Ted: Okay, wait. If you guys are really us, what number are we thinking of?A character needs to prove his identity or situation to another character who's not likely to believe him. Maybe he's from the future, or he's some other character reincarnated. Maybe he has to pass a God Test. He might even have to prove his identity to his past or future self. That's when he breaks out the Trust Password — something that will prove his identity beyond a shadow of a doubt. Trust Passwords come in several forms. Very often, it will be Something Only They Would Say. It can be set up in advance as a Covert Distress Code or a form of Spy Speak (often in sign/countersign form). It could be specialized knowledge of certain facts or events. Or it could be an object that has a special significance (such as an object that's been separated in half, so if you have one half only someone you trust has the other). The trope can be Played for Laughs quite easily — the skeptical character might not believe an otherwise airtight password, or the password might be common knowledge such that anyone can easily impersonate the character. And it's not related to Safe Word. (Usually.)
Future Bill and Ted: 69, dudes!
Bill and Ted: Whoa.
Future Bill and Ted: 69, dudes!
Bill and Ted: Whoa.
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- A Tim Horton's commercial had two people crossing the border into Canada proving that they're true Canadians by showing that they know how to correctly pronounce the chain's big yearly promotion: "R-R-Roll up the R-R-Rim to Win!" The next two in line can't do it.
Anime & Manga
- This occurs frequently in the Haruhi Suzumiya series:
- In Bamboo Leaf Rhapsody, Kyon gives Yuki the piece of paper that she gives him in three years' time as he travels through time himself. This convinces her to synchronize her memories with her future self, effectively merging her current and future selves into the same person. And then she returns the favor immediately afterward, taking off her glasses the same way she does three years later.
- In the fourth light 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 in desperation, he introduces himself as John Smith, the alias he used as a time traveler helping him out three years ago. After he does that and the world is returned to "normal", he keeps this as an emergency trump card and finds himself using it to blackmail Yuki's boss.
- Adult Mikuru convinces Kyon of her identity by showing him a distinctive mole on her breast — except Kyon didn't know about that yet, even though Mikuru claims he pointed it out to her. Kyon uses this information to point this fact out to the younger Mikuru, turning the whole thing into a Stable Time Loop. It pops up later, too — he recognizes a fake Mikuru because she doesn't have the mole, and he tries it again in Disappearance on an alternate universe Mikuru, who calls him a pervert and punches him out.
- In YuYu Hakusho, Yusuke possesses Kuwabara to try and tell Keiko that he's trying to come back to life and needs his body. She doesn't believe him, and he tries to come up with a Trust Password. So he gropes her breasts (or in the TV version, peeks up her skirt), causing her to reflexively slap him and yell "Yusuke, you jerk!" And in the end, she tells him that was unnecessary after all — she recognized his body language.
- Naruto plays with the trope. Sasuke Uchiha, having just defeated an impostor by recognizing him through sheer luck, has enough of a scare to give Trust Passwords to Sakura and Naruto. But in Naruto's case, the password is so complicated that he knows Naruto will never remember it — so when an impostor impersonating Naruto shows up and correctly recites the password, he knows it's a fake.
- Ghost in the Shell: Stand Alone Complex:
- A terrorist known as "Angel's Wing" (or "Angel Feathers" in the English dub) gives a Trust Password to his blind, wheelchair-bound daughter so that she'll know it's him even if she doesn't recognize it. At the end of the episode, he finds himself captured in a chapel, when his daughter happens to enter. She gives her half of the password ("What is the angel planning to do today?") Batou knows both halves of the password, and he could give the right response (if only to make her happy), but he doesn't — he tells her, "The angel... isn't planning to go anywhere."
- One of the final episodes in 2nd Gig has the Big Bad trying to send Rangers after Section 9. They immediately stand down after running into Batou, a former Ranger; they can tell this immediately because all Rangers have the exact same Electronic Eyes.
- One Piece:
- After learning that one of the villain's lackeys can duplicate appearances, Zoro comes up with a solution: each Straw Hat should wear a white cloth over their arm to hide the true trust sign, a large X written on their arm.
- In the Dressrosa arc, Luffy is reunited with someone he had thought long dead. Said individual starts asking about the time they stole Dadan's liquor to confirm his identity, but Luffy is glomping him in joy before he can finish the sentence.
- All You Need Is Kill has a variant. Rita is part of a controlled time loop; after every battle, she is sent back in time one day to improve the outcome the next time around. Every battle, she asks everyone she can the same weird question (a different one every battle), like "Is it true that in Japan green tea is complimentary after a meal?" That way, if another looper ever shows up, they can prove their identity simply by answering the question before the battle even starts.
- In Steins;Gate, Makise Kurisu provides Okabe a trust password ("My Fork") to convince her past self that he came from future using technology she finished building a few minutes ago. Humorously, the password she uses is a very old 2ch meme roughly meaning "lover", so past-Kurisu is horrified when Okabe uses it (fortunately for her, Okabe doesn't seem to recognize the meme).
Okabe: What you want most right now is "My Fork"!
Okabe: You already have "My Spoon", apparently.
Kurisu: I am going to kill my future self five hours from now...
- In Mushoku Tensei future Rudeus convinces Rudeus of his identity, and thus to trust his warnings, by saying something only he could know: their Japanese name.
- In The Seven Deadly Sins, Gilthunder's supposed Badass Boast turns out to be a trust password. It's a phrase learned from Meliodas, to be used if they're ever in deep trouble.
Gilthunder: I am now stronger than any of the Seven Deadly Sins!
- From Superman comics:
- In The Death of Superman, the real Superman proves his identity to Lois by naming Clark Kent's favorite movie, To Kill a Mockingbird.
