Trust Password

"Ted: Okay, wait. If you guys are really us, what number are we thinking of? Future Bill and Ted: 69, dudes! Bill and Ted: Whoa."

- Bill and Ted's Excellent Adventure

Maybe you claim to be from the future, or maybe you say you are some reincarnation. Whatever it may be, Mr. Suspicious isn't buying it. So what do you do? You tell or show him the one thing that absolutely proves your claim beyond a shadow of a doubt.

A subtrope of God Test. Compare Something Only They Would Say and Only the Knowledgable May Pass. May be in Spy Speak if it's a sign/countersign challenge.

Crazy Prepared characters naturally have at least one of these.

Anime and Manga

 * In the fourth Suzumiya Haruhi novel, Kyon finds himself in an alternate universe where the SOS members are ordinary humans. Haruhi won't even give him the time of day until.
 * Previously played straight in Bamboo Leaf Rhapsody: after . Inverted immediately after when
 * Played with multiple times in a different context..
 * Probably worth mentioning
 * And in Disappearance,
 * Yusuke of Yu Yu Hakusho regrets not having one of these when he possesses Kuwabara, and not a single person believes he's actually Yusuke.
 * He ends up identifying himself to Keiko by grabbing her breasts (or, in the edited-for-TV version, peeking up her skirt), which elicits a reflex response of "Yusuke you jerk!" and a slap, at which point she finally accepts that it's him.
 * And that it was unnecessary because she recognizes his body language. Oh well.
 * Played With in Naruto. Sasuke Uchiha, having just defeated an imposter through recognizing him by luck, gives one to both Sakura and Naruto to say. However, this is actually Playing With the trope because he knew that the real Naruto wouldn't be able to remember it. He (correctly) calls out another imposter based on this several minutes later.
 * In a filler episode of Ghost in the Shell: Stand Alone Complex, a terrorist known as "Angel's Wing" ("Angel Feathers" in the English dub) gives one of these to so that she'll know it's him even if she doesn't recognize him. At the end of the episode,
 * In One Piece, after learning that one of the villain's lackeys can duplicate appearances, Zoro comes up with a solution; To distinguish each other, each Straw Hat should wear a white cloth over their arm
 * Subverted when Bon Clay ended up . It was Something They Would Never Say that gave him away.

Comic Books

 * During the The Death of Superman storyline, the real Superman proves himself to Lois with Clark Kent's favorite movie:.
 * In later comics, Superman's codeword to Lois is "Beef bourguignon with ketchup". Though in this case, its a way to let Lois know through an intermediary that he's safe. A farming tradition. Subverted when Parasite poses as Lois and is able to use the password.
 * During the Obsidian Age arc in the Justice League of America, in which the entire League is killed and their souls trapped by an ancient Atlantean sorceress, its revealed that Batman's passcode for Nightwing is "The Daring Young Man on the Flying Trapeze"; a reference of course to Dick Grayson's boyhood career.
 * Also, song lyrics.
 * Inverted by Genre Savvy players in the Knights of the Dinner Table comics. Often, players will set up a 'Doppleganger Password'; should a character become separated from the group and later rejoin, they can be asked to provide the password in order to prove that they're not an infiltrator in disguise.
 * Used to great effect by the Black Hands group. During an annual Hackmaster tournament, Newt asks Stevil for the password and, not getting it, peppers his body with crossbow bolts and dumps his body down a well. All to the good, except that Newt accidentally (or "accidentally") forgot to set up the password with Stevil ahead of time, giving him an excuse to take him out of the game. Nitro doesn't buy the excuse and, when the Black Hands are eliminated, orders Newt to his house for 'remedial training'. And to bring a sack lunch.

Fanfic

 * In the Harry Potter Peggy Sue fic Backwards with Purpose, Harry convinces Dumbledore of the time travel by stating what Dumbledore saw in the Mirror of Erised.
 * Ed and Sam of qntm's Ed Stories both have time travel passwords, for use when they need to prove their identities to themselves. They make use of these passwords in Be Here Now: 4 of 5.
 * Twisted in Futari wa Pretty Cure Blue Moon. Facing down an illusory copy of Yoko controlled by Millusion, Cure Sunday asks her to answer a question about herself. The twist is that giving the right answer proves her as the fake, because Yoko didn't know it.
 * Kyon: Big Damn Hero starts with Kyon using his Trust Password so Haruhi will believe him and help Yuki.
 * In the Bleach fic Hogyoku Ex Machina, Yamamoto gives Ichigo one of these to prepare for when Rukia will be Ret Goned in the future (i.e. the third film).
 * Ichigo convinces Gin that he's from the future by explaining the true power of Gin's bankai, which Gin never revealed to anyone.
 * Winter War: How do you convince someone who's been Aizen's prisoner for months that you really are you?

