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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.
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.
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.
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.
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.
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.
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).
Iff we have achieved all this, then the question becomes mundane. A few possible methods...
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.
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.
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.
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.
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.
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.
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.
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.
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...
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!
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.
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.
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.
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:
There are three basic principles to make type 3 work:
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.
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.
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.
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.
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.