- In later comics, Superman sets up a code phrase with Lois: "Beef bourguignon with ketchup" (a farming tradition). He uses it to let Lois know that he's safe even when he can't contact her directly. Unfortunately, Parasite is also able to use the password when posing as Lois.
- Justice League of America: During the Obsidian Age arc, in which the entire League is killed and their souls trapped by an ancient Atlantean sorceress, it's revealed that Batman's passcode for Nightwing is "The Daring Young Man on the Flying Trapeze"; not only are they song lyrics, but they also refer to Dick Grayson's boyhood career.
- Mortadelo y Filemón have an arbitrarily catalogued number of trust passwords. A Running Gag is for the password to be very offensive to some group or collective that happens to be within earshot. A beating ensues.
- One particularly extreme password was reciting Don Quixote in its entirety. Filemón tries unsuccessfully to convince the guard on the other side of the door to let him skip the password for once. He has a Photographic Memory and knows it by heart, but by the time he's finished reciting it, he's grown a Time Passage Beard. And then the door collapses, and the guard turns out to have died of old age.
- In Paper Girls, 2016!Erin asks several very personal questions in a row to verify that 1988!Erin is who she claims to be. Subverted in that 1988!Erin doesn't know the answer to the first one (her social security number), and the last one isn't a Trust Password at all, it's a genuine question that 2016!Erin has always wondered about.
- Knights of the Dinner Table
- Used by Genre Savvy players in the comics. Often, players will set up a "Doppelganger 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.
- It's 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.
- 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 this 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 turn Ret Gone in the future (i.e. the third film). It also ends up being useful for identification against Aizen's illusions, though Ichigo still has to scramble as he struggles to remember it.
- 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? You hand him his zanpakutou, which will do the convincing for you.
- In scifigrl47's Toasterverse, Tony has one of the escaped scientists sing the Teapot Song to verify his hand in their escape.
- In the Hard Reset sequel Hard Reset 2: Reset Harder, when Celestia hears that Twilight is a fellow time looper, she tells Twilight a password to unlock secret knowledge in Celestia's brain. When Twilight repeats the password to Celestia later on, she's paralyzed by a powerful spell, and Celestia immediately kills her after revealing that the "secret knowledge" is the rest of the password. It's repeating the entire combination that convinces Celestia of Twilight's story.
- In PnF: Stolen Identity, Phineas does an impromptu one on Ferb's replacement, asking him about the day they first met. It's different from the typical example in that the first time he asks this question, he doesn't realize he's talking to an impostor (rather he thinks Ferb's gone mad) and wants reassurance that Ferb is still the person he loves. Later the real Ferb answers the question correctly (Phineas insisted on saving Ferb some cake), much to Phineas' obvious relief.
- In the Unacceptable Sitch Series (specifically "Under the Milky Way Tonight"), Señor Senior, Sr. gave his attorneys a list of passwords to prove he was of sound mind and free of coercion. Specifically, any future changes to his will would include all but one of them.
- Played for laughs in The Wrong Reflection: Tess randomly demands of Eleya at gunpoint why Tess joined Starfleet, to make sure Dal Kanril hasn't tried to replace her. After going "WTF?" Eleya answers, and then asks Tess if she really thought Eleya wasn't herself.
- In A Different Halloween, if Lily Potter uses the word "darling" at the start of a phone conversation with James, it means that she's safe and coercion-free.
- Diaries of a Madman: Navarone, when dealing with shapeshifters alongside his adopted daughter Taya, gives her a few of these for verification. He also has his true name, which ostensibly isn't accurately recorded in his journals, replaced instead with "Anonymous".
- The RWBY fanfic Four Deadly Secrets has Neo and Ruby exchange one, in the form of sign/counter-sign when they first meet.
Neo: So breathe easy, sister.
Ruby: Free air is easy to breathe, sister.
- The Bridge: Gigan is unsure if the being he's talking to is Monster X or Kaizer Ghidorah and asks him to tell him two things only his best friend would know. X proves himself by talking about how the two of them stopped the radio incident with the Kilaaks and what Gigan did the moment they first met. And X calls Gigan his best friend, while Kaizer would never do that because he Hates Everyone Equally.
- Stars From Home:
Charles: You have a birthmark on the underside of your left breast and freckles on your back. And scars from two bullets, and an otherwise… flawlessly beautiful body.
- When Hank's cure first works, Scott assumes the not-blue-and-furry Hank is a imposter. Hank tries twice before managing to convince Scott of who he is — because he knows why Scott doesn't like The Andy Griffith Show.
- A few chapters later, Charles has to prove his identity to Ruth when he uses Hank's serum to walk. His initial attempt at using personal details does not go well.
Ruth: Charles Xavier is not the only man I have screwed. This is not private information.
- In the One Piece Self-Insert Fic This Bites!, after the initial encounter with Mr. 2, Cross forms one of these with the rest of the crew, and makes it twofold for extra security. It's utilized in Chapter 21:
- In Avenging Harry Potter Harry holds a phone conversation with a worried Hermione shortly after being rescued from captivity.
Hermione: If I solemnly swear that I'm up to no good how do we end?
Harry: We've managed the mischief.
- In First Knight, when traveling through the screwed up space-time continuum in the Overlook Hotel, guardians/caretakers like Xander formally greet alternates of people they've met to check if they're real and if they're hostile. When an alternate Warren comes across Xander's group in the stairwell, he refers to Xander as a guardian and only relaxes after Xander explains he's just the caretaker.
- Harry Potter and the Methods of Rationality uses this a few times, most notably in the first few chapters. McGonnagall is flabbergasted when Harry first mentions that he actually has one, just in case of an unforeseeable eventuality when he'd need to use it.
Films — Animation
Films — Live-Action
- Bill & Ted:
- Bill & Ted's Excellent Adventure has the protagonists' future selves prove their identities thusly:
Ted: "If you guys are really us, what number are we thinking of?"
