I need a way to send messages that can only be viewed once, and then become permanently unreadable (physical media destroyed, encrypted, any method as long as the message cannot be feasibly recovered by anyone). How can this be accomplished?

UPDATE: This is happening in a world with roughly the same technology level as us. The messenger and recipient both understand the secret nature of these messages.

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    I assume the intended recipient wants the message to disappear? Otherwise nothing can stop them from e.g. taking a photo of it while they read it. – Draconis Aug 14 at 2:36
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    Hello Adrian! The answers are good, but they make a ton of assumptions. What is the tech level or time period relating to the question? What resouces are available to the sender and recipient? How much trust do we have in the recipient? What is the tech level of your enemies? Etc. – JBH Aug 14 at 3:25
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    Making it a verbal message and killing the messenger after delivery would seem to be reasonably secure. – Mawg Aug 14 at 13:08
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    They have solved this problem in Mission Impossible. Just look at any TV episode or a movie. It is always a magnetic tape or a variation which always "poofs" after playing the message and smoke comes out. – Fixed Point Aug 14 at 15:21
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    [This message has already been read and has self-destructed.] – David Richerby Aug 15 at 11:21

20 Answers 20

up vote 42 down vote accepted

There's lots of physical solutions, but every one of them has some fatal flaw. There's information theoretic and physics based reasons why its really tricky to make something which can be read once, but only once.

The best solution is used in the US government today. It's called "EYES ONLY." It's really simple. If I want to send a message to you, I write it on an EYES ONLY document, carry it to you, show it to you, then take it to a shredder to be disposed of.

Beyond that, it's all just games. When you consider an opponent that is willing to mill the layers off a smart card and attach wires, guided by an electron scanning microscope, or an opponent willing to measure micro-amps of power consumption to attack your message, physical security just isn't what it used to be.

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    Never write anything down. (delete me) – Mazura Aug 14 at 3:30
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    @Mazura I had to, as part of the OP's requirments to view the message. Your approach is far safer. – Cort Ammon Aug 14 at 3:32
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    This. You can have magical/technical solutions but in the end they'll all be defeated when an intern delivers the failproof read-once magipaper to the wrong office, and there's no handwaving that. – AmiralPatate Aug 14 at 8:11
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    As is often found when companies try to secure media via encryption, if there is ever a point where your message can be perceived by a human, you have at least one weak point in your protection. – Tofystedeth Aug 14 at 21:50
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    Just hope the document isn't cached in the printer buffer, or the printer is compromised ;-) – Gertsen Aug 15 at 10:50

The fastest way is writing a message on edible paper (such as rice paper) using food colorant as ink. The reader finishes reading the message and immediately eats it.

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    Exactly. I was going to suggest edible paper under cookies, frequently seen in German Christmas cookies. Thus you can both conceal and destroy the message! – Ludi Aug 14 at 10:31
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    Even things like this fail if "someone" has a camera nearby and takes a picture of it. Doesn't even need to be the reader, doesn't even need to be intentional (e.g. a security camera). – Draco18s Aug 14 at 17:20
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    @Ludi I feel like I would accidentally eat the message before reading it in that case. – Nic Hartley Aug 14 at 17:37
  • @NicHartley point taken. It only works when you expect messages of that kind. – Ludi Aug 14 at 21:22
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    This message can be read plenty of times, at least until someone destroys it. At which point you might as well print it on regular paper and set it on fire after reading. – Erik Aug 16 at 11:13

Take a quantum leap!

The theoretically safe way to send a secret is to use quantum physics. See Quantum key distribution

Basically, you utilize the fact that a quantum state cannot be measured without being distorted. So if you encrypt your message in quantum states, as soon as the recipient reads (measures) it, it will become distorted gibberish.

This means a tech level slightly above current, but no magic or alien physics required.