Film
"M: [buzzing intercom] Miss Moneypenny, give 007 the password we've agreed with Japanese S.I.S. MoneyPenny: Yes sir. [to Bond] MoneyPenny: We tried to think of something that you wouldn't forget. James Bond: Yes? MoneyPenny: "I, love, you". Repeat it please, to make sure you get it James Bond: Don't worry, I get it. [Later...] Tiger Tanaka: Permit me to introduce myself. I am Tanaka. Please call me Tiger. James Bond: If you're Tanaka, then how do you feel about me? Tiger Tanaka: [the code response] I... love you. James Bond: Well, I'm glad we got that out of the way."
 * Bill and Teds Excellent Adventure (see page quote) has the future selves prove that they are who they are by identifying what number the present selves are thinking of.
 * Given their personalities, it's not that hard to guess...
 * Turned on its head in Bill and Teds Bogus Journey when the "Evil Robot Usses" correctly guess how many fingers Ted is going to show them; it's entirely possible he only held up the amount they said because they said it.
 * It's common to use detailed knowledge of future events to prove you're a time traveler. 2015 Biff Tannen demonstrates to his 1955 self that he and his Sports Almanac really are from the future by predicting an extreme long shot football game result in Back to the Future Part II.
 * Although Old Biff makes a point of not telling his younger self who he really is, he demonstrates knowledge that only Young Biff would have, such as how to start his car.
 * More important example: from Part I: Doc will not believe Marty is a teenager from the future until Marty proves it. He only believes Marty when the latter says things he could know only from Doc himself, particularly Doc's accident and vision that later led to the creation of the time machine itself.
 * Similarly, the protagonist of Frequency convinces his father in the past that he really is speaking from the future by detailing a baseball game that hadn't happened yet. Protagonist Sr. buys it when he sees the game happen exactly as described the next day.
 * Terminator and Terminator 2 never confirm this, but it seems likely that John Connor uses "Come with Me If You Want to Live" as a password for "good" Terminators to use when meeting him, a phrase he undoubtedly knows from stories of how his parents met.
 * Used again in The Sarah Connor Chronicles; Cameron uses this as her way of identifying herself to John when she first saves his life.
 * Brutally subverted in Ghost Town. Ricky Gervais' character has started telling the wife of a dead husband (whose ghost he can see) facts that he shouldn't know. So she makes him prove that Ricky's really talking to the husband with a recurring nightmare the husband had that he only told her about.
 * In the first X-Men 1 movie, after being fooled by Mystique one too many times, Cyclops demands that Wolverine prove he is who he says he is. Wolverine's response: "You're a dick." It works.
 * In Cooperstown, a former baseball player claims to be seeing the ghost of his deceased friend. To figure out if he's lying or not, another character asks him several questions that she thinks Harry won't know, but that the ghost would know. As it turns out,
 * Star Trek actually gives an example where the Trust Password doesn't work. Spock Prime greets Kirk with "I have been and always shall be your friend". Except Kirk and Spock aren't friends yet in this timeline, so it kind of falls flat. He finally resorts to using a mindmeld instead.
 * In 12 Monkeys, Cole can't figure out if he's actually from the future or if he's just crazy. He figures it out when
 * From You Only Live Twice:

"Mike: It's me, Mike O'Donnell, your best friend! You have an undescended testicle. Ned: Googleable! Mike: You helped me cheat on a math test, but I got caught. Ned: Public records! Mike: You asked Princess Leia to Junior Prom! Ned: Covered by the local news!"
 * Big has the most complicated example, with a fast sort of song and dance routine.
 * Subverted in 17 Again. Mike tells his best friend Ned several incriminating details that only he could know while being attacked, but Ned justifies how the weird, soaking wet man in his house could get access to all the information he gives.