Future Bill and Ted: "69, dudes!"
Bill and Ted: "Whoa..."
[quadruple air guitar]
- In Bill & Ted's Bogus Journey, Ted tries this on the boys' "evil robot usses" by giving them a How Many Fingers? test. They pass, although since Ted was hoping for them to pass, he may just have flashed whatever number of fingers they said he was holding
- Bill & Ted's Excellent Adventure has the protagonists' future selves prove their identities thusly:
- Back to the Future:
- In the first film, Marty tries to convince 1955 Doc that he's from the future and he needs Doc's help. He first tries to reveal "future" history, which Doc laughs off because he refuses to believe that Hollywood actor Ronald Reagan is really President. Marty successfully gets Doc to believe him by telling him something only Doc would know — the vision that led to his invention of the time machine.
- In Back to the Future Part II, 2015 Biff doesn't actually want his 1955 self to recognize him, but he does get his past self to trust him and take the Sports Almanac by demonstrating knowledge only young Biff would have, such as how to start his car.
- In Frequency, the protagonist convinces his father in the past that he's really from the future by detailing a baseball game that hasn't happen yet. When the father sees the game the next day happen exactly as described, he believes.
- 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. He says drowning, but it turns out that's way off, and the husband was purposely lying to Ricky so that he would stay away from the wife.
- In Edge of Tomorrow, Rita devises one for Cage when he first convinces her that he can time travel, allowing him to quickly convince past her in subsequent loops. She also decides to reveal her middle name "Rose" to him, probably to make it even easier for him in subsequent "runs".
- X-Men Film Series:
- X-Men: After being fooled by Mystique one too many times, Cyclops demands that Wolverine prove he is who he says he is, under threat of being eye-beamed. Wolverine's response: "You're a dick." It works.
- X-Men: Days of Future Past: Logan convinces 1973 Charles to hear him out by telling him some details about his childhood that he learned from future Charles and that the younger Xavier hadn't told anyone up until this point in time:
Logan: I know your powers came when you were nine. I know you thought you were going crazy when it started, all the voices in your head, and it wasn't until you were twelve that you realized all the voices were in everyone else's head.
- 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, Harry was seeing the ghost of his friend. The questions proved it.
- Star Trek gives an example where this doesn't work. Spock Prime greets Kirk with the line, "I have been and always shall be your friend". Except that event happens so far into the future that Kirk and Spock aren't even friends yet in this timeline, so Kirk doesn't get it. Spock 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 he tells Kathryn to make a phone call for him to a phone he knows will be monitored in the future. Kathryn leaves the message Cole had been told about in the future, which confirms he's from the future.
- In You Only Live Twice, Miss Moneypenny abuses her role as creator of Bond's trust password to get him to tell her something she really wanted to hear from him. It doesn't work and just makes everything kind of awkward:
M: (buzzing intercom) Miss Moneypenny, give 007 the password we've agreed with Japanese S.I.S.
Moneypenny: Yes sir. (to Bond) We tried to think of something that you wouldn't forget.
Moneypenny: "I, love, you." Repeat it please, to make sure you get it.
James Bond: Don't worry, I get it.
Tiger Tanaka: Permit me to introduce myself. I am Tanaka. Please call me Tiger.
Bond: If you're Tanaka, then how do you feel about me?
Tanaka: I... love you.
Bond: Well, I'm glad we got that out of the way.
- The most prominent trust password is extremely complicated, with a fast song and dance routine.
- When Josh calls his mother, pretending he's a kidnapper, his mother asks him to name the song she used to sing to him when he was younger. Josh tries to dodge the question, but when she insists, he sheepishly sings the song, the title theme of The Way We Were.
- Rob Schneider's character in The Hot Chick (really Rachel McAdams' character's brain in his body thanks to a "Freaky Friday" Flip) recites a handclapping game about how All Men Are Perverts (with Subverted Rhyme Every Occasion) to prove who he really is to his best friend.
- 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 The Bourne Ultimatum films, it's shown that all Treadstone agents have "Safe" and "Under Duress" passwords. Nicky Parsons is expected to give her "Under Duress" password when Bourne tracks her down; she doesn't, signaling her defection to Bourne's side.
- 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.
- Parodied in several The Pink Panther movies. Clouseau will ask Dreyfus what his code name is, or what the password is. Dreyfus has to think, then replies in a fury that he doesn't have a codename or that there is no password. Clouseau confirms that only the real Dreyfus would know that, but this just angers Dreyfus further.
- In Airheads, to determine whether a record executive actually is an exec and not an undercover cop, Chazz asks him, "Whose side did you take in the Van Halen/Roth split: Van Halen or Roth?". He incorrectly answers "Van Halen", outing him as a cop. Chazz still gives him one more chance, asking "Who would win a wrestling match: Lemmy or God?". He first answers "Lemmy", then hastily switches to "God", before Rex tells him, "Wrong, dickhead! Trick question! Lemmy is God!", and he's finally driven away.
- In Terminator Genisys, when Sarah encounters Future John Connor, she suspects him to be a shapeshifted terminator. Connor gains her trust by citing details of the past that only John Connor would know. Subverted, it turns out he's a Terminator-human hybrid made from Connor's body.
- In Ghost, Patrick Swayze's deceased character is trying to convince his girlfriend that a medium can speak for him. He tries to tell her that he loves her, but this backfires, since he was reluctant to say those words in life. He quickly corrects himself and tells the medium to say "ditto", which is how he would usually respond to his girlfriend telling him that she loves him.
- In The Sixth Sense, Lynn Sear accepts that Cole is telling her the truth — he really can see and talk to dead people — after this exchange:
Cole: [My grandmother] wanted me to tell you...
Lynn: Cole, please stop...