  • Is this what the Chinese “Micius” satellite did? In which case, I guess its current :) +1 – Mark Aug 14 at 11:42
  • See Greg Egan's "Incandescence" for this used in practice by post-humans sending their digital selves around the galaxy. en.wikipedia.org/wiki/Incandescence_(novel) – Caleb Jay Aug 14 at 19:28
  • @Mark Wasn't that photon entanglement which from my understanding is trying to send a message faster than the speed of light. I don't think the quantum state itself can be measured without changing the result (I remember there was a joke in Futurama about this, related to horse racing) – Shadowzee Aug 15 at 2:47
  • @Shadowzee I am not sure about the satellite, it was something with encryption though... You're right that measuring a quantum state changes the result (except for pure states). Entanglement can have effects faster than light speed, but you can't transmit any information. That last part is too complex to explain in a comment box (and I'm probably not smart enough, anyway). – Mark Aug 15 at 6:29
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    @Guran that doesn't help at all, you need it to be non-copyable too – somebody Aug 16 at 6:06

Back in the distant past, movies weren't rented off the internet via Netflix or iTunes, they were rented via physical media. In those ancient times, you walked into a shop, picked some movies off the shelf, went up to the counter, and handed over some money after agreeing to return the media on a given date (or be charged late fees). This is a model for the young people today to understand, pretty much like I struggle with the idea of my grandparents having to boil water on a large wood stove for a bath, but I digress.

The point of it was that during those times, there was an experiment in certain quarters for a rental model that didn't involve returning the media.

What they did was put kiosks out where you could walk up, 'rent' a movie which would trigger you receiving a disposable DVD of the movie. Why disposable? Well the idea was that the DVD was sealed in a black plastic bag with the name of the movie on it. The moment you opened the bag, the DVD was exposed to light, and there was a photosensitive pigment in the plastic of the disk that permanently turned the plastic in the disc opaque, meaning that the dye in the burnable disc couldn't be seen by the DVD player anymore.

This took around 24-48 hrs to happen.

It didn't take off because of several concerns, but mostly digital distribution coming on line.

The thing is, this kind of pigment system could allow people to watch a DVD once, and then it wipe itself. If you had the pigment actually triggered by the DVD laser for instance, it could wipe itself as it was read. You wouldn't be able to rewind if you missed something, but it would certainly meet your brief. On top of that, this kind of pigmentation triggering wouldn't mean that you're restricted to specific machines to play the message; in theory, you'd be able to play it pretty much anywhere.

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    This is a cool idea, but the pigment still exists: couldn't someone with sufficient resources slowly deconstruct the disk, reading the pigment off it to make a new copy? – Draconis Aug 14 at 2:51
  • @Draconis Probably, but if you used the activated form of the same pigment to record the DVD in the first place, then separating the pigments through deconstruction shouldn't help you. You might want a different writing laser to alter the strength of the pigment burn when recording, and you'd have to do the whole thing in complete darkness, but it's possible. – Tim B II Aug 14 at 2:57
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    @Draconis also if you're willing to put that much effort into it, you could just copy the disk before the pigment sets in – dn3s Aug 14 at 5:30
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    @dn3s Which probably is (one of) the fatal flaw(s) in this approach. – Mast Aug 14 at 8:40
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    Point a camera at the television. Play disk. Copy made. DVD piracy made easy. (See also: "you wouldn't steal a car" and "don't copy that floppy".) – Pharap Aug 16 at 13:12

Pyrophoricity

Write the message using pyrophoric substances which ignite upon contact with air. Do the writing inside a box filled with an inert gas and seal it in an opaque & impermeable substance - or perhaps rolled up in a sealed glass bottle.

The recipient opens it up and has a few seconds to read the message before it spontaneously combusts. No matches required.

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    "Your mission, should you choose to accept it..." – RonJohn Aug 14 at 2:56
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    This also usefully doubles as an anti-tampering mechanism. If you open the lock wrong, or if the package is abused too much, the message will be burned up by the time you get it open. – Cadence Aug 14 at 3:28
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    Open it in a nitrogen container, take a picture, reseal. – Mark Aug 14 at 11:38
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    This idea looks much better on paper than in practice. Most pyrophoric substances won't burn the whole paper, leaving holes where letters were. Quickly splashing the message with water will stop the burning. Also, the message is likely to be toxic. – Tomáš Zato Aug 14 at 12:43
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    Now if you made the not-letters the inked part of the message, when it catches fire, you'll be left with a loose collection of letters. Of course, that doesn't address the nitrogen container or the water. – Draco18s Aug 14 at 17:27

Cryptography geek-out

Short answer: cannot be done, in the strictest sense of information theory.

