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Anime and Manga
- Dance in the Vampire Bund: At one point, Mina makes a public announcement, speaking in ancient Sumerian as a challenge to another vampire, but with fake subtitles in Japanese.
- Death Note: Misa is a Kira-fangirl, but doesn't know who he is. So she uses her own Death Note to kill people and force the news to send messages. Light is annoyed, since while people who don't know about the notebooks won't be able to figure out what they're talking about, it is still far too public for his taste.
- Detective Conan: In the two-part episode "The Secret of the Sun, Moon and Stars", Professor Agasa and Shinichi Kudo (really Conan Edogawa) suggest that the drawings of the sun, moon, and stars found on various objects in the home of Agasa's late uncle were a code much like the Dancing Men (see the Sherlock Holmes examples, below).
- Fullmetal Alchemist: Riza Hawkeye uses this to deliver a message to Roy Mustang while she's under surveillance. She gets into a mundane conversation with him over lunch in which she talks about the activities of various people she knows. Mustang is later able to take the first initial of each name Hawkeye mentioned and spell out her message.
- Monster: Tenma needed to speak with a former college classmate, so he put an ad in the paper that simply said "Let's discuss our memories of cheating", a reference to a test at medical school that the two happened to discuss the last time they ran into each other.
- One Piece: At the end of the manga's fifth saga, the main character returns to the scene of a major battle, ostensibly to honor the dead. Reporters on the scene photographed him, and his crew, scattered around the world, saw the article, and realized it wasn't the kind of thing their captain typically did, and noticed a simple message written on a tattoo.
- The Promised Neverland: Books shipped into the orphanage by William Minerva contain hidden messages to inform the kids of the orphanage of the true nature of the world outside while slipping past Isabella's vigilant security:
- The first messages discovered are contained within the stamps on the inside front covers: The stamps, which depict Minerva's owl emblem, have imperfections on the rings that look like the results of hand-stamping but actually form dots and dashes to create words in Morse code—putting the books in chronological order of publication creates a message urging the kids to escape as the place is actually a farm run by demons who want to harvest and eat them.
- A number of his books are also adventure novels, which turn out to be guides on how to deal with, escape from, and fool monsters and other dangerous creatures that live outside of the orphanage.
- One book's Morse code stamp does not seem to fit with the others, being a seemingly random bunch of numbers. By looking through the books themselves, namely their page numbers and the order of the words on those pages, they discover it's the password for a pen smuggled into the orphanage that unlocks its GPS capabilities, which displays locations in a coordinate system that only Minerva uses that the kids also have to figure out. Within the box the pen came in is yet another encrypted message, telling the kids his location (in those coordinates) and to find him if they ever need help.
- Tintin: In 1934 Hergé drew a story named The Blue Lotus in which Tintin travels to China. Hergé's friend, a Chinese foreign exchange student named Zhang Chong Ren told him a lot about Chinese culture and society, including the then current situation in Asia, where Japan had military occupied China. He also wrote all the Chinese signs, billboards, ideograms and texts seen in the backgrounds. As a Bilingual Bonus only Chinese people could read these. This also might explain why the book wasn't censored from the start because many of these texts are anti-Japanese slogans, like for instance: Boycot Japanese products, Abolish unfair treaties and Down with Imperialism. Upon realising the anti-Japanese tone of the story, Japan's diplomats stationed in Belgium issued an official complaint and threatened to take their complaint to the Permanent Court of International Justice at The Hague. Zhang congratulated Hergé, stating that it would only further expose the actions of Japan in China to further international scrutiny and would make Hergé "world-famous".
- In Chapter 23 of This Bites!, Cross utilizes the SBS to send a coded message to Sabo, phrased so that nobody other than his brothers and the Revolutionaries could discern its recipient, to tell him about his brothers.
Film — Live Action
- In Men in Black, Agent K explains that tabloids, which are assumed to be hoaxes by muggles, are in fact based on true events behind The Masquerade (since tabloids have less Weirdness Censor than "serious" newspapers). Later, when agent K retires, Agent J notices an article with Agent K's photo and an article about a postal worker who returned to his old job after years in a coma, revealing Agent K's fate, which then becomes a major plot point in the second film.
