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I am an alien AI probe orbiting Earth. I know your civilization. I have been watching you for millennia. And today I decided to talk to you for the first time. How it happened is not important.

I know your major languages (more or less, since you can read this) and I can use every form and means of communication you want. But...

  1. I do not want to enter the atmosphere of your planet
  2. I want to speak with only one interlocutor who speaks for all of you: a commission or an individual ambassador. It is up to you.

Now, after the first contact, you chose for your safety to disclose the first-contact information, yet you also want to keep the communication with me safe. No one must be able to intercept the communications, entering and leaving. I agree and I can ignore every unwanted call or visit.

What is the best way for you to do it? You can use all the technology you already have (in orbit or not) or build something ad-hoc.

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    $\begingroup$ Define "no one must be allowed intercept". This greatly affects the solution. 1) Are third parties allowed to know that there is communication? 2) If yes, are third parties allowed to know what kind of communication there is? 3) If yes, are third parties allowed to know when communication is happening? 4) I will assume that the contents must be kept secret at all times. $\endgroup$ – MichaelK Mar 14 '17 at 11:39
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    $\begingroup$ "one interlocutor who speaks for all of you" <- that's where your story becomes unlikely :D $\endgroup$ – Secespitus Mar 14 '17 at 12:43
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    $\begingroup$ Why mustn't anyone listen in? Why would you trust the interlocutor with your secrets? In Clarke's Childhood's End communication with the Overlords was open (and quite dull). The few in-person meetings with the Interlocutor did not discuss secret material nor nefarious strategy. $\endgroup$ – user535733 Mar 14 '17 at 14:03
  • $\begingroup$ @user535733 You are an easily impressed species and it is easy to imagine misunderstanding when you talk to 7 billion of individuals. $\endgroup$ – Lupetto Mar 14 '17 at 14:10
  • $\begingroup$ @Lupetto is the risk high for misunderstanding? If so, why intiate contact at this time? Why not wait for the species to grow up a bit more? $\endgroup$ – user535733 Mar 14 '17 at 14:13
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Since what you want is secure comunication, I think you should generate a pair of encryption keys. That way, even if someone does intercept the transmission between you and the embassador or commission, it will be encrypted anyways, and this should guarantee that you and your interlocutor can speak freely. As for how to convey the message, well, you could piggy-back on the ISS comms.

Being an alien AI you might not be familiar with the concept, so i suggest a visit to the folks over at Security SE and Cryptography SE

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    $\begingroup$ This is the right answer, to clarify, both parties generate a public/private key pair. The alien sends out the public key. The humans designate the representative and publish a public key. The first message from earth is encrypted with the aliens public key so only ser can read it. And signed with the human representative's private key. The alien can read the message and verify that it was sent by the representative. The alien can craft a response that only the representative can read and sign it likewise. After which symmetric keys can be securely exchanged if computation limits warrant. $\endgroup$ – jorfus Mar 15 '17 at 3:19
  • $\begingroup$ "... Here, Terran, is my secure 256 bits long RSA key ..." ;-) $\endgroup$ – Edheldil Mar 15 '17 at 8:56
  • $\begingroup$ @jorfus While this might be the correct way to have a secure conversation - how will the AI know whom it should choose to have the conversation with? E.g., if some faction does not agree on the chosen representative, then either Earth will never reply (as there will be permanent argumentation regarding who will represent Earth) or the disagreeing factions will simply publish their own key and try to beat the others at establishing contact with the AI. What if the other faction send an signed and encrypted message first, which states "do not trust the other ones claiming to represent Earth"? $\endgroup$ – Mrkvička Mar 15 '17 at 13:19
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Teach the chosen interlocutor your language.

If it is sufficiently alien, it should be undecipherable to other humans.

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    $\begingroup$ This is no guarantee and is certainly far more time consuming than just using encryption. $\endgroup$ – Shufflepants Mar 14 '17 at 15:53
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    $\begingroup$ @Shufflepants maybe, but there is historical precedent for this. The United States used the Navajo Code Talkers during WWII for communications because their language wasn't written down so even if the communications were intercepted, they couldn't be deciphered. It would take time to teach a few humans the language necessary for communication, but it would be nearly undecipherable for anyone else. $\endgroup$ – Cody Mar 14 '17 at 16:14
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    $\begingroup$ @Cody Yes there's precedent, but they only did that because their radios were analog (micro-circuitry not having been invented yet) and so they had no way to implement encryption on their radios for real time fast communication. Notice that they don't do that anymore, they use encrypted digital signal radios now because it's easier and much harder to crack. $\endgroup$ – Shufflepants Mar 14 '17 at 17:59
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    $\begingroup$ @Jorfus Translating a known language is totally different from decrypting an unknown one. And yes I deliberately chose to call it 'decrypting'. If you truly have to start from scratch, if you truly have no context whatsoever then I don't think there's any sensible place to start. We needed the Rosetta Stone giving us a starting point to decipher Egyptian hieroglyphics because it was so totally different from the living languages. $\endgroup$ – Falc Mar 15 '17 at 7:22
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    $\begingroup$ @Cody Compulsory XKCD on Navajo code talkers $\endgroup$ – Mrkvička Mar 15 '17 at 10:04
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The same way mission control communicates with people in space, via an encrypted radio transmission.

Although we on earth can communicate with the ISS using ham radio, mission critical communications are handled on an encrypted channel.

You, as the visiting alien should be able to handle this. Or you can drop in on the ISS itself, where crew members will be available to facilitate your call.

