The pigpen cipher (sometimes referred to as the masonic cipher, Freemason's cipher, Rosicrucian cipher, or Tic-tac-toe cipher) is a geometric simple substitution cipher which exchanges letters for symbols which are fragments of a grid. The example key shows one way the letters can be assigned to the grid.
The use of symbols is no impediment to cryptanalysis, and this system is identical to that of other simple monoalphabetic substitution schemes. Due to the simplicity of the cipher, it is often included in children's books on ciphers and secret writing.
The exact origin of the cipher is uncertain, but records of this system have been found which go back to at least the 18th century. Variations of this cipher were used by both the Rosicrucian brotherhood and the Freemasons, though the latter used it so often that the system is frequently called the Freemason's cipher. They began using it in the early 18th century to keep their records of history and rites private, and for correspondence between lodge leaders. Tombstones of Freemasons can also be found which use the system as part of the engravings. One of the earliest stones in Trinity Church Cemetery in New York City, which opened in 1697, contains a cipher of this type which deciphers to "Remember death". (Cf. "memento mori".) George Washington's army had documentation about the system, with a much more randomized form of the alphabet. And during the American Civil War, the system was used by Union prisoners in Confederate prisons.
The core elements of this system are the grid and dots. Some systems use the X's, but even these can be rearranged. One commonly used method orders the symbols as shown in the above image: grid, grid, X, X. Another commonly used system orders the symbols as grid, X, grid, X. Another is grid, grid, grid, with each cell having a letter of the alphabet, and the last one having an "&" character. Letters from the first grid have no dot, letters from the second each have one dot, and letters from the third each have two dots. Another variation of this last one is called the Newark Cipher, which instead of dots uses one to three short lines which may be projecting in any length or orientation. This gives the illusion of a larger number of different characters than actually exist.
Another system, used by the Rosicrucians, used a single grid of nine cells, and 1 to 3 dots in each cell or "pen". So ABC would be in the top left pen, followed by DEF and GHI on the first line, then groups of JKL MNO PQR on the second, and STU VWX YZ on the third. When enciphered, the location of the dot in each symbol (left, center, or right), would indicate which letter in that pen was represented. More difficult systems use a non-standard form of the alphabet, such as writing it backwards in the grid, up and down in the columns, or a completely randomized set of letters.
Using the example key in the above image, the message "X MARKS THE SPOT" is rendered in ciphertext as:
In popular culture 
The Pigpen cipher has been used in several works of popular culture including Dan Brown's 2009 novel The Lost Symbol, both in the book itself, and also to provide a puzzle in the artwork of the U.S. version of the bookjacket. The Trap, a 2009 nominee for Lancashire Children's Book of the Year, uses a variation of the Pigpen cipher.
In the game Assassin's Creed II, the cipher is used in the hidden glyph puzzle number 10, titled "Apollo". Here the cipher is one of many hidden messages tucked away in paintings and photos of historic events or people. Though not essential to the play of the game, the encrypted messages, some of which also use Morse Code and binary code, provide clues to the game's back-story.
The BBC series Sherlock has created a website to correlate with the events in the series - The Science of Deduction. Under the "Hidden Messages" tab, "Sherlock" lists three coded messages he received from a stalker, and the third is in pigpen cipher.
In Club Penguin, a modified pigpen cipher is used for "Secret Agents".
In the beginning of Linkin Park's "In the End" music video, the doorway that Chester emerges from is inscribed in Pigpen symbols
- Barker, Wayne G., ed. (1978). The History of Codes and Ciphers in the United States Prior to World War I. Aegean Park Press. ISBN 0-89412-026-3.
- Gardner, Martin (1972). Codes, ciphers and secret writing. ISBN [[Special:BookSources/0-486-24763-9|0-486-24763-9 [[Category:Articles with invalid ISBNs]]]] Check
- Kahn, David (1967). The Codebreakers. The Story of Secret Writing. Macmillan.
- Kahn, David (1996). The Codebreakers. The Story of Secret Writing. Scribner. ISBN 0-684-83130-9.
- Newton, David E. (1998). "Freemason's Cipher". Encyclopedia of Cryptology. ISBN 0-87436-772-7.
- Pratt, Fletcher (1939). Secret and Urgent: The story of codes and ciphers. Aegean Park Press. ISBN 0-89412-261-4.
- Shulman, David; Weintraub, Joseph (1961). A glossary of cryptography. Crypto Press. p. 44.
- Wrixon, Fred B. (1998). Codes, Ciphers, and other Cryptic & Clandestine Communication. Black Dog & Leventhal Publishers, Inc. ISBN 1-57912-040-7.
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