Edward Snowden, the former contractor who leaked National Security Agency secrets publicly in 2013, is now getting attention for an odd subject: aliens.

In a podcast interview with astrophysicist Neil deGrasse Tyson, Snowden suggested that alien communications might be encrypted so well that humans trying to eavesdrop on extraterrestrials would have no idea they were hearing anything but noise. There's only a small window in the development of communication in which unencrypted messages are the norm, Snowden said.

"So if you have an alien civilization trying to listen for other civilizations, or our civilization trying to listen for aliens, there's only one small period in the development of their society where all of their communications will be sent via the most primitive and most unprotected means," he said.

But those holding out hope for contact from extraterrestrials can breathe easy: Humanity's current search for alien intelligence doesn't rely on an intelligible message, say scientists with the SETI Institute, which is dedicated to the search for life in the universe. The real hunt, they say, is for the medium.

"We're not looking for the message," said Seth Shostak, director of the SETI Institute's Center for SETI Research. "We're looking for the signal that tells us that somebody has a transmitter." [13 Ways to Hunt for Intelligent Aliens]

Signal received

To be fair, Snowden was speaking off-the-cuff about encryption in general; it's not likely he expected to be chatting about aliens or has done an in-depth study of how the search for extraterrestrial intelligence has evolved.

But data encryption is beside the point, Shostak said. So far, most of the hunt for alien signals has used radio waves, based on the theory that radio is a relatively easy and cheap way to send signals a long way through space.

The SETI Institute uses powerful radio telescopes on Earth to search for narrow-band signals, or signals focused at one spot on the radio dial, Shostak said. Lots of natural bodies make radio noise, he said, but the only thing that makes a narrow-band signal, as far as scientists know, is a transmitter.

Thus, a focused band of signal is a waving flag, signaling, "Hey, there's somebody out there who can build a radio transmitter," Shostak said. The message itself might be indistinguishable from noise if it were well encrypted, but it would still, obviously, be a message.

Eavesdropping on aliens

In his comments, Snowden went on to suggest that if humans overheard aliens communicating amongst themselves — if humans were to pick up the alien version of a telephone call or television broadcast — it might be so well encrypted that it would be invisible among the radio chatter of the natural universe.

But that's not necessarily the case, Shostak said, because even general broadcast signals would have narrow-band components that humans might notice.

At the moment, the question is largely moot, said Doug Vakoch, a researcher at the SETI Institute in charge of interstellar message composition. (Yes, this means he's in charge of thinking about how to talk to aliens.) The technology is simply not there to overhear broadcasts not directed at earthlings, Vakoch told Live Science. [Incredible Technology: How to Search for Alien Civilizations]

"Even our radio and television signals that are streaming off into space would be undetectable by us if they were out at the nearest star system beyond Earth," Vakoch said.

In another few hundred years, technology might develop far enough so that eavesdropping over mind-bendingly long distances might be possible, Vakoch said. In other words, Snowden's conjecture about encryption could pose problems for people searching for alien life hundreds or thousands of years from now.

But encryption isn't the biggest challenge for eavesdropping earthlings.

"If another civilization wants to conceal its identity, it doesn't even have to worry about encryption," Vakoch said. "If you look at telecommunications as it is developing here on Earth, we have been noisy in the past. We had a lot of TV and radio going out into space. Now, as we shift to communication by fiber-optic or by telecommunication satellite, there is less of this leakage going off into space."

Thus, if an advanced civilization is looking for us, it does indeed have a short window to do so — regardless of encryption. That means any alien civilization that overhears earthlings or reaches out to us is likely to have been listening and transmitting for thousands, or even millions, of years, Vakoch said. Otherwise, it's simply too likely that earthlings and aliens will miss each other in the vastness of space and time. For now, the best hope remains searching for direct messages sent deliberately by clever extraterrestrials, Vakoch said. [10 Alien Encounters, Debunked]

Conversing with E.T.

If aliens were sending a direct message, they'd probably put a bit of thought into how it might be read. Humanity certainly has.

In 1974, scientists beamed a radio message to space from the Arecibo Observatory radio telescope in Puerto Rico. This "Arecibo message" was a 3-minute data broadcast of 1,679 bits that can be reconstructed into an illustration of 73 lines made up of 23 characters each that looks like something out of an Atari game. In fact, it's a representation of the numbers 1 through 10; the atomic numbers of several elements important to life on Earth; information about DNA; a representation of the human form; a graphic of the solar system; and a graphic of the transmitting telescope.

Scientists composed the message with care. There are only two numbers that can be multiplied to give you 1,679, Vakoch said, and both are prime numbers (73 and 23), which should give a smart E.T. a hint at the message's dimensions. (If they've figured out radio waves, they probably know something about math, Vakoch said.) Aliens might still be flummoxed by the message's content, but they'd be able to see symmetrical forms in the correctly reconstructed image, letting them know they were correct in putting it together.

"We want to do anti-cryptography," Vakoch said. "We want to create a message that's as easy as possible to decode."

The ideal message to space would also be anti-efficient. Instead of transmitting data once in a small form, Vakoch said, you'd want to make it redundant and draw the conversation out so that your listener would have some way to check his or her (or its) work. Redundancy would also help correct for any errors that might occur during transmission over long distances.

"The really nice thing about that exchange between Neil deGrasse Tyson and Edward Snowden is it raises the whole issue about how and why we might want to encode information," Vakoch said.

Other ways to find life

Radio isn't the only way to find aliens, though it remains the cheapest and most promising at the moment. There are SETI experiments searching for flashing lights, whether visible spectrum or longer-traveling infrared, Shostak said.

"The thing about that is, they have to be deliberately aiming your way with the light, because otherwise, it's very expensive," he said. "And I assume that Klingons worry about cost."

Light also fades over shorter distances than radio waves obscured by interplanetary dust.

Another possibility, though still out of reach of Earth technology, is scanning exoplanet atmospheres for signs of life. It's tricky, Shostak said, because many unnatural emissions that would signal life cause problems. Humans, for example, only dumped chlorofluorocarbons (CFCs) into the atmosphere for a few decades before realizing the damage to the ozone and reducing CFC emissions. Fossil-fuel use sends high levels of carbon dioxide into the atmosphere, but high levels of carbon dioxide aren't fingerprints of intelligent life, Shostak said — you can find tons of carbon dioxide on Venus.

One telling bit of galactic trash might be heat, Shostak said. It's possible that an advanced civilization running a huge amount of machinery would give off more infrared (heat) signals than normal. However, right now, humanity can only detect anomalous infrared in huge amounts, comparable to the entire star shine of a galaxy, Shostak said.

Recent exoplanet discoveries have heightened hopes that intelligent life is out there, though. Research published in 2013 in the journal Proceedings of the National Academy of Sciences estimated that some 22 percent of stars have Earth-size planets in their habitable zones.

"It's not one in a million," Shostak said.

The discoveries don't tell you where to point the antennas, he said, but they suggest it might not matter where you point them. Life could be anywhere. And there's always the chance humanity will stumble across it by accident.

"It's not impossible that the first discovery of intelligence will be something you didn't expect to find," Shostak said.

Editor's Note: This article was updated to change the word "encode" to the correct term "decode."

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