43, 15, 51, 23, 59, 31, 3, 39, 11, 47, 19, 55, 27, 63, 36, 8, 44, 16, 52, 24, 60, 32, 4, 40, 12, 48, 20, 56, 28, 64
  A second transposition is needed to obscure the letters.  Following the chess analogy, the route taken might be the knight's move. 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.
caption: A trellis or chessboard cipher.
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