- In Star Trek II: The Wrath of Khan, Spock tells Kirk on an open channel, "If we go by the book, hours could seem like days". To anyone else, this might sound like a case of Lawful Stupid, but Kirk, who'd been discussing regulations about coded messages with Spock earlier, knows that this means to decode the next message, replace the word "days" with "hours".
- In Serenity, the code that makes River flip out is hidden in an ad spot for Fruity Oaty Bars.
- In Mercury Rising, a secret agency publishes messages in secret codes in the puzzles section of newspapers to determine how difficult they are for amateur code crackers. When a nine-year-old autistic boy solves one thought to be particularly secure, conflict ensues between factions with opposing ideas about what to do about it.
- A variation in I, Robot with Dr. Lanning's hologram. It doesn't say much of use while in public, but the whole point was to alert Det. Spooner without anyone else noticing.
- Even more understandable and closer to the trope when you realise that Lanning couldn't simply encode the answers into the hologram because he was being constantly watched at the time, so he encoded the hologram with a "that's the right question" response and had it respond with that whenever Spooner's question indicated he was on the right track to solve the mystery.
- In Get Smart, KAOS turncoat Dalip is able to tip off Max to Siegfried's plan via a cryptic request on American Top 40, playing on Dalip & Max's shared love of Ryan Seacrest.
- In Angry Video Game Nerd: The Movie, the Atari Eee Tee games have a level design emulating Area 51.
- In Hackers, Crash Override is arrested while holding a floppy disk with important, incriminating information, but manages to slip the disk into a trash bin. When taken outside, he sees his ally Cereal Killer in the crowd and goes off on a rant, "They're TRASHING our rights, man! They're TRASHING the flow of data! They're TRASHING!...." Cereal gets the message, and is next seen rummaging through the trash bin.
- Reservoir Dogs: In the film Mr. Brown explains what he thinks is the hidden message in the lyrics of Madonna's song Like a Virgin.
- The Art of War (2000). A female agent is shown printing posters for a Lost Cat, even though said cat is in her room. She's murdered before she has a chance to put the posters up, but the protagonist finds them in her apartment and deciphers the message encoded in each poster.
- The Gauntlet: The Mafia keeps track of The Bet about whether witness Augustina "Gus" Malley will make it to the trial on Phoenix via a made-up horse race and a made-up horse called "Malley No-Show".
- Wag the Dog: One part of the story regarding the soldier Schumann (a fake American POW for a fake war in Albania) is a photo of him in captivity with his sweater ripped in a pattern that "turns out" to be a Morse Code message saying that he's okay and asking for his mother to stay strong.
- In The End of Eternity, a stranded time traveller publishes an advertisement in a newspaper containing a picture of a mushroom and the phrase:
All theTalkOf theMarket!"
- In Harry Potter and the Order of the Phoenix, Harry alerts Snape while making it possible to convince Umbridge he was shouting gibberish, by yelling "He's got Padfoot at the place where it's hidden!" to the entire room. Snape understands that "Padfoot" was Sirius' old nickname and "the place" was a location that Snape knows Harry's been seeing in his dreams.
- Robert A. Heinlein's novel Friday. In Vancouver, Friday is reading the personal ads in a newspaper and sees an ad that says "W.K.-Make your will. You have only a week to live. A.C.B." More than a week later she sees another ad in a Vicksburg, Mississippi newspaper that says "W.K.-Make your will. You have only ten days to live. A.C.B." Her traveling companion Georges realizes that the messages are a code — the first message meant "number seven" (1 week = 7 days), while the second message meant "number ten".
- Sherlock Holmes stories feature multiple examples. Conan Doyle seemed to like this one.
- In "The Adventure of the Red Circle", someone places ads in the London Daily Gazette' "agony column" to send secret messages.
- "The Adventure of the Bruce-Partington Plans" also features messages in an agony column as a clue, this time in the Daily Telegraph.
- In "The Adventure of the Dancing Men", a series of dancing stick figures appeared in several locations visible to anyone who passed by. Holmes decides the figures represent letters and decodes the message.
- In the Mary Russell mysteries, Holmes and Russell frequently use the "agony column" of the London Times to send messages to each other in a kind of code.
- In the Ngaio Marsh novel Death in a White Tie, one partner in a blackmail scheme sends a coded message to his accomplice via a personal advert in a newspaper: "Childie Darling. Living in exile. Longing. Only want Daughter. Daddy." Taking the first letter of each word yields C.D. Lie low. D.D. [To]Columbo Dimitri Lie low. [signed] Daniel Davidson.