Long answer: okay, so it can be done... but there are a few requirements that depend on humans behaving correctly (which is always the biggest weakness of any "secure" system).

The players

  • Alice, head of the organisation known as The Organisation
  • Bob, works for Alice out in the field, and is expecting a message from her
  • Carol, works for Alice as her communications technician
  • Eve, works for The Others, and really wants to get her hands on Alice's message to Bob

The premise

  • Alice and Bob are not in the same location
  • Alice needs to get a message to Bob
  • Alice needs that this message is read/heard/viewed only once by Bob
  • Alice needs that Eve does not get hold of the message
  • Carol is the one that provides Alice with the tech needed to get the message to Bob

The requirements

  • Alice and Bob have the secrecy of the message as their priority.
  • Carol is incorruptible. If Carol is in any way compromised, this makes the system she has set up for Alice compromised as well. Carol must be perfectly trustworthy.
  • Bob does not record/copy the message, nor does he talk in his sleep, nor does he give in to any kind of method aimed at extracting the message from him.
  • The message can be seen/heard/viewed only by Bob. Do not use leaky headphones Bob... Eve can hear you.
  • Eve cannot intercept the message before Bob gets it. This of course means that the system that Carol has set up must be perfectly secure, and also that the incorruptibility requirement extends to all couriers that Carol employs.
  • Bob needs to follow his message extraction protocol perfectly.

Iff we have achieved all this, then the question becomes mundane. A few possible methods...

  • Carol created edible paper. Bob is required to eat the paper after reading. Mmm.... chocolate flavour... nice one Carol. (See also Christmas Snow's answer below)
  • Carol has set up a secure phone line that cannot be overheard (a one-time pad protocol solves that easily). Alice states the message plainly to Bob. This same goes for any kind of service that streams the message to Bob.
  • Carol has reprogrammed an MP3 player so that for every byte of audio it reads from its memory, it erases that byte through secure methods (i.e. overwrites it a thousand times with random data). By the time Bob hears that corresponding byte, it is already gone.
  • A good answer to the wrong question. The OP does not require that Eve cannot read the message, only that Eve cannot read it after Bob has read it. (Or that Bob cannot read it if Eve has intercepted it for that matter) – Guran Aug 14 at 8:20
  • @Guran If Eve can pick up the message before Bob does, then the message has been received more than once. If you mean that it is permissible that the message is readable by anyone and we do not give a hoot about who did it, the system is pretty much nonsensical. And it does not matter anyway because the methods described are just as viable. – MichaelK Aug 14 at 8:24
  • Yes that's the tricky bit. Remember, it was required that the message can be read exactly once. It was not required that the message can be read only by the intended recipient. – Guran Aug 14 at 8:29
  • @Guran So what? The methods to achieve the one-time-read remain the same. Sure, in case of the edible paper Eve/Mallory can just leave the paper in place and not eat it; the protocol is compromised by their non-compliance. But compliance is a requirement no matter what because even if the message is destroyed, Eve/Mallory can just restate it whenever they want. So we have to assume compliance, or this is not achievable. – MichaelK Aug 14 at 8:49
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    @Gryphon We do not talk about what happened to Dave... it was not pretty. The exact details of the... "incident" are in the message. – MichaelK Aug 14 at 14:53

I'm assuming that you mean a read-once transmission medium that ensures no trace of the message is left behind once it is read.

I'm also assuming that we are not dealing with in-transport security here, so that it's safe to assume that the message has been safely delivered to the recipient.

So that the only requirement is that the recipient or anyone else can't provide or accidentally leave behind any evidence of the message content.

Note that it's not possible to prevent the recipient (or a last minute eavesdropper) from copying the content as it is read. At least with modern technology. Any terminal delivery mechanism intended to transfer the message from the transport medium to the recipient's brain (e.g. audio playback or letters on a page) can reliably be captured by technological means (audio recording, photography) as it is being delivered. Even the signal from something esoteric like a direct cranial stimulation device can be recorded and used to deduce the message contents.