Cole: She wanted me to tell you she saw you dance. She said, when you were little, you and her had a fight, right before your dance recital. You thought she didn't come see you dance. She did. She hid in the back so you wouldn't see. She said you were like an angel. She said you came to the place where they buried her. Asked her a question. She said the answer is: "Every day." What did you ask?
Lynn: Do... do I make her proud?
- Ministry of Fear offers a Spy Speak example. The whole story unfolds after Stephen accidentally utters the code phrase, telling the palm reader to ignore the past and tell him the future. She then tells him the weight of the cake which is a prize in a weight-guessing game. Cue a long chase as Nazi spies try to get the MacGuffin hidden inside the cake that Stephen won.
- At the start of The A-Team, Hannibal has just carjacked B.A.'s van, notices B.A.'s Ranger tattoo, and recites the Ranger Creed to get him to help.
- The Assignment is about a US naval officer who has an uncanny resemblance to Carlos the Jackal, so he's recruited in a CIA/Mossad scheme to discredit the terrorist. Another terrorist who knows the real Carlos accidentally runs into this doppelganger at Heathrow Airport. The protagonist tries to bluff his way out by pretending to be Carlos, but when the terrorist responds, "I need to buy a newspaper", the protagonist realizes too late that it's a password to which he doesn't know the countersign — his life is only saved by the intervention of a Mossad agent who gets killed in the process. Afterwards his CIA handler mentions a similar incident where he was forced to kill a man who didn't respond with the correct countersign, and later uses this story to tell the difference between the protagonist and the real Carlos.
- Captain America: Civil War: Bucky proves that he's back to his old self after being re-brainwashed and causing an international incident by remembering that pre-serum Steve used to stuff newspapers in his shoes; he's the only living person other than Steve who would know that, and he wouldn't remember it if he were still under the effects of the kind of brainwashing HYDRA uses.
- Kingsman: The Secret Service: After Eggsy's father's Heroic Sacrifice, Harry Hart pays a visit to the deceased's family and gives Eggsy a medallion with a number they can call to request a "favor", along with a choice of words they have to say to the speaker.
"Oxfords not brogues."
- T2 Trainspotting: Played straight with Renton and Sick Boy's improvised sectarian song; subverted with "1690" as the widely-used and easily-guessable bank card PIN.
- In the final scene of Sharknado 5: Global Swarming, Fin Shepard runs into a grown man who claims to be the Future Badass version of his son Little Gil. Fin is understandably skeptical, until the man says, "Semper Paratus. A Shepard is always prepared."
- 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. 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 as a dream, but Old Borges proves That Was 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 trust passwords for himself and Butler in case the Fairies wipe their memories. Artemis' own password is a video of himself saying what happened, as Artemis would only believe something he said himself. Butler's is Artemis telling him his first name: Domovoi.
- In the last two Harry Potter books, the Ministry of Magic suggests people set up a 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 (such as Arthur accidentally outing his secret Affectionate Nickname for Molly), but becomes deadly serious in Deathly Hallows, as the circumstances are much more dire, and people will turn on their loved ones if they don't answer correctly immediately.
- 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, had he been female.
"Well, I'd wanted to call you Aurora..."
Griffin's heart sank.
"But your mother had her heart set on Celeste. So it was going to be Celeste."
- In Marge Piercy's He She And It, the heroine suspects the VR representation of her husband in traction after a probably fatal incident is not real. 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 password as part of Mara's loyalty test.
- Played with in The Hourglass Door trilogy: 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 Patricia C. Wrede's The Raven Ring, Karvonen is captured by the shapeshifting bad guy, and manages to convince him that (a) the other characters have a password (they don't) and (b) it's in heroine Eleret's native language, which the shapeshifter doesn't speak. This results in "Karvonen" walking up to the others and carefully reciting the phrase "Karvonen says to tell you I'm the shapeshifter."
- In Alistair Maclean's spy novel The Dark Crusader (published in the USA as The Black Shrike), the hero includes the nonsense word Bilex in all communications with headquarters to show that the message is genuine and not written under duress. After he is captured, the enemy claim to have sent a fake message that all is well, which presumably would not include the codeword. The twist is that the hero's boss is actually in league with the enemy, something he realises when his boss takes no action in response to an obviously fake message... which was probably never actually sent anyway.
- In Brotherhood Of The Rose by David Morrell, the CIA are following Saul until he makes contact with his surrogate brother Chris and they can both be killed. When Saul sets up the meeting, an underling urges spymaster Elliot to let them grab Saul and sweat the meeting place out of him. Elliot turns this idea down, pointing out that Chris and Saul have known each other since they were children and have codes and passwords dating back to then, so they've no way of being sure that Saul couldn't pass on a hidden warning.
- In the non-fiction Don't Tell Mum I Work on the Oil Rigs, She Thinks I'm a Piano Player in a Whorehouse, the author Paul Carter is sent to work in Nigeria. He's given a sentence which the driver picking him up at the airport must give before he gets in the cab to avoid a Not My Driver trope ending in robbery and murder. The sentence is somewhat garbled in translation, but close enough.
- In Carrie Vaughn's Kitty Norville series, a medium successfully contacts Harry Houdini, famous in Real Life (as noted in its section below) for leaving codes he would use to try and contact people from beyond the grave, believing that it was all bunk and trying to discredit it. He tells the medium, "Everyone who knew my codes is dead. No one will believe you. But thanks for trying."
- Subverted in the Liavek story, "A Well-Made Plan" into Something Only They Would Say. A Body Swapped character shows up at his own front door and tries to convince his butler that it's really him. The butler asks for this trope, in the form of what the item that is the source of his magical power is (information that every magician keeps a closely held-secret, due to the way magic works in this universe). Instead of naming an item, the character explodes in fury, refuses to reveal it, then comments that since he'd never told the butler in the first place, there was no reason for the butler to expect him to answer. This is the response that the butler was looking for, and he gladly lets him in.