- In the Young Bond novel Double or Die, Bond's teacher (who is also a cryptic crossword compiler) is kidnapped. The kidnappers allow him to submit his final crossword as failure to submit it would have alerted people to the fact he was missing. He uses the crossword to conceal clues as to his location.
- Agatha Christie
- In one book, she wrote of a spy who was behind enemy lines and managed to find a really crucial piece of information but had no way of passing it on, so in the end he sacrificed himself, committing a series of murders and getting convicted — it seemed like random acts of violence but those who knew he was a spy figured that the victims' personal data was used as a code.
- In one of the Tommy and Tuppence stories, Tommy is able to tell Tuppence "These new clients are suspicious. Contact our police liaison and follow me when I go off with them" right in front of the clients, by disguising it as an order for dinner.
- In John Twelve Hawks' (a pseudonym) The Fourth Realm trilogy, this is how the Harlequins communicate with each other and with Travelers. They leave graffiti or other notes in public places that have easily detectable second meanings decodable only by other Harlequins or Travelers.
- In Philip K. Dick's Radio Free Albemuth and its more polished rewrite VALIS, a subliminal message is sent to the public in the form of song lyrics so that the government won't intercept it but those who know the truth will be able to spread the message.
- In The Anubis Gates by Tim Powers, time travelers in the early 19th century get each other's attentions on busy city streets by whistling Beatles songs.
- In Trial by Journal, Lily sends messages to the presumed dead Perry by calling him 'Hansel" (his character in a play) in the newspaper. In return, he calls her LAW, and sends replies via the painting everyone thinks a gorilla is making. It makes sense in context.
- In Robert Anton Wilson's Illuminatus!, a secret society places personal ads reading "In thanks to St. Jude for favors answered — A.W." as a code to their other members.
- In Orson Scott Card's Shadow of the Hegemon Petra and Bean communicate by starting an e-mail signature meme: an image file of a dragon with a coded message in it and the words "Share this dragon / If you do / Lucky end for / Them and you", which reference the Dragon Army and Ender Wiggin.
- In Red Dragon, Hannibal Lecter places a personal ad filled with Bible verse numbers in a tabloid as a coded message (the numbers refer to the nth letter on the xth page of his edition of The Joy of Cooking) to Francis Dolarhyde. The FBI decrypts it, and realizes that it told Dolarhyde where Will Graham lives; they send Dolarhyde another message to lure him into a trap, but he recognizes it for what it is.
- A variation is used in Heir to the Empire. Mara Jade needs to send a message to her boss, Talon Karrde, but there's no way to do so without the message being intercepted by an Imperial Star Destroyer in orbit. Luke Skywalker (who is with her at the time), suggests using a "counterpart encrypt" between his astromech droid (also with them) and his X-wing (in Karrde's possession). The unusual encrypt works; the X-wing's computer decodes the message easily, while everyone else is stumped by it (although the Empire, while unable to read the message, does at least manage to figure out that the message is using a counterpart encrypt). Karrde later sends a message back using the same method, via the X-wing.
- There' a different version in the same book; Winter tells Han about Ackbar's 'kids' acting up, and he inquires about the 'neighbors' — the New Republic politicians and the Empire, respectively. Problem is, this wasn't a code that they'd worked out beforehand — actually, they never worked it out at all. Han knows Winter means there's been some problems, but not just what they mean. This incident was, bizarrely, a case of Real Life Writes the Plot. Long before the book was released Timothy Zahn told some trusted friends about it and met them at a convention to discuss it further, only to realize that they were surrounded by scifi geeks who would know what he meant if he started talking about Luke and Leia and Han and Chewie, and then he'd be in trouble. So on the fly he called them Brother, Sister, Friend, and Copilot... and it worked.
- The protagonist of Mother Night by Kurt Vonnegut works as a radio propagandist for the Nazis in World War II, but he is actually a spy for the Allies. He's given a list of things they want him to find out about the Nazis, and after he finds them out he communicates the answer on his radio show by, say, coughing in the middle of a certain sentence if the answer is "yes" and not coughing if the answer is "no", or by using a certain word he wouldn't otherwise use, etc.