So let's try to ensure that a message can't be extracted from the original delivery medium (or what's left of it) after it has been read, not before, and certainly not "as" it is being read.

Unfortunately, our current understanding of information tells us that you cannot actually destroy information, as every current state of the universe depends on every past state, and given enough information about the current state, you can always trace it back into a past one. The only known way to really get rid of it is to chuck it into a black hole, and even then, you probably only send it elsewhere on a one-way trip.

But that is probably more than good enough, so that's the principle to adopt.

The principle of good enough

You can't make it impossible, but you can make it practically hard enough so that it is no longer worth it to try to reconstruct the message from the remains of the medium.

Let's think of two somewhat accessible contemporary approaches that would make it hard enough so that it is much more (orders of magnitude) expensive in terms of time, money and effort to reconstruct a destroyed message than to package it in the self-destructing medium.

Photography materials

Photograph your message onto film and personally print it onto photographic paper in the darkroom. Do the developer bath properly but skip the fixer/stopper bath. Dry it as it is and seal it into a light-proof envelope. Once the recipient opens the envelope, they have a few seconds to a few minutes (depending on the paper grade and the amount of ambient light) to read the message, before the paper completely turns black.

Reconstructing the message from this black paper would take a whole crew of graduate level chemistry researchers equipped with tunneling electron microscopes, looking for tiny traces of molecules being reshaped in slightly different environments. They could probably reconstruct a simple message with heuristics in about a couple of months of dedicated work. It would take a few times more effort to accurately reproduce something with precise numbers, like the coordinates of a secret installation. All assuming that the message wasn't encrypted in the first place.

Besides, if they make a single mistake in their process, they could easily destroy all their progress, and make it even harder to get back on track.

Don't forget to properly destroy your originals and negatives as well. And always encrypt your secret messages.

If this kind of activity is common in your world, there could probably be some device like a polaroid camera that works on these principles. You take a photograph of whatever represents your message, and this device produces a special print in a light-sealed envelope. The contents chemically turn dark a few minutes after exposure to light.

Deliberately defective computer memory

By design, the bit (binary digit) circuitry that holds a single one or zero in computer memory gets reset every time it is read. This is normally counteracted by rewriting the read data onto the bit circuit every time it is read. A purpose built chip deliberately lacking this rewriting mechanism would practically become a read-once medium.

A potential gotcha here is how to deliver the once-read binary data to a human. Displaying it on a screen requires additional processing and at the least holding it on some other memory in the form of pixel data. Either the whole device and software has to be of trusted design, or it could be compromised.

A simpler way could be storing digital audio in the 1-bit 192kHz Super Audio CD format or something similar on the memory, which can be converted back into a sound signal with very simple analog circuitry, involving just a capacitor, amplifier and speaker.

And the data would be gone as it is being played back.

Looking for the traces of the data on the leftover chip would be a similar effort to deciphering the photographic approach, this time employing a different set of experts and their lab.

If you encrypt the audio data, then you lose the ability to play it back directly on simple circuitry. You have to decrypt it first. Solution, there are dedicated chips for that as well.

Extra combo: Special CD-R

Let's combine the two ideas…

You could also maybe record audio on a special hyper-sensitive rewritable CD, set up to be so sensitive that the pickup laser in the player mangles it beyond recognition while reading it.

Such a sensitive CD will probably have to be transported in a sealed package as other ambient IR sources (like the sun) could easily damage it. This is all the better; if someone who doesn't know this intercepts and opens the package, they are left with just a thin circular piece of transparent plastic.

The recipient will know better, and put the disc into the player in a dark room.

  • Back when I actually was current, the dynamic RAM available would lose its contents really fast, so it had to be constantly rewritten. The problem is how to keep the data in the chip long enough to get to the destination. – David Thornley Aug 15 at 20:22
  • NVRAM should do the trick probably. Or a battery, plus a custom flip-flop circuit that writes random noise after read. – edgerunner Aug 15 at 20:35

If you're sending a physical message, write it on flash paper (nitrocellulose) and set it on fire after reading. This paper burns so well that it doesn't even leave ash behind; there's literally nothing to recover the message from.

