Public Secret Message

Alice needs to send a message to Bob without Carol overhearing. Unfortunately for Alice, she can't simply meet with Bob in private to discuss it, and sending a private message is either physically impossible or would arouse too many suspicions. The solution? Send a message to everyone, but encode, encrypt, or word it in a way that only Bob will understand the message. Alice might disguise the message in such a way that Carol will think it's an relatively innocent item, such as an advertisement (Wikipedia lists this technique as "Steganography"), word it so that everyone but Bob will misinterpret the meaning, or simply make the message so hard to decode that only Bob could do it.

Subtrope to Double Meaning. Compare/contrast Sarcastic Confession, Hidden in Plain Sight, Overt Rendezvous.

Examples of Public Secret Message include:

Anime and Manga

• In 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" (on tests).
• In 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 notes won't be able to figure out what they're talking about, it is still far too public for his taste.
• One Piece featured the main character returning 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, realized it wasn't the kind of thing their captain typically did, and noticed a simple message written on a tattoo.
• Dance in the Vampire Bund has an instance where Mina makes a public announcement, speaking in an ancient language as a challenge to another vampire, but with fake subtitles in Japanese.
• In the two-part Detective Conan 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).

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 become 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.

Literature

• 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 the
Talk
Of the
Market!"

• • While this means little to locals, it is likely to draw attention from other time travelers, since the atomic bomb hasn't been invented yet.
• 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 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 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 John Twelve Hawks' (a Pseudonym) 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 when 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.
• The protagonist of Mother Night by Kurt Vonnegut works as a radio propagandist for the Nazis in World War Two, 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.
• Dominic Flandry needed to secretly alert the Terran Empire that the ruler of the Mongol-settled planet Altai was conspiring with the Merseians. He got Terra's attention by "vandalizing" an Altaian monument with very colorful paint spelling out a nonsense word. Well, it was nonsense to the people of Altai: "Mayday."

Live Action Television

• In Time Trax Lambert would send messages to the future by placing coded personal ads in the newspaper.
• Sue Thomas FB 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.
• There's another example of a publicly-published crossword containing a hidden message for a particular person in Oliver's Travels.
• 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.
• 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.
• A Real Life instance (several, actually): 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 the Wrath of Khan); and Noonien Soongh, Data's creator in Star Trek the Next Generation.

Tabletop Games

• Dungeons and 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 for GURPS, 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." And then, in June 2012, an entire game's fanbase facepalmed at the release of Power-Ups 5: Impulse Buys.

Web Comics

• 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.

Web Original

• LIS DEAD is seemingly an Inversion; it's encrypted and secret to nobody except for a certain mysterious 'Him'

Western Animation

• The Simpsons:
  • In "My Mother the Carjacker", Mona Simpson encodes secret messages to her son in the newspaper, in food articles, using the first letter from each word.
  • Homer sent a message to Lisa in the New York Times Crossword Puzzle. Defictionalized by the NY Times running that same puzzle

Real Life

• 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 encryption 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.
  • The Axis used analog computers such as the Enigma Machine to send radio messages they believed were too heavily encoded for the Allies to decipher. This provided incentive for the development of electronic computers by the Allies.
  • Some allied units used "code talkers" - radio announcers who could encode secret messages into a common language such as English, then translate them into a language which was virtually unknown in the Axis forces (typically a Native American language) before the broadcast.
• "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.