- In Replay, by Ken Grimwood, in one loop, Jeff and Pam use ads with various loop-derived future references to try to make contact with other replayers.
- In A Series of Unfortunate Events VFD members communicate with "Sebald code", in which every eleventh word between two bell rings (or two mentions of bells ringing, if the communication is text-based) is the message. The character of Gustav Sebald, the Volunteer who developed the code, was a famous film director and sent messages with the code in his movies.
- In A Running Duck (aka Fair Game) by Paula Gosling, the protagonists open a newspaper and are shocked to find an open death threat against them. The antagonist got the newspaper to print it by claiming it was an advertising gimmick that would be followed up in the next edition.
- In Brotherhood of the Rose by David Morrell, a CIA agent is apparently calling his contact via a public phone booth, but is actually reading a line of graffiti written on the wall of the booth as a coded message.
- In Semper Mars by Ian Douglas, after UN troops take over the Heinlein Base and "relocate" all US Marines there to a tiny remote outpost, Major Mark Garroway sends a coded message to his daughter Kaitlin back on Earth, asking her to pass a message to "Uncle Walt". Since he knows the UN is likely to block any message with certain key words, he peppers the message with enough American and Marine-specific references to slip under the radar. The message is "Walt, you sorry-assed son of a bitch, listen up and listen good. The blue boys pulled a Pearl Harbor 1207 GMT 27 May. The boss is down, but okay. Forcibly relocated to Red Planet. Have capped guards and secured cat. Am marching on Derna, with complete openness." "Blue boys" refers to UN troops, who wear blue uniforms, "Pearl Harbor" refers to a sneak attack, "the boss is down" means Garroway is in command, "forcibly relocated" means "imprisoned", "Red Planet" is a reference to the Heinlein Base (Red Planet is a novel by Robert A. Heinlein), "have capped guards and secured cat" means that Garroway and his people have managed to escape, kill their guards, and obtain transportation. "Marching on Derna" is a reference to a key piece of USMC history - the Battle of Derna in 1805, which was preceded by a 600-mile march through the desert. This lets the brass know that Garroway and his people plan to make a similar trek across the Martian desert to secure the city of Mars Prime. While it's true that anyone with access to internet would eventually be able to figure out the message, the reason for wording it thus is to avoid automated filters in place by UN.
Live Action Television
- In Time Trax Lambert would send messages to the future by placing coded personal ads in the newspaper.
- Sue Thomas: F.B.Eye: Used by terrorists in the episode "The Lawyer." A terrorist in jail sends a message to his organization to change their plans for an attack by having his lawyer read a statement on the news about how sad he is to miss his nephew's birthday party. The FBI team brings in a special expert on these types of hidden-in-plain-sight messages to help decode the communications.
- In an episode of the Adam West Batman, Batman talks to one of the villains over a broadcast radio station, but requests that all other citizens of Gotham switch off to avoid hearing his private message. Naturally they oblige.
- In Rubicon, a secret society publishes messages encrypted in crossword puzzles in newspapers.
- On Chuck, Steven Bartowski provides Ellie Bartowski with a method for contacting him through coded messages in the personal ads.
- Chuck also takes the microphone at Sarah's high school reunion, puts a spotlight on a couple of people in the audience and starts reminiscing about high school with them. Neither Chuck nor those men attended Sarah's high school; he was just identifying criminals for Sarah and Casey, who he didn't have time to approach personally.
- In Oliver's Travels, Oliver's friend who writes crosswords under the name of "Aristotle" goes missing after getting on the wrong side of a conspiracy, but Aristotle's crosswords continue to be published, and at one point Oliver realizes that the most recent one contains a hidden message indicating who's behind the conspiracy.
- In the Doctor Who episode "Blink", the Doctor records message that a DVD publisher he befriends adds as an Easter Egg on some very specific DVDs (corresponding to the entire DVD collection of one character) decades later. It makes no sense whatsoever, until late in the episode where it's revealed to be The Tape Knew You Would Say That: The character's reactions to the scene form a perfectly cohesive conversation with the Doctor (who has a transcript of her reactions) regarding the TARDIS and the Weeping Angels.
- "Bad Wolf" was a subtle message sent all through time and space by Rose, who she sends to send her past self to rescue the Doctor.