- On 30 Rock, Nathan Lane shows up claiming to be Jack's brother, and Liz is suspicious because Jack has never mentioned a brother and this man pronounces their last name "Dona-hee" as opposed to "Dona-gee." Then he comments that she would be prettier if she didn't scowl, and she knows he's related to Jack.
- In the first-season finale of Agents of S.H.I.E.L.D., Skye convinces Mike she has his son by having Ace repeat what they said to each other in the pilot.
- In Battlestar Galactica (1978), 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 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 turns out to be "Paladin" — i.e. "defender of the faith", which as his boss notes is an oddly appropriate one for a Catholic FBI agent.)
- 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.
- Buffy the Vampire Slayer was never known to use this trope straight, which makes sense, as it was written by Joss Whedon (who also gave us Wolverine's "You're a dick" line):
Giles: Cordelia, it's me! It's me!
- Giles tries to prove his identity:
Cordelia: How do we know it's really you and not zombie Giles?
Giles: Cordelia, do stop being tiresome.
Cordelia: It's him.
- Xander does the Snoopy Dance to prove he's Xander for Willow.
- Xander tells a story about one of his early birthdays to prove his identity. He wanted a fire truck but didn't get one, which made him sad, but then the neighbor's house caught fire for real, and he got to see real fire trucks. He adds that he always thought Willow might have set the fire ("if you did you can tell me"). The problem with this is (a) he's trying to prove he's not his Evil Twin, who would know this anyway, and (b) Willow wasn't aware that there was a twin and thus had no reason to doubt he was Xander.
- In "That 70's Episode", Prue and Piper convince their past selves to trust them by opening a trick drawer in a cabinet.
- Piper attempts it in another episode, telling their mother they can be trusted because they know she's pregnant with Phoebe. It backfires because not only does she not know, she believes she's infertile.
- Doctor Who:
- "BAD WOLF" were Series One's Arc Words, and they tended to show up even after the "arc" was over. More than once, someone could get the Doctor's attention as to the severity of the situation (and whether they could be trusted) by saying those words to him.
- In "Human Nature"/"The Family of Blood", Martha Jones, a medical student from the future disguised as a maid, uses her knowledge of anatomy to prove that she's more than she seems.
- In "Forest of the Dead", Professor River Song whispers the Doctor's name into his ear to prove that she knows him in the future.
- In "Journey's End", Rose encounters two Doctors and has to decide which one to trust. One tells her "I love you", which means he's not the real Doctor — who infamously Cannot Spit It Out — but she chooses the doppelganger anyway, because that's what she's always wanted to hear.
- "The Impossible Astronaut": when Amy needs to get the Doctor to trust her, she tells him a detail only they would know: the Doctor's Wacky Cravings from their first meeting:
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?
The Doctor: You're lying.
Amy: I'm not lying.
The Doctor: Swear to me. Swear to me on something that matters.
Amy: Fish fingers and custard.
The Doctor: My life in your hands. Amelia Pond.
- An unintentional variation occurs in Farscape. John is having a normal conversation with Aeryn until he mentions her baby and she doesn't know what he's talking about. This gives away that she is a bioloid duplicate and the real Aeryn has been kidnapped.
- In Jessica Jones, Trish and Jessica decide they need some sort of signal to indicate Jess is not under Kilgrave's mind control. Since Kilgrave would make Jessica pretend like everything's normal, they decide to use a phrase that Jessica would never say: "I love you."
Trish: You say it, you're still you.
- In one episode of Lois & Clark, a time-traveller is able to enlist Clark's aid by whispering to him, "I know you're Superman, and I need your help."
- In season 4 episode "The Constant", 2004-Daniel tells Desmond to tell 1996-Daniel that he knows about Eloise, and give him some important numbers for an experiment. This proves to Daniel that Desmond is traveling in time and has spoken to a future version of Daniel.
- Played with in season 5. Hurley is afraid of inadvertently being God Tested and revealed to be a time traveler when he's unable to answer a question that someone living in 1977 should be able to (which in his case goes as far as "who's President in 1977?") Pierre Chang eventually gets suspicious and trips him up easily.
Pierre: What year were you born?
Pierre: So you're 47 years old?
Pierre: And you fought in The Korean War?
Hurley: Ha! No such thing!
- 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!" As befits their general methods, this is horribly ineffective. When one of them does encounter the shifter, he just says it with prompting, and fails to notice the shifter's look of confusion.
- Quantum Leap: Sam has leaped into an illiterate murderer on the run who is holding a woman and her daughter hostage. He decides to drop the masquerade, telling her he's a doctor from the future in a Time Travel experiment. She doesn't believe him. Then he notices her medical textbook, and she reveals that she's in medical school. So he has her quiz him on medical stuff to prove that he's telling the truth. At first she still doesn't believe him, saying that he looked at the book already, so could have memorized the information. Sam quickly fires back: "When was the last time you met an illiterate speed-reader?"
- In an episode of Salute Your Shorts, Camp Anawana is playing a very intense game of Capture the Flag with a rival camp. Pinsky ends up going to Donkeylips for help. Donkeylips and Z.Z. aren't sure if he's real or an impostor, so Donkeylips decides to ask him something that he told him earlier in the episode.
Donkeylips: What are the four S's of warfare?
Pinsky: Strength, strategy, and surprise.
Z.Z.: It's Pinsky!
Pinsky: That's the fourth S.
- Time Travel was the whole point of 7Days, 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". Interestingly, it was never used this way after the first episode, once the team knows full well who he is — it's just a signal for them to put him through to the right people.
