Is This How Edward Snowden's Back-Up Files Are Encrypted?

A couple of plausible theories
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Now we have a bit more clarity on what Edward Snowden meant last week when he said, "The US government is not going to be able to cover this up by jailing or murdering me. Truth is coming, and it cannot be stopped."

It turns out Snowden has given copies of the files he purloined from the US National Security Agency, his former employer, to "many different people around the world," according to Glenn Greenwald, the Guardian journalist who first published some of the materials provided by Snowden. But the files are encrypted, so the people who have the documents can't read them. "If anything happens at all to Edward Snowden, he told me he has arranged for them to get access to the full archives," Greenwald told the Daily Beast.

How might that work? Snowden could be using any of a number of complicated cryptographic gambits.

Cryptography is a gatekeeper. It allows us to check our bank accounts, sign into email, and browse Facebook without worrying that any of that data can be intercepted by others (the NSA surveillance revealed by Snowden notwithstanding).

The simplest form of cryptography that Snowden and his allies could be employing would involve one person keeping an encrypted copy of the files and someone else holding the key necessary to decrypt it. But that method is vulnerable, relying on the trustworthiness of the person who has the key, and it doesn't sound like what Snowden has done.

More complex, more secure, and more interesting would be a form of "secret sharing." Essentially, the files can only be unlocked if each member of a group shares his portion of the encrypted information; or, alternatively, if several people are given encrypted portions and a combination of, say, any three of them is sufficient to unlock the files. For instance, this illustration represents the data shared by three people as intersecting planes; the point where they intersect represents the secret that is unlocked when they're shared:

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It's obviously more complicated than that, and there are other methods Snowden could be using. How