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  • $\begingroup$ :D - You're right, I was too broad. Edit in bold. $\endgroup$ – Lupetto Mar 14 '17 at 11:32
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Use a highly directional form of transmission once you've selected your interlocuter. Ideally, you'd get as low as you're comfortable going and use a laser to communicate directly down. You'll also want to shield this laser so that its source can't be seen, so create a tube (drill through a piece of asteroid, if you have to) to shield it from all other directions. Feel free to also use the other answers, but you can't decrypt what you can't see or hear, and that's what makes laser-based communications so secure.

Quick math says that if you're in LEO (160km), your starting point for a horizon is 1437km (900mi / roughly the size of the Louisiana Purchase) but this only matters if you're communicating with a transmitter that is completely non-directional. If you're using a laser, you can dictate how large the area that can see your transmission is, down to just a few square feet. If you had a specific enough setup, you could probably make your transmission visible to just one eye of one person, but I'm not sure how to determine what you'd need in terms of a laser array or collimater at that point.

A downside to this is that maintaining station over a single point above the earth takes a huge amount of delta V (rocket fuel). If you're an AI made by an alien race that has reactionless drives, you don't care about this.

If propellant use is an issue, you can go up to geostationary orbit and still do this, but you'll have to aim much more carefully.

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  • $\begingroup$ Sounds cool, I need to know more. Yet I do not quite understand the shield-thing. $\endgroup$ – Lupetto Mar 14 '17 at 15:39
  • $\begingroup$ Best way to picture it is pick up a flash and it point it at the ground. It lights up a certain area of the ground, right? Now tape a paper towel roll to the head of the flashlight and it lights up a much smaller area. The paper towel roll is acting as a shield or "gobo" (goes between) in photography jargon. The shield shouldn't really be necessary if you're using laser communications, but it would help to prevent any odd reflections that someone might use to read the signal without being in the area that's lit up. $\endgroup$ – William Mar 14 '17 at 19:04
  • $\begingroup$ @Lupetto - I should clarify that this really only works at absurdly high frequencies such as in the terahertz range, which happens to be where light is. You can't do this with an HF/shortwave radio without some serious scaling problems. On the bright side, humans have an intuitive understanding of how light works, so it's a lot easier to visualize, and it's not really hard to make large amounts of light. Still, you probably want to use a laser (coherent light) and it wouldn't hurt to change frequencies every so often to mess with people trying to listen in. $\endgroup$ – William Mar 14 '17 at 19:08
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As structure created by a highly developed civilization you probably know that in age when quantum computing is arising classic cryptography is not reliable anymore, because of computational complexity guarding cryptographical algorithms is dropping thanks to ability of quantum machines to inspect great quantities of states, exponential to number of its qubits

While new field of quantum cryptography is arising, giving much more reliable way to generate keys known only by communicating sides, not by man-in-the-middle. It based on the Heisenbergs uncertainty princible. Third person trying to observe quantum information during shared key generation process will necesarry affect this state, which could be noticed by both sides.

From engeneering site quantum communication is very hard on the Earth because of high density of matter, electromagnetic field fluctuations and high average temperature. In space, even on the Earth orbit we could at least keep necessary materials near to absolute zero in the superconductor state and send some quantum beams directly through space, fixing some fluctuation errors with error correction algorithms

I guess emerge of quantum computing could be the reason why you've decided to speak with us at this exact moment, when technology becomes available but only for few intelligent people

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tl;dr- Provide your human contact with as many bits of one-time pad cipher as they need. This results in perfect message secrecy; other humans won't ever be able to break it, even with hyper-advanced quantum computers.

Provide one-time pads

Encryption via a one-time pad is perfect and unbreakable, as long as:

  1. the one-time pad was generated using a process that attackers can't reverse-engineer; and
  2. each part of the one-time pad is discarded after use, never to be reused.

Humans have plenty of methods for generating high-quality informational entropy, i.e. one-time pads that can't be reverse-engineered. But, as an alien AI, you probably have crazy patterns and methods that we couldn't begin to hope to reverse-engineer, meaning that you're an excellent source of extreme-quality entropy.

Don't use normal encryption methods

The best encryption methods are pretty much unbreakable, we think, as far as modern human knowledge and technology go. But why bother? You can use perfect encryption via one-time pads.

Add junk data if you want

The one big information leak from one-time pad communication is the amount of information and time it was transmitted. This is, if someone sends you a message that's 100-bytes long, then spies can know that you received a 100-byte message. They wouldn't know what it says, but since they know it was sent, that's still technically an information leak.

So, add junk data to fill out messages up to some certain size, let's say 100TB. Then, all a spy knows is that a message was sent that was up to 100TB in size.

This would be impractical for humans, since getting 100TB of high-quality single-use entropy for each message would be really annoying and costly, but since you're an alien AI, it's probably basically free for you.

Schedule regular transmissions

Have your human contact send messages at regularly scheduled times, even if they've got nothing to say and just end up sending pure junk data.

This way, no one knows when you're actually sending messages. All any spy can tell is that they're not sending you more than the message size at the regularly scheduled rate.

Use a hidden transmission channel

Using the above tactics, no spy could ever read your communications or know when you're sending them. At most, they know that you're not sending more than, say, 100TB of messages every communication cycle; which, since that's already pretty much a given, doesn't matter.

But, if you want to go one step further, you could do a directional transmission channel. This would have the advantage of spies not even having the chance to realize that there is some sort of secret communication going on.

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If i were you i would send my own drone down to the surface and communicate through that, that way they would know for sure they are not being punked by a pirate radio signal and a kid with Mad FX Skills. Let the humans decide who steps in front of the drone.

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  • $\begingroup$ My zero-aggression policy makes that impossible. $\endgroup$ – Lupetto Mar 14 '17 at 13:02
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Here is an article on quantum communications that should fit the bill. China is putting up a satellite to test it soon.

The message isn't sent using quantum entanglement (we aren't there yet) but the decryption key is.

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