If you're sending an electronic message, don't ever store it on a hard disk or similarly persistent medium. (This rules out most current communication protocols.) After reading it, turn the device off and wait for a few minutes; after this point, no real trace is left in RAM.

  • How can I make sure the flash paper doesn't get burnt during transportation? – Adrian Zhang Aug 14 at 2:48
  • @AdrianZhang You can buy flash paper online and have it shipped to your house, so I'm assuming transportation isn't much of an issue for it. If ink residue on the packaging becomes a problem, stack a few more blank pages on either side. – Draconis Aug 14 at 2:49
  • Sending any electronic message opens you up to eavesdroppers. Even if it's not over wifi; anyone on an ethernet network can see every packet on the network. You can encrypt them of course, but I don't think that really fits the original question. Eavesdroppers can still see the encrypted message, they just can't decrypt it. – Ryan_L Aug 14 at 2:53
  • @Ryan_L True, but this question seems to be about recovery after the fact, not interception. Any message can be intercepted. (If nothing else, point a camera over their shoulder when they read it.) – Draconis Aug 14 at 2:55
  • @Ryan_L everyone on an ethernet segment can see each others' packets. On switched networks each switch port creates a separate segment, unlike older networks with hubs that create one large, shared segment. switches are ubiquitous these days and hubs are now very rare though. There are plenty of risks communicating over the internet or even a LAN, but this is extremely rarely one of them. – dn3s Aug 14 at 5:37

There are many physical and chemical processes that can theoretically be reversed but are basically impossible. This may be as simple as completely burning a note that had a message on it, or completely destroying a hard disk using etching acid.

A fun tidbit is that RSA Laboratories generated the famous RSA Numbers for their factoring challenge by generating them on a computer with no network connection of any kind and then destroying the hard drive.

Another fun idea to think about is the black hole information paradox. If physical information is permanently lost when dumped into a black hole, then it would be physically impossible by any means to ever retrieve the information.

Self-shredding audio tape

The problem with written message is that anyone can photography or screenshoot it during the lapse of time it is displayed. An audio message will remain invisible to the eye, yet conveying the same amount of informations for the recipient.

Of course, i'm thinking about Mission: Impossible for an audio device, but not the kind you listen in public with a loud speaker like in the earlier series: I think more of a plastic-casted device that cannot be opened without destroying its contents, with a magnetic tape that only plays when connected to a particular set of earpieces (that only the recipient has, acting as a physical key as well as a biometric one if you need).

When the earpieces are in place, the magnetic tape begins to unfold, delivering the message to the recipient and the part that has just been heard goes into a small shredder (or a small burner), making a second listening impossible.

Then when the message is about to end, an automatic system opens a small vial of acid, destroying any remaining parts. Then it burns, folds into a crumble of plastic and throws itself into another shredder (that explodes).

Also, the message does not stop until it has been completely delivered, whether the recipient hears it or not. Kind of a Prophecy Record like in Harry Potter, in fact.

Schwarzschild Compressors

Let me introduce you to my new and revolutionary invention, the Schwarzschild compressor. This handy device, no larger than a small backpack, will compress whatever matter you put into it well below its Schwarzschild radius, causing it to collapse into a miniature black hole that will itself succumb immediately into a Hawking radiation decay.

The device is built to withstand the energy release of the collapsing black hole. In fact, this is how the battery is charged to provide energy for the next use (complemented by negative matter decoupler).


Alternatively, one can also use...

Antimessage Generator

For a bargain price of $999.99 you can now purchase an Antimessage generator. It will take a physical carrier of a message, and generate the needed anti-matter to annihilate the message entirely. The resulting high energy photon released in the process can be used to charge your mobile phone, too!


For a bargain price, and only today, you can get both a compact Schwarzschild compressor (compress up to 50 gram of matter!) and an Antimessage Generator (up to 20 grams) for only $2001!

  • Alternatively, use your post-it note to vaporise your adversaries and everyone they care about in a quick flash of gamma rays :D – edgerunner Aug 14 at 19:46

Embed ink in wax layers, a few words at a time.