- In "Power of the Daleks", the rebels place a secret message detailing the time and place of their next meeting hidden in a notice placed on a public noticeboard.
- Michael Weston of Burn Notice has been known to create or receive coded messages in public locations. One earlier example showed him writing a cryptic note to another spy on the sidewalk in chalk.
- In an episode of Get Smart, The Chief (disguised as a singing waiter) communicates a message to Max and 99 by slipping code phrases into the song he is singing.
- The Wild Wild West: James West communicates with his partner, Artemus Gordon, through the use of coded symbols written on a menu in one episode and via Morse code punched into a newspaper in another.
- In the first season of Dexter, the Ice Truck Killer leaves little clues in his kills that makes it very clear to Dexter that he's speaking to him personally and not the police.
- In iCarly, in the episode "iPsycho", Carly and the gang need to get a help message out so someone will rescue them from Nora. She's watching their every move, so they ask if they can send a birthday video to their friend Gibby. With Nora watching, they make him a video which contains a secret message.
- "Every fourth!" "Word!" "Every fourth!" "Word!"
- JAG: In "The Black Jet", Jack Keeter has downed in Iran with secret airplane and is detained by the authorities. When first meeting Harm and Mac, accompanied by an Iranian official in prison, he says “No Martin Baker” several times over. Harm later decodes the message: Martin Baker is the manufacturer of ejection seats, and thus Keeter didn’t eject but landed the plane and was captured elsewhere.
- The Pretender: Argus asks for Jarod's help in this way by placing an ad in the newspaper.
- In the German series Löwengrube, the religious (and somewhat naive) mother-in-law of the protagonist mentions one day that the people who beg at her door were so pious nowadays. Since he works for the criminal police, he remembers what he recently learned about hobo signs — and indeed, there is one at her door, meaning "act as if you were pious".
- In one episode of Andromeda, the ship receives a message from Dylan to arrive in a certain system. Tyr says that with Dylan's behavior and constant blinking, it's obvious he's not telling something. Rommie replies that he is telling something — the number of blinks can be deciphered as a code for a military protocol ordering them to arrive prepared to fire at the ship holding him.
- In The Hour, Freddie discovers that a secret code that is being spread through crossword puzzles in the newspaper, though you need a special key to find it.
- In the Star Trek episode "Bread and Circuses", Kirk calls up to the ship just as the enemy's guards threaten him with guns, so instead of telling Scotty to beam down a security detail, he says "Condition Green." Although that sounds like it means "all is well", it's a duress code meaning "We're in trouble, but take no action at this time."
- Murdoch Mysteries:
- In the episode "A Study in Sherlock", David Kingsley (a.k.a. Sherlock Holmes) insists Moriarty communicates with his gang via coded obituaries in the local paper. Murdoch and Brackenreid are dubious, but as which much else in the case, the young Sherlock proves to be correct.
- In "The Artful Detective", Brackenreid notices an extra race published in the racing form; he's familiar with the track and draws Murdoch's attention to the discrepancy. The team compares the horse names to the profiles of several recent murder victims and finds several of the "horse" names are each descriptive of their victims. They come up with the hypothesis that the victims and a few other people are actually killing each other in a Deadly Game, and they stake out the racing form's publisher. Sure enough, the "horses" they have matched to their corpses are missing from the next "running" of that non-existent race, but a new entry is added: "Artful Detective". Guess who soon finds himself attacked in the street?
- Person of Interest.
Finch: When you said your communication with the Machine was limited, you didn't say that it wasn't talking to you at all.Root: If she talks, Samaritan would see. I get whispers. New cover identities hidden in the static of a phone, a map and a message encrypted in an infomercial. She was supposed to remake the world. Now, God's on the run.
- Privacy terrorist group Vigilance communicates between its various cells via this method, as they're Properly Paranoid about government surveillance (methods include a message written on a wall in infrared paint, and a code embedded in a spam advertisement).
- In Season 4, the Machine does this to communicate with Team Machine without Samaritan (which now monitors all CCTV cameras and phones) finding out.
- In Season 5, it's revealed that Samaritan is communicated with its operatives via Fake Static in radio transmissions, forming an encrypted morse code.
- The Equalizer. In "The Distant Fire", McCall finds a postcard pinned to a noticeboard at a bar he frequents. He scribbles out parts of the writing on the back to reveal a message hidden there by an old adversary.
