Open code forms a branch of Linguistic
Steganography of invisibly concealed secret writing (as opposed to
semagrams which are visibly concealed secret writing).
Masked Open Code
Masking a message as open communication requires an agreement prior to the sending of the message as to the true meaning of what is actually said. This has been used for centuries and is often found in games. This can be anything from how a person holds a cigarette, scratches their head to gestures and expressions. This can often be found in partnership games such as contract bridge or whist. The phrase "Have you seen Bob?" would mean "I have hearts" because it starts with 'H'. This method was often used in Victorian days at whist clubs. The British team was suspected of exchanging similar signals at the wold bridge championships in 1965 at Buenos Aires, however, nothing could be proved.
Secret markings have also been used for centuries. A system of marking for traveling scholars in the middle ages existed with much the same goals as the writings of the hobos. Along their travels, hobos would mark on fenceposts, shacks, backs of road signs and other out of the way places. These marks would inform his fellows of the benefits and dangerous of a particular area. While to most people, these were just odd scratches, to someone who knew how to read them they were a valuable set of information. To an extent, jargon can be used by a social group in a similar manner as hobos used symbols to both communicate more effectively or possibly disguise what is actually being said.
Jargon can be used to mask names of people in messages or convey a deeper meaning. The world of crime is filled with jargon: 'hole' for prison, 'hot' for recently stolen property, 'snow' or 'sugar' for cocaine, and 'grass' or 'weed' for marijuana.
This form of masking has difficulty when trying to be used for more universal messages - a censor can quickly spot the awkward language. One of the favorite tricks of a censor is to alter a message yet retaining its obvious message. In World War I, a censor altered a message from "Father is dead" to "Father is deceased." The response back was "Is father dead or deceased?"
Allegory can also be used to mask messages, though the history of open code is full of 'amusing' mistakes. In 1915, German spies bought cigars in batches of thousands from shipyards - each 1000 representing one battleship. In 1944, Velvalee Dickinson, a seller of fine dolls and Japanophile was discovered to have been using her correspondence on broken dolls to send secret messages about the state of the war. In Breakfast at Tiffany's, Miss Golighty spent a night behind bars for helping a gangster conduct his cocaine trade through "weather" reports, as improbable as "snow in New Orleans" would be.
Cues
The most important special case of masking is the cue. This pre-arranged phrase indicated when something important is about to be said. In World War II, this method was often used by both sides. The BBC broadcast to the French resistance a the first verse of the poem Chanson d'Automne by Paul Verlaine on June 1, 1944. The second half was sent on June 5th. While the Germans had already been made aware of the code and its significance, the alarm did not reach the 7th army, stationed on the Normandy coast.
The Japanese had a similar system with the overseas weather reports. The message "east wind, rain" when said twice indicated "war with the U.S.A" and "west wind, clear" indicated the start of hostilities with Great Britain.
Null Cipher
A classic rule for veiled messages is "the nth character after a particular character" This was very popular with soldiers in World War II and often was the first character after a space. This typically leads to stilted wording which is easily detectable.
I favor yoghurt of undeniable chocolate. Another naked race ensured and Danny thought he ignored Sheila.
Once this is lined up, on a page, it jumps out on the page.
A story of a similar method of a soldier in the U.S. Army is told. He arraigned with his parents to tell them the location where he had been posted by sending several letters - the first letter of the first word after the greeting. This isn't so bad from the perspective of cryptography and steganography, except that his cover was blown when his parents wrote back "Where is Nutsi? We can't find it in our atlas." -- He had forgotten to date the letters.
This system can also be translated into music as nodes directly or part of a theme. The i-th note of the scale occurs k times indicating the indicating the k-th letter of the alphabet is to be inserted in the i-th position. In the organ chorale "Vor deinen Thron" by Johann Sebastian Bach, in the key of G major, g occurs twice (B), a once (A), b three times (C), and c eight times (H) in the four bar flourish.
Pig Latin and other similar systems of word modification also fall under this system of steganography.
Grills
The essence of this is to take an existing grille with holes in it and write out the message within its spaces. Around this message, other text can be written. While it is simple and easy to understand, it suffers from the fact that both sides must have a physical device to be able to read the message - something not always easy to do when in prison. Modern variations on this impose more rules on it including binary numbers in which a word with an even number of vowels is used to represent a 0 and an odd number of vowels used to represent the digit 1.