A layer of opaque wax, print the last few words of your message on it, put the next layer of wax on, print the words just before, and so on until you get to the top layer: "Good Morning Mr. Phelps." Add more layers to the top and bottom if you don't want the salutation and closing to be visible. Embed fibers or other interlayer structure to the process and heat to the softening point to prevent delamination.

The recipient scrapes off the wax, layer by layer, revealing a few words at a time. In order to get to the next words, he has to scrape off the previous words.

  • add something to ensure that he cannot carefully lift off the entire layer intact, which is what I would try to do. – Tom Aug 16 at 10:21

On some old (90s) mobile phones, you were able to send an SMS to screen memory. This would get delivered to the recipients phone and open automatically on the screen. If you pressed any buttons, it would disappear and go back to whatever was on the previous display. The message would not be saved anywhere.

  • Hi BoB, welcome to Worldbuilding.SE! If you haven't had a chance to check it out yet, don't miss the tour. I like this answer, and I think it could really be enhanced by some explanation of how this works, maybe adding a link out if the explanation is very technical. I hope you enjoy your time here on WB.SE! – realityChemist Aug 15 at 13:33

Just say it

Pass the message verbally, either in person or via some encrypted communication channel. The sound waves hit the recipient's ear and dissipate moments later, making sure no one else can hear the message.

Snapchat video message should work. The only way to view it after sending it is to record it, which in reality could be done with any of the above suggestions as well and will always be a risk.

Distinguish the three cases

This is a practical, real world problem that a lot of industries have tackled, and there is a vast amount of literature on the subject if you really want to dig deep.

There are a lot of good answers here, but both the OP and most answers failed to clearly distinguish three cases which are important in practice:

  1. The intended / first recipient cannot be trusted to play fair. In this case, as others have noted, the problem is impossible: there is no way to make it fundamentally impossible for the recipient to simply broadcast the message as he listens to it -- not even by quantum crypto methods.
  2. The intended / first recipient is absolutely trustworthy and thoroughly competent. In this case, we flip all the way from "impossible" to "easy." There are a whole bunch of ways to destroy the medium so that it can never be recovered.
  3. The intended / first recipient is not trying to break the system -- at least, not trying hard -- but either won't try to make it work, or is not competent to do so. This is the actually interesting case that applies in the real world. It is essentially what -- cough, spit -- DRM is about. It can't be solved absolutely, but it can be made so that it is much easier for Bob to comply with the protocol than to casually cheat or accidentally screw up. The system will fail from time to time, but hopefully, you can keep your losses low.

There are three basic principles to make type 3 work:

  • The system must be automatic, so it doesn't rely on Bob not being a doofus, or being totally honest. To break the system, he must actively attempt to break its technical measures, or co-operate in allowing a stranger to do so.
  • The system must have graceful failure, also known as lack of brittleness: Cracking one device only provides access to messages intended for that device.
  • Losses arising from breach of a device must accrue to its owner. Now, Bob has nothing to gain from cracking his device, or giving it to hostile parties.

If you can meet these conditions, the problem may not be totally solved but it's good enough to make a living.

The issue with a lot of the methods above is that if the message is meant for eyes then it can be photographed instantly before it disappears or changes, and if it’s meant for ears it can be recorded or listened in on. The safest way to send a message that can only be read once is to add layers of complexity to each step to minimize the probability of it being copied or read by somebody else. You can do this by add multiple layers of encryption requiring several messages in order to learn the decryption in order to be read, each message sent on a different medium and having different methods of disappearing after being read like the above methods. The probability that somebody would anticipate ALL of those methods in succession and be able to copy it for evidence would be extremely unlikely.

Encrypt and encode the message in a laser beam, and bounce it off something deep in space such that it takes a couple hours to get back to Earth. The recipient has to know exactly when and from where the tight beam messages are coming to be able to intercept it. That would be a scheme that is shared between the two parties ahead of time. Once the message hits Earth, its gone forever.

The process could technically be done with radio waves or perhaps microwaves as well, but lasers are cooler of course.

  • At ranges of multiple light-hours, your beam is diffraction-limited (and your reflector is somewhere past Saturn). No matter how well-collimated it is at source, by the time it comes back, it's going to be gigantic. You need arcsecond precision to even hit the Earth at one light hour. The result of this: If your beam is powerful enough to be read by your recipient, it's powerful enough to be read by half the planet. – Yurgen Aug 15 at 23:26
  • If you know where its coming from and exactly when, and can identify it as a message from all other radiation, and then decrypt it, yes, half the planet can read it. But the question was about self destructing messages, and once the beam hits Earth, its gone. – GrandmasterB Aug 16 at 3:42
  • Or just record 24/7 and do the analysis offline. Your adversary is nation-state, and data storage is cheap, so that's not a significant barrier (compared to installing a moon-sized retroreflector in the outer solar system). – Yurgen Aug 16 at 19:20
  • I agree, if they recorded every bit of light and radiation across the entire spectrum coming from every single possible direction, and if they can identify the message (which in theory could be spread across multiple frequencies), and then if they can decrypt it, and do that all within a couple of hours, they can intercept the message and act on it. – GrandmasterB Aug 20 at 16:55
  • They don't need to record from every direction. They know where your giant retroreflector is because it's impossible to miss. Also the observatories you need to monitor it are orders of magnitude cheaper than the rocket you used to install it. Encryption is irrelevant because you have to show why your scheme isn't just a strictly more expensive version of a 1970s communications satellite. – Yurgen Aug 20 at 17:06

I remember a scene from one of the Mission Impossible movies.

There was a pair of sunglasses which played an audio-file once someone puts them on. The audio-file was basically some guy explaining what Tom Cruises next mission is. At the end, the voice said that the sunglasses would explode in about 5 seconds. Just enough time for Tommy to get rid of the glasses.

This method of course has one major flaw: Is the recipient of the message capable of disposing the glasses after getting the message? (without getting injured, without injuring someone, without getting noticed etc.) I don't know what your plan is, but this could work out for you.

Another thing that some of todays messaging-apps do is deleting the messages a few seconds after the receiver was reading the message. If everything is working out as intended (The recipient is connected to the internet while reading the message and the app does send a signal to the sender after the message was read), this is technically a safe method. Those messages are usually encrypted in a way that only the sender and receiver can read these messages. No one in between who manages to capture the transferred data is capable of decrypting and reading the message.

Anyway, there are is one flaw I could think of:

Could the recipient want to keep a copy of the message?

The person could just take a screenshot or do a recording of the screen before the message deletes itself. Some apps do notice if a screenshot was taken and tell all participants if this happened. If this happens once, the sender knows that the recipient is not trustworthy and should not send any further messages. Before knowing if the recipient is trustworthy, the sender could split up the message in a few batches in a way that every batch alone does not make sense if someone was reading just that single part. After each part was read and deleted, the next part would be sent. But in that case, you might as well record the screen with another camera, so the app wouldn't notice.

Every single safe systems major flaw are the people who operate them.

Method 1: Sand sculpture

The message is drawn on a plate or pane of glass with fine black sand on a white sand background. Then a plane of glass is pressed down and the edges are sealed. To erase the message, simply separate the plates and the sand pours out onto the ground.

Method 2: RAM

Although RAM has been shown not to immediately lose its contents when power is cut, overwriting data in RAM erases the previous value for all practical purposes. A dedicated message-viewing device could be constructed that, maybe after authenticating the reader somehow, displayed the message on an LCD display then overwrote its own RAM and displayed garbage data on the LCD, and then potentially catching fire for good measure.

For added security the entire message can be encrypted with a key known by Bob and a second "plausible deniability" message with a different key also be embedded in the device.

Method 3: Spontaneous teleportation

Ok, this requires teleportation technology to be discovered. However, essentially, the message is teleported away from Bob once he reads it to a secure location where it can be archived or destroyed.

One way to achieve this is for Bob to carry a pocket teleporter with him, which has a built in magnifying lens viewer, then teleport the message into the teleporter to send it to him, and teleport it away once he has had sufficient time to read it.

If it's the kind of "lock onto an object" type of teleportation technology (a la Star Trek) then the message could teleport itself away, or a remote teleporter could lock onto it and take it.

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