- Murder, She Wrote: In "The Dead File", a cartoonist hides blackmail threats in a daily comic strip.
- Many rock bands have been accused by Moral Guardians of putting hidden messages in their album covers or the lyrics of their songs, some of which would only be deciphered if you played the records backwards. The Beatles are the most famous example. Though sometimes these hidden messages were clearly intended by the artists themselves, others were nothing more than extremely far-fetched fan theories.
- Frank Zappa included secret messages and clues in the album art work and lyrics of his songs that are still being decyphered by fans and music historians world wide. He also had at least one secret word in each concert that he gave, usually inside jokes between him and his band members.
- Dungeons & Dragons 3E had a skill called "Innuendo" for this exact purpose. It was removed in 3.5E (turned into a part of the "Bluff" skill), presumably because it was too specific to be worth spending skill points on.
- The staff at Steve Jackson Games were not allowed to release the title of one game supplement in the Power-Ups series, and the forum speculation was that they were trying to create a sudden burst of interest on release to fuel impulse buys. The author even said "I've been told that the secrecy around Power-Ups 5 does, indeed, have something to do with impulse buys." An entire game's fanbase facepalmed at the release of Power-Ups 5: Impulse Buys.
- In Iji, Dan communicates with Iji by speaking, in English, through every speaker in the complex. Since the aliens never bothered to learn any human languages, they can't understand him, and since he uses every speaker, they can't use the active speakers to track Iji.
- City of Heroes has villains getting bank robbery missions in such a manner, by looking in the newspaper for encoded messages from their Brokers.
- The Illuminati in Deus Ex use this interestingly. They communicate primarily by heavily encrypted e-mails, which they know their oppponents will pick up and read. However, the information isn't in the e-mail, it's in the encryption key. Each key means "Meet me at place X, at Y o'clock, on day Z".
- In the Bad Boys Love route of HatofulBoyfriend, Nageki tells his "brother" Hitori that he's fine when he's clearly in pain due to his illness, causing Hitori to ban him from using the phrase "I'm fine" around him. Nageki only ever uses it again years later in a letter to Hitori from St. Pigeonation's, to let him know that something is very wrong.
- This xkcd makes fun of the public messages in Redwall.
- Girl Genius: here a bunch of Jagers are posing and Chewing the Scenery, probably loud enough to hear for not only a few panicked bystanders, but half of the town. Dimo's first phrase, however, had a very specific meaning, which the only intended listener recognized immediately (and was somewhat shocked). And it's something they say openly only when they are sure no outsider can possibly overhear.
- Clockwork: Before leaving for Arcadia under heavy coercion, Cog leaves a letter for his best friend Boris. It appears to be just a friendly effort to ease Boris' objections to his decision, but the first letters of each line spell out "HELP ME".
- LIS_DEAD is seemingly an inversion; it's encrypted and secret to nobody except for a certain mysterious 'Him'
- Joked about in RockedReviews. While reviewing Metallica's album St. Anger, he theorizes that there must be a hidden message to the album due to its harshness and distinct departure from Metallica's usual sound. The final track on the album (kinda) proves him right when James Hetfield repeatedly shouts the word "KILL" as loud as possible. This scene is accompanied by Luke seeming hypnotized and picking up a box cutter briefly before putting it down.
- The Simpsons:
- In the second season of The Secret Saturdays, Argost starts sending Zak secret messages regarding cryptids as part of television show Weird World.
- Crops up in an episode of Gerry Anderson's New Captain Scarlet when Captain Ochre is captured by Mysteron Agents and forced to check in with headquarters at gunpoint. She uses a pre-arranged duress code-phrase to alert Spectrum that she's in trouble, and Captains Blue and Scarlet are dispatched in a Rhino APC to rescue her. Unfortunately, the Mysterons are a lot smarter in the remake, and not only did they see it coming, they were counting on it so they could jack the Rhino and blow up a nuclear power station with it.
- Before the advent of other means of private communication, many people would place encrypted (using simple substitution ciphers) adverts to lovers in newspapers. Some mathematicians (such as Charles Babbage) were known to keep their minds sharp by breaking them (and in at least one case place similarly encrypted advice not to elope).
- Hobos left marks called "Hobo Signs" on people's fences telling other hobos about the nature of those living there.