- In Sleepy Hollow, Ichabod and Abbie use a fist-bump to identify each other in Purgatory, as Abbie had recently taught it to Ichabod. Also while in Purgatory, Abbie correctly identifies a Crane imposter since the fake didn't pronounce her title using Crane's insistent British pronunciation: "Lef-tenant".
- In "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.
- 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). In the same incident, O'Neill convinces Lt. Hammond that the team knows him in the future by telling him he watched the first moon landing two weeks prior from his father's bedside after a heart attack.
- The team finds a crystal skull inside a chamber filled with radiation, and their attempt to examine it results in Daniel being made [[Intangibility 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:
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.
- Similarly, Daniel is inclined to believe that a teenager that got on the base is O'Neill because of the exasperated way the kid shouted "Daniel!" In the same episode, he convinces a group of seasoned pilots that he really is O'Neill (a full-bird colonel) with a well-known phrase. The majors and captains visibly straighten in their seats and lose their smiles.
- Parodied a few times in one episode where the team is dealing with aliens that can shapeshift into other people:
Jack: Daniel, are you you?
Daniel: Yes, are you?
Daniel: Never mind.
Jack: Wait, how do I know you're the real Daniel?
Daniel: (exasperated) Because.
Jack: (beat) Yeah, okay...
- The free Jaffa use the phrase "Shal kek nem ron" as a password to identify other members of the resistance.
- 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).
- Star Trek:
Alexander: I was three years old. She was dying when we found her. She barely managed to whisper my name. And then she took my hand and placed it in yours. And she died. And then you howled in rage, and said "Look upon her. Look upon death, and always remember." And I always have.
- 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 a three-dimensional chess move "Queen to Queen's level three, sir" — to which only Kirk and Spock knew the appropriate countersign (Queen to King's level one).
- 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 his younger self into becoming a warrior. To prove his identity, Worf asks future!Alexander to say what his mother said before she died.
- 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".
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.
- Sam and Dean have a series of passwords and routines established for if they get separated, captured or arrested. The phrase "Funky Town" means the brother who says it is being held at gunpoint. "Poughkeepsie" means drop everything and run. A note addressed to "Hilt" and signed "Steve McQueen" means the brother who sent it will create a distraction while the brother who received it makes an escape.
- In "The End", Dean is sent to the future and proves his identity to Future!Dean by telling a story that only he would know:
- When Dean travels back in time, he has to convince his mother, Mary, that he is her son sent from the future. He convinces her by telling her that she always sang "Hey Jude" to him as a kid because it was her favorite song.
- In The West Wing, the word "Sagittarius" is used to denote a person who knows about Jed Bartlet's multiple sclerosis.
- In one episode of Without a Trace, the episode's missing person is a preteen girl who's been kidnapped as part of a pedophile ring. At the episode's outset, while talking about her quirks, the girl's uncle tells Malone that his nickname for the child is "Chicken Little". Much later, this comes in handy when Malone, having determined where the girl's abductor is, poses as a pedophile customer and gets to talk to the girl, but he can't say anything too suspicious to her since they're both still in the antagonist's line of sight. What does he wind up saying to gain her trust?
Malone: Chicken Little says hi. (the girl immediately looks hopeful)
- In Brad Paisley's "Letter to Me", he says that if he could send a letter back to himself when he was still in high school, he'd prove to his past self it was really him by telling him to look under the bed for the Skoal (chewing tobacco) can and Playboy no one else knew he had.
- In Call of Duty: Modern Warfare 2, the trust password for identifying ally under EMP is "Star", to which the proper response is "Texas", as seen in Second Sun stage.
- In Escape from Monkey Island, Guybrush asks his future counterpart to guess what number he's thinking of. Since you "remember" what future-Guybrush said to past-Guybrush, you can answer him.
- In Prince of Persia: The Sands of Time, the Prince 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 Geist: Since Raimi has the ability to Body Surf, he has to prove his identity to Bryson by reproducing their Secret Handshake.
- Near the ending of Planescape: Torment, the only way you can knock some sanity into the Paranoid Incarnation is to talk to him in the obscure language of Uyo, one of the things he used to lock away some journals of his. After all, if he killed everyone who ever knew the language, how could the new guy in the crystal be anything other than a more lucid aspect of himself? 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 aid an anti government group called "The Forest Owls" and is 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.
- In the Portal 2 Perpetual Testing Initiative DLC, Cave Johnson realizes that it's very difficult to discern who is the original Cave, and who's from an Alternate Universe. He therefore announces that his trust password is "chariots", and only messages where he says that word come from the real him. Of course, he then discovers that there's an alternate him who just likes to say "chariots" randomly, so he changes the password to him saying it twice in a row: "chariots, chariots". You will never hear so many references to ancient Greek vehicles.
- In Full Throttle, at one point Ben is about to be painfully killed by a gang led by Maureen, who believes that Ben killed her father Malcolm. Of course, Ben's innocent, and to prove it, you have to call Maureen by her childhood nickname, "Diapered Dynamo", which Malcolm told you shortly before he died.
- Fire Emblem Awakening has several for most child characters. Generally they prove their identity with a future version of a unique item, generally a wedding ring but in one case Falchion. Amusingly, one character points out the problem with the item passwords; it is possible that the person presenting it murdered the time-traveller and stole their mom's wedding ring.
- There's a way to use this in Millennia Altered Destinies to get the most powerful weapon in the game. Occasionally, when you perform a temporal jump, you will end up in a green mist with a mirror of your ship in front of you. You can contact your other self, and he will tell you a number. The next time you're in the same mist, you can tell your other self the same number. He will then teleport a set of plans onto your ship that can be given to a sufficiently-advanced race to build.
- Mass Effect:
- Quarians are issued one of these to give when returning to the migrant fleet. They are also issued another phrase that indicates they are returning under duress and their ship should be fired upon before it gets too close.