 * In one of the The Bourne Series films, it's shown that all Treadstone agents have "Safe" and "Under Duress" passwords.
 * Groundhog Day takes it to extremes. To prove that he really is repeating the day over and over again, Phil walks Rita through a diner, giving the trust passwords for everyone in the diner, none of whom remember telling him those things. She still doesn't quite believe him until he shows he can also predict what people are about to say.
 * Averted in one or more of the Pink Panther movies. Clouseau asks Dreyfus what his code name is, or what the pass word is. Dreyfus has to think they replies in a fury that he doesn't have a codename or there is no password. Clouseau confirms that only the real Dreyfus would know that, causing Dreyfus to become even more angry.
 * In Airheads, to determine whether a record executive actually is an exec, not an undercover cop, Chazz asks him "Whose side did you take in the Van Halen / Roth split: Van Halen or Roth?". When he answers "Van Halen" instead of "Roth", he's concluded to be a cop, but Chazz gives him one more chance, asking "Who would win a wrestling match: Lemmy or God?". As he first answers "Lemmy" before answering "God", Rex tells him "Wrong, dickhead! Trick question! Lemmy IS God!", and he's finally driven away.

Music

 * There's a country rock song where a man sings about a letter he would to his younger self if it was possible. His trust password to himself is to mention the porn magazine he has under his bed. Supposedly, he never told anyone about at when he was that age.

Literature
""I was thinking of Aurora..." Griffin's heart sank. "But your mother had her heart set on Celeste, so Celeste it was going to be.""
 * In The Time Traveler's Wife, Henry ultimately convinces many people of his ability to spontaneously time travel by disappearing in front of their eyes. On one occasion he tells a doctor all the biological information on the doctor's then-unborn child in the hopes of getting some treatment, but it's the disappearing that clinches the deal.
 * Jorge Luis Borges' short story The Other has a clever version of this trope. In it, an older Borges meets his younger self on a bench by a river. The older tells the younger details about their life that no one else could know. Young Borges dismisses this because he could be dreaming. His dream self would of course know all the details of his own life. A few minutes later, Old Borges proves that it is not a dream by reciting a line of French poetry he is sure his younger self has never heard nor could have dreamed up, and showing him a piece of money with a recent date on it. He later realizes that the note he showed his younger self doesn't actually have a date on it -- meaning that the younger Borges did in fact dream it, but the older one did not.
 * In Artemis Fowl, Artemis sets up not one, but two of these in case his memory ever gets wiped by the Fairies. The password for him is a video of himself saying what happened, as Artemis only would believe something he said. Butler's is
 * In the last two Harry Potter books, the Ministry of Magic suggests people set up a Trust Password with their loved ones so as to identify someone disguised with Polyjuice Potion. It's generally treated as a joke in Half Blood Prince, but becomes deadly serious in Deathly Hallows. (Not only are circumstances more serious, but people have figured out that it's best to make the Q&A spontaneous.)
 * In Firewing, when Griffin finally meets his father Shade, he doesn't believe it's him (due to the many illusions of the underworld). He first asks him about Shade's past adventures, but Shade gets the answers wrong because the stories Griffin believes are exaggerations of the true events. Griffin then asks him what his name would have been, would he have been female.


 * (Not an exact quote, just the best that I could type from memory.)
 * In Marge Piercy's He, She & It, the heroine suspects the VR representation of her husband in traction after a probably fatal incident. She asks him a personal question, and his answer is the lie that she and her real husband put in a government file for just such a test.
 * In ''Mara Daughter of the Nile": Sheftu gives Nekonkh a Trust Password for Mara: "Tell her I have not forgotten what I said last night when I took her in my arms." Nekonkh is repulsed by Sheftu's cold-bloodedness, as he is to use the Trust Password as part of Mara's loyalty test.
 * Played with in The Hourglass Door trilogy (As it's full of time travel). In the second book, Leo tells V to go back in time and tell his past self that, "the lady of light" sent him, and that it is time, "to honor his vow," so that Past Leo won't kill Future V. In the third book, main character Abby goes back in time and saves Leo, who calls her his lady of light, and she asks him to promise to do something for the one who asks him to honor the vow. It gets more confusing from there.
 * In The Science of Discworld II after traveling back in time most of the wizards make a point of saying something to their past selves to prove that they are time travelers rather than doubles created by the elves. The exception is Rincewind, who is so jaded by everything that's happened to him in his very eventful life that he just walks up to his past self and says hi.
 * In H. Beam Piper's Four-Day Planet, there's a moment when the narrator remarks that if, when his father dies, a medium claims to produce a spirit message from him, he won't accept it as genuine without Dad's Catch Phrase, "Now wait a minute; it's here somewhere." This isn't any kind of Foreshadowing, just a bit of humor about his father's way of misplacing things and rummaging around looking for them.

Live Action TV
"Amy: Trust me. The Doctor: Okay. Amy: You have to do this. And you can't ask why. The Doctor: Are you being threatened? Is someone making you say that? Amy: No. The Doctor: You're lying. Amy: I'm not lying. The Doctor: Swear to me. Swear to me on something that matters. Beat Amy: Fish fingers and custard. The Doctor: My life in your hands. Amelia Pond."
 * In the classic Battlestar Galactica Classic series, when Starbuck escapes from the Cylons by stealing one of their ships, he proves it is him to the Galactica by "waggling" the ship back and forth (since the ID transmitter he was given was damaged).
 * In the Series Four Doctor Who episode "Forest of the Dead," Professor Song  to prove that she knows him in the future.
 * Subverted in later episodes where it is implied that that she might not be someone he ought to trust after all.
 * And in the season finale.
 * "She told me to warn you. She said two words: . Well, what does it mean?"
 * And from the series six premiere:

"Daniel: Repeat after me: He's standing right beside me. Nick: He is standing right beside me. Jack: Well, he's lost a few pounds... Daniel: Jack, don't be an ass. Nick: Jack, don't be an ass. Jack: ...Daniel?"
 * In Lost's "The Constant," 2004-Daniel tells Desmond to tell 1996-Daniel that
 * In an episode of Stargate SG-1, the team accidentally travels back in time to 1969. Since this was a Stable Time Loop, General Hammond knew to send them with a sealed note explaining things to his rookie past-self. When he finds it, Lieutenant Hammond believes it and helps them escape imprisonment (and remembers to write the note "again" 30 years later.)
 * Also in the same incident, O'Neill convinces Lieutenant Hammond the team know him in the future by telling him he watched the first moon landing two weeks ago from his father's bedside after a heart attack.
 * In another episode, the team finds a crystal skull inside a chamber filled with radiation and their attempt to examine it results in Daniel being made invisible and immaterial. The rest of the cast has no idea what happened to him until it is revealed that Daniel's grandfather Nick (who had once seen a similar skull and was living in a mental institution) can see him. Jack is skeptical, but is convinced by this exchange:

"Dean: Rhonda Hurley. We were, uh...19. She made us try on her panties. They were pink. And satiny. And you know what? We kind of liked it."
 * In Stargate: Continuum, the trope is subverted twice: O'Neill refuses to believe the alternate timeline SG-1 (partly because what Daniel tries to use—his son's accidental death—didn't happen in the alternate timeline), while Landry cuts them off as they are about to do this by telling them he believes them (having previously seen the tapes of their interrogations).
 * In an alternate future of Star Trek: Enterprise, the now elderly Captain Jonathan Archer is being cared for by his... caretaker... T'Pol. However, an injury prevents him from remembering all that's happened since he was last on the Enterprise. T'Pol says to him that she fully understands that he might consider all this to be an illusion or an elaborate deception. To alleviate his fears she tells him the story of an old girlfriend he wanted to marry back on Earth. The stunned Archer wants to know just what kind of 'relationship' he and T'Pol have that he'd ever tell her the story. She'll only say their relationship has "evolved".
 * In the Star Trek: The Original Series episode "Whom Gods Destroy", a shapeshifting madman assumes Kirk's appearance and tells Scotty to beam him back aboard. Much to his chagrin, Scotty challenges him with "Queen to queen's level three, sir"—which only Kirk and Spock knew the appropriate countersign for.
 * Time Travel was the whole point of Seven Days, and the team set up a code phrase for the protagonist to use so that they'd always know when he had shown up from the future, and they weren't dealing with a hoax. The password used is Frank contacting Backstep and identifying himself as 'Conundrum'.
 * This password is only useful for the first episode, when the team has no idea who he is. After that, he's really just using 'Conundrum' as a codename and to get put through to the right people, because they a) all know Frank, and b) notice when their giant time-traveling sphere vanishes from the hanger.
 * In one episode of Lois and Clark: The New Adventures Of Superman, a time-traveller is able to enlist Clark's aid by whispering "I know you're Superman, and I need your help", to him.
 * In the Charmed episode "That 70's Episode", Prue and Piper convince their past selves to trust them by opening a trick drawer in a cabinet.
 * In the Supernatural episode "The End", Dean is sent to the future and proves his identity to Future!Dean by telling a story that only he would know:

"Giles: Cordelia, it's me! It's me! Cordelia: How do we know it's really you and not zombie Giles? Giles: Cordelia, do stop being tiresome. Cordelia: It's him."
 * In an episode of Buck Rogers in the 25th Century, Buck hijacks a Draconian fighter and must prove to another pilot that he's from Earth. The pilot is someone Buck had previously been stranded in the desert with and had shared stories of the past. In this case, Buck proved who he was by giving a description of OJ Simpson. Since Buck had left Earth in 1987, he had no idea that OJ turned out to be a double murderer and since the show was filmed prior to this, neither did the writers.
 * And in Buffy the Vampire Slayer, this classic exchange. Makes even more sense when you realize the above X-Men "You're a dick" line was also penned by Joss Whedon.

""Paladin." Defender of the Faith."
 * In another episode, Xander does the Snoopy Dance to prove he's Xander for Willow.
 * He also tells a story about one of his early birthdays where he didn't get a firetruck and was sad, but then the neighbours house caught fire so real trucks came and he always thought Willow might have set the fire "and if you did you can tell me". Played with in that he's doing all this to prove that he's not his Evil Twin who the rest of the Scoobies have gone off to fight. A) His twin would know all this stuff anyway, given that Xander had been duplicated and B) Willow wasn't even aware there was a twin so she had no reason to doubt he was Xander. She just let him talk because he didn't give her the option not to.
 * In one episode of Bones, a boy is kidnapped. Booth asks the dad what password he can use so the boy will trust him during the rescue. (It's Paladin, and the rescue scene is awesome and heartwarming.)
 * And the appropriateness of this trust password for Agent Booth, a Roman Catholic FBI agent, is lampshaded by his boss:


 * An episode of Star Trek: The Next Generation had Worf visited by a Klingon who first claims to be an advisor, and then later admits that he is Worf's now-grown son Alexander, traveling back in time to urge current!Alexander into becoming a warrior. To prove his identity, Worf asks future!Alexander to repeat his mother's last words. He gives a detailed account.
 * In The West Wing, the word "Sagittarius" is used to denote that person knows about Jed Bartlet's MS.
 * In the series 2 premiere of Misfits, the gang have to deal with a shapeshifter. In order to make sure it's really them, they greet each other with "Monkeyslut!"
 * An unintentional variation occurs in Farscape. John is having a normal conversation with Aeryn until he mentions and she doesn't know what he's talking about. This gives away that she is a
 * In the Smallville episode "Transference", Clark Kent and Lionel Luthor swap bodies. In Lionel's body, Clark convinces Ma Kent it's really him by telling the story of how he discovered Super Speed at age 6.
 * Subverted in "Apocalypse". Clark tries to prove he's a friend to Chloe by revealing some of their past experiences, but since he's in an alternate universe where they have never met, it fails.

Video Games

 * In Escape from Monkey Island, Guybrush meets his future self from Twenty Minutes Into the Future (Or you meet your past self from Twenty Minutes in the Past). Either one asks "If you're really me, then what number am I thinking of?".
 * Prince of Persia: Sands of Time proves his story to the princess (events that never happened due to the events in the game) by telling her a word invented by her mother, Kakolukia.
 * In Betrayal at Krondor, James sends Gorath, a dark elf, to warn Prince Arutha of an attack, and tells him to use the phrase "There's a party at Mother's" to convince Arutha that the message is indeed from James, as it's a phrase they used years earlier in their adventures together and Something Only They Would Say.
 * A non-verbal example in the game Geist. Since Raimi has the ability to Body Surf, he has to prove his identity to Raimi by reproducing their Secret Handshake.
 * Near the ending of Planescape: Torment,  The change from "paranoid psychopath" to "scared puppy" is heartbreaking.
 * Early on in Final Fantasy VIII the Player Party is sent on a mission to aide an anti government group called "The Forest Owls" and are given a Password to confirm their identity. Upon reaching the rendezvous point and saying the password to the group's representative (regardless of whether or not you gave the correct response), he takes you to meet the other members.

Web Comics

 * Bob and George hangs a lampshade on Bill and Ted when time travel becomes a regular occurrence in the comic; of course, since Mega Man and Bass are legendarily stupid, Hilarity Ensues.
 * In Home On The Strange, Tanner sets one of these up with Izzy, to the latter's bemusement.
 * During their adventures in the Punyverse in Sluggy Freelance, Bun-Bun cons Lord Grater into believing he works for his boss, Zorgon Gola. Lord Grater responds "if you know everything about me, what am I thinking about right now?" Bun-bun responds that the infomation is classified, which instantly appeases him.
 * another time in "Oceans Unmoving". (Big spoiler alert.)
 * Panthera uses this. When Onca tries to tell Tigris that  However, when Pardus shows up, she tells him that Leo told her   Pardus knows Leo wouldn't tell her unless he had to to validate a message to the others.
 * Double-subverted in The Inexplicable Adventures of Bob. In the dragon Hibachi's first appearance, he claims to be a friend of Mrs. Primrose who'd been sent to pick up the box she had left with Bob. Bob is understandably suspicious, and demands the seemingly-useless Trust Password of her favorite color. Turns out it's such an obscure color (a specific discontinued shade of paint), and has such an involved story attached to it that it really would be impossible for anyone who didn't know her to guess.
 * In Homestuck, Jade implements a system of these, thought it's less about trust and more about trying to keep her conversations relatively linear since the Trolls tend to skip around a lot regarding time shenanigans.

Web Original

 * In Red vs.Blue: Reconstruction Agent Washington verifies that he is from Command to the Reds by knowing Sarges ultimate secret Password.

Western Animation

 * Aladdin, with "Do you trust me?", which works both when he's a street rat and a prince, which also starts to show that they're the same person.
 * This was partly accidental - Aladdin didn't intend to reveal his true identity to Jasmine, but she came to suspect they were one and the same,
 * In the Justice League episode where Flash and Lex Luthor switch minds, Flash proves he's really himself again by starting to reveal Green Lantern's old nickname.
 * In Ben 10 Alien Force, Past!Gwen demands Future!Gwen say something only they/she would know. Future!Gwen whispers something to her, prompting a disgusted reaction. We never find out what it was. Dwayne MacDuffie refused to comment, saying it was "personal."
 * In the My Little Pony Friendship Is Magic two-parter A Canterlot Wedding, Twilight and Cadance have a rhyme from way back when.

Truth in Television

 * According to tradition, Joan of Arc whispered some secret to the Dauphin to convince him that she had really been divinely inspired to help him claim the throne. According to additional tradition, what she whispers is that God wants him to be king of France, though some prefer to believe that she described an embarrassing birthmark on his backside.
 * Magician Harry Houdini had spent much of his later career debunking mediums and others who claimed to speak "from beyond the grave". He arranged a number of code phrases (one being a song called "Rosabelle") as identifiers for his wife Bess if such communication was possible. He died in 1926, well before Bess. No one was ever able to deliver a message she was satisfied was genuine.
 * Funny how "paranormal experts" never bring that up.
 * They must have a good reason...
 * Used well in Carrie Vaughn's Kitty Norville series, where a medium contacts Houdini, and is told "Everyone who knew my codes is long dead. Stop trying."
 * Don't forget about that password you have to tell the computer to make sure it's really you.
 * Careful parents often arrange these with their children, so that the children can verify that anyone who claims to be sent by mom and dad actually was (as in the Bones example above). Some PSAs (or similar productions) recommend this if the family ever gets separated.
 * Surprisingly more common than most people think. It's extremely easy to set up, and it never hurts to be Crazy Prepared or Genre Savvy.
 * There was a website many years ago (it's long since dead) that was touted as a "time-traveler's support network." Meeting places and times in various cities were designated where a "volunteer" from the project would wait for someone to say a one-time use passphrase and render aid. The database of locations, times, and phrases was said to exist in a sealed document held by a law firm which would be turned over to any time-travel project in the future.
 * It may be long since dead now, but if you're going back in time that shouldn't really be a problem.
 * As in the Bourne example above, secret agents normally have security checks they can insert into a message to verify that it's real. During World War II, one British agent captured by the Germans deliberately gave his captors the wrong security check. He expected his bosses to realize that the messages coming from his radio were false. His bosses didn't pay attention. Das Englandspiel (also called Operation North Pole) resulted in the capture and execution of approximately fifty Allied agents, and didn't end until the Germans themselves called it off in a clear-text message to London..
 * Shibboleths are words whose pronunciation is so unique that only native or extremely fluent speakers can pronounce them correctly.