- During World War II, the Allies would send messages to friendly resistance fighters, spies and other clandestine groups behind Axis lines by making announcements over civilian BBC broadcasts. While the broadcasts were open and able to be listened to by the Axis, the messages themselves would be meaningless phrases whose meaning would only be understood by the intended receiver.
- Although, as the example of the first two lines of a Verlaine poem broadcast shown in the movie The Longest Day broadcast separately on two separate days before June 6, 1944, demonstrates, someone in the Abwehr did correctly guess the import of such messages (in this case: "the landing in France is imminent").
- The use of phrases and code names even in both clear and encrypted communication also falls into this territory, for instance the Japanese high command sent the message "Climb Mount Niitaka" to Admiral Nagumo to launch the attack on Pearl Harbor.
- An odd-looking advert for the dice game "Deadly Double" had gotten the FBI suspicious, as it had all the trappings of a Public Secret Message announcing the Pearl Harbor attack. It was apparently just a coincidence, though.
- Some US units used "code talkers" — radio operatives who would translate their messages from English into a language that was unknown to Axis forces (typically a Native American language, for some of which, including Navajo, there were no published textbooks or dictionaries). As the absence of messages might have clued off the allies, code talkers were encouraged to just have "private chatter" on days with less communication to take care of. Similarly, terms that would usually be loanwords in Navajo as well as proper names were rendered as calques or complicated cirucmlocutions - it helps that Navajo grammar both enables this and makes it frustratingly hard to understand to non-native speakers. For example, the code word for submarine was "iron fish".
- Numbers Stations are clearly sending some sort of coded message, but it's meaningless to anyone but the intended audience.
- One of the features of "public key encryption" is that you can publish one key, letting anyone send you a message that no one without the other key can decipher. Or conversely, you can encrypt with the private key so that everyone can read your messages knowing they could only have come from you. And you can do both at the same time.
- Gene Roddenberry had served in World War II with a man named Kim Noonien Singh. They lost touch after the war. Roddenberry named two characters after Singh, hoping to attract his attention — Khan Noonien Singh in the "Space Seed" episode of Star Trek: The Original Series (and Star Trek II); and Noonian Soong, Data's creator in Star Trek: The Next Generation.
- A fairly common (and easy) method of sending secret messages in the past was to write an innocuous letter then write between the lines using a clear fruit juice, such as lemon or apple. The recipient would need to know that there was a message and by heating the paper, the fruit-juice words would be "cooked" and become brown and thus visible. This is believed by many to be the origin of the saying "to read between the lines".
- One unusual National Weather Service forecast from 2013 contains an Open Secret. In one section, the first letters of each line spell P-L-E-A-S-E-P-A-Y-U-S, please pay us. Because of a US government shutdown, the employees who prepared the message were working without pay. News organizations exposed the secret, but only after the NWS had sent the message.
- In The '90s, a blackmailer in Germany became somewhat famous. Since one of the messages in the newspaper had the text "Dagobert [Uncle Scrooge's name in Germany] greets his nephews", the press nicknamed him "Dagobert" after this.
- Viktor Suvorov mentions that during his work, he was supposed to leave a message for an agent by sticking a drawing-pin into a pole. Unfortunately, there were so many pins already that they had to devise a separate system — he or his wife went to a park and marked certain letters on a certain plaque with lipstick.
- A "duress code" is a covert signal to call for help without the knowledge of the persons threatening you. For instance, home alarms often have two shutoff codes: the real one, and the one you enter if a robber is forcing you to. That's the code that silently alerts the police.
- In World War II, if an agent wanted to visit a safe-house, he'd simply call the agent at the safe-house and ask if it was genuinely safe to visit. If it was safe, the answer would be "no", but the duress answer would be "yes." (If you're surrounded by gun-wielding Nazis, which would you rather they hear you say?)
- After his plane was shot down over Vietnam, US Navy pilot Jeremiah Denton was taken prisoner by the Viet Cong, who later put on a televised press conference to show how well they were treating enemy captives. Denton repeatedly signaled the word 'TORTURE' in Morse Code by blinking his eyes.
- Similar to the above, when the crew of the USS Pueblo was captured by the North Koreans in international waters, the Koreans distributed a picture of the crew to show that they were well cared for during their stay. Looking closely, several crew members appear to disagree.