- In the second game, during the first meeting with Tali, Shepard can mention the geth data that they gave her during the first game, wondering if it helped her complete her Pilgrimage. As Tali was the only other person who knew that Shepard had disobeyed orders by giving her a copy of classified intel, this proves that Shepard had indeed come Back from the Dead.
- In the Citadel DLC for the third game, Shepard can prove to Traynor that it was their evil clone that just fired her and threw her off the Normandy, by describing the toothbrush that Traynor mentioned as wanting to buy during their first conversation. Alternatively, a Female Shepard in a romance with her can do this via a "Shut Up" Kiss.
- One example from the third game had David Archer, the Tragic Villain from the Overlord DLC in the previous game, vouching for Shepard when the school he was sent to is under attack by Cerberus.
David: The square root of 906.01 equals—
- Played for Laughs in Halo 3. There's an Easter Egg (guest-starring the cast of Red vs. Blue) where one Marine has to give another a password to prove that he isn't a Brute.
- In Undertale, if the player loads the game immediately after receiving a pacifist judgement from Sans and receives it again, he will notice that you seem to have heard his speech before; to put you to the test, he gives you a Trust Password that he's been setting up just in case he ever needed to verify if someone's a time traveler. If you load again, the playable character will tell Sans the password, "I'm a stupid doodoo butt," and he will, in turn, give you the true trust password that will verify, once and for all, that you're a time traveler. Load again and the playable character will give Sans the second password, "I'm the legendary fartmaster," and he will, in turn, give you the key to his room.
- In Dragonsphere a group of guards bar you from entering a land of shapeshifters, even though you are their king, out of concern that the "you" who ends up returning may in fact be a shapeshifter who took your place. This is resolved by giving them an item from your inventory (any item will do) so that they will know that the real "you" will ask for that item upon your return. Ironically, you actually are a shapeshifter all along, you just don't realise it at first.
- SOON: Dr Fang decided on a "time travel password" as a kid just in case someone had to prove time travel was real.
Atlas: A password? Why didn't you think of that?
- Bob and George pay homage to Bill & Ted's Excellent Adventure this way when Time Travel becomes a regular occurrence in the comic; since Mega Man and Bass are legendarily stupid, it goes about the same way.
- In Home On The Strange, Tanner sets one of these up with Izzy, to the latter's bemusement.
- Sluggy Freelance:
- During their adventures in the Punyverse, 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 information is classified, which instantly appeases him.
- In "Oceans Unmoving", (major spoilers), it's inverted. Bun-Bun can't appear himself to his past self as himself, because he knows that if he were to see someone claiming to be himself from the future, he'd figure it was a trap and kill them, expecting that if it were really him he would have expected that.
- In Panthera, when Onca tries to tell Tigris that Ovid isn't the evil corporation they've been told it is, Tigris thinks Onca betrayed Panthera and attacks her. However, when Pardus shows up, she tells him that Leo told her Pardus dyes his fur. Pardus knows Leo wouldn't tell her that 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 it has such an involved story attached to it, it really would be impossible for anyone who didn't know her to guess.
- In Homestuck, Jade implements a system of these, though 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.
- Subverted in one conversation with Karkat, who can't remember the password, but does remember that it's insulting. He rattles off a truly impressive self-loathing monologue, which convinces Jade anyway, because Karkat is the only person who hates himself enough to put that much effort into insulting himself.
- In Skullkickers, when Shorty and Baldy make a stop in a tavern while hunting a doppelganger, Baldy suggests establishing a trust password, so that they will know if the doppelganger is impersonating the other. Shorty suggests some passwords, but Baldy dismisses all of Shorty's suggestions and kills the false "Shorty" on the spot. As it happens, they'd already established such a password.
- Something Awful goons answer the question "Do you have stairs in your house?" with the countersign "I am protected."
- Worm has passwords as standard when fighting Strangers (who can mess with perceptions) and Masters (who can control other people's bodies). The most prominent example of this is after the protagonists capture and bodyjack Shadow Stalker in order to infiltrate the Wards. When Weld uses the standard password, they're able to make Shadow Stalker reply with the standard countersign indicating that she's uncompromised — but they are at a loss when Weld asks who it was Shadow Stalker recently got in trouble for harassing. The incorrect response turns an until-then-successful infiltration into a firefight.
- In the "Sparks Nevada, Marshal on Mars" episode of the Thrilling Adventure Hour, the first time Croach the Tracker sees Sparks Nevada after returning from the dead, Nevada needs more than just Croach's word that it's really him and demands something only the two of them would know about. To Nevada's dismay, Croach chooses to use the time Nevada massaged his egg sacks in the second episode.
- In the Red Panda Adventures episode "The World Next Door", Baboon McSmoothie needs to do this twice. The first time, to convince the Red Panda he's from an Alternate Timeline, he gives the Panda a message from his counterpart with nothing but the Red Panda's real name on it. That convinces him McSmoothie is telling the truth, but isn't enough to secure the Red Panda's help stealing a prototype device from a Nazi scientist that, while destined to become one of the series greatest villains, hasn't done anything yet. What convinces the Panda to help is a file created by his counterpart detailing the supervillain Death Trap that killed his Flying Squirrel.
- Batman: The Brave and the Bold:
Batman: Your favorite color is blue, you used to sleep with a night light, and you're deathly afraid of monkeys.
- In "The Criss-Cross Conspiracy!", Batman is trapped in Batwoman's body. Nightwing asks him to prove it by saying something only Batman would know.
Nightwing: It's him.
"It really is you, isn't it?"
- When Batman is projecting himself astrally after being Buried Alive (It Makes Sense in Context), he possesses Speedy and starts telling Green Arrow about what's going on. Arrow initially thinks it's Speedy doing an impression, before Bats grabs him by the hem of his shirt and threatens him to his face.
- In Ben 10: Alien Force, Past Gwen demands Future Gwen say something only 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 Dynomutt Dog Wonder, when Manyfaces has tricked Dynomutt into believing that Blue Falcon is an impostor, the hero proves himself by showing his friend that he knows Dynomutt's "one ticklish spot".
- In the Family Guy episode "Prick Up Your Ears", Stewie tries to catch the Tooth Fairy. He sets up a trap, which catches Brian.
Brian: Stewie, what the hell? Get me down from here.
Stewie: No, way, man! How do I know you're not the Tooth Fairy in disguise?
Brian: Your middle name is Gilligan.
Stewie: Not good enough!
Brian: You think my girlfriend's a moron.
Stewie: So does everyone!
Brian: You have a picture of Chris Noth in your wallet.
- In Justice League, when Flash and Lex Luthor switch minds, Flash proves he's really himself again by starting to reveal Green Lantern's old nickname.
- My Little Pony: Friendship Is Magic:
- In the two-parter "A Canterlot Wedding", Twilight and Cadance have a rhyme from way back when. When the false Cadance doesn't know it, Twilight smells a rat. When, due to Fake Cadance's manipulations, she nearly blasts the head off the real one, the Real Cadance does this to prove it's really her this time.
- It proves useful in "The Times They Are A Changeling." The Crystal Empire is on guard against a changeling. They don't know if the visitors from Ponyville are really them, so Cadance initiates the rhyme and Twilight reciprocates almost immediately, calming the guards down.
- In the sixth season finale, "To Where and Back Again", Starlight Glimmer and her team, venturing into a kingdom full of Changelings, use the phrase "klutzy draconequus" as a trust password. Discord isn't exactly pleased. However when the phony Discord doesn't say "draconequus" after Starlight says "klutzy," she, Trixie, and Thorax immediately know what's up.
- In the two-parter "A Canterlot Wedding", Twilight and Cadance have a rhyme from way back when. When the false Cadance doesn't know it, Twilight smells a rat. When, due to Fake Cadance's manipulations, she nearly blasts the head off the real one, the Real Cadance does this to prove it's really her this time.
- In the Sonic Boom episode "Hedgehog Day", Eggman asks Sonic and Tails what he can say in order to convince them quickly that the time loop exists. After time resets itself again:
Tails: There's no such thing as a time—
Eggman: Last night, you dreamed you were being chased by a giant sock puppet.
- In South Park, Cartman wants to be a Human Popsicle so that he won't have to wait for the Nintendo Wii to come out. He badly overshoots his goal and ends up in a Bad Future, where there's a war going on which prevents him from playing the Wii. He calls his past self to fix things and tries to convince him of his identity by explaining things only he would know, liking drinking Ovaltine and putting Clyde Frog in the closet before going to Butters and trying to freeze himself. But his past self merely thinks someone was spying on him and is not convinced.
- Star Wars Rebels:
- Trust Passwords are naturally commonly used in the military and to detect spies:
- A common way to accomplish this was with a shibboleth, a word whose pronunciation is unique enough that only certain speakers can say it correctly. It's old enough for the term to come from The Bible. It was used commonly in World War II, most deviously with the word "lollapalooza", which abused the Japanese inability to pronounce the letter "L" to allow Americans to weed out Japanese soldiers.
- One World War II variation was used to Bluff the Impostor. The guard would state, "The land of the free", to which an American would reply, "And the home of the brave" — an easy reference to the national anthem. Then the guard would state "The terror of flight", and if the reply was "And the gloom of the grave", the guard knows it's really a German soldier — because only a spy trying too hard to pass as an American would know the third verse of the anthem.
- Secret agents are typically given several security checks they can insert into a message to verify that it's real. That said, they don't always work; a British agent captured by the Germans during World War II tried this, but his bosses didn't pay attention, and it led to Das Englandspiel (a.k.a. Operation North Pole) and the capture and execution of fifty Allied agents. What's scarier is the possibility that the bosses did get the message but considered their cover more valuable than the agent's life.
- In modern military operations, ground troops will use code words to describe a situation to an extraction unit (usually a helicopter) rather than describing it outright; this allows them to call off an extraction if they're captured without tipping anyone off.
- According to tradition, Joan of Arc convinced the Dauphin that she had really been divinely inspired to help him claim the French throne by whispering a secret into his ear. That secret ranges from a prayer the Dauphin had said in private (which Joan could learn only from God Himself), to God wanting the Dauphin to be king, to describing 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 debunked several mediums who tried to contact his mother and made the mistake of having her address him as "Harry" (which was his stage name, when she would use his real name, Erik) or speaking in English (when she was more confident in Hungarian or Yiddish). Houdini also arranged a number of code phrases (one being a song called "Rosabelle") with his wife Bess which he promised to use if he died before her and tried to contact her after death. He died in 1926, well before Bess, and no one was ever able to deliver a message she was satisfied was genuine.
- 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. PSAs also recommend this for if the family gets separated. This is the basis for a few fictional portrayals where the hero tries to save a kid in danger but needs the password from the parents.
- Some birds teach these to their unhatched chicks, which then use the password in their begging calls. If the parents return to their nest and do not hear the password, they'll know that the nest has likely been taken over by brood parasites and will abandon the nest.
- And in general, more people than you'd think have trust passwords just because they're Genre Savvy, Crazy-Prepared, or extremely nerdy. One old website was touted as a "time traveler's support network" which designated secret meeting places and times in various cities, where a "volunteer" would help anyone who used the correct password. The website is long since dead (not that this should be any problem for a time traveler anyway).
- Some people even have passphrases for themselves in the off-chance that they somehow end up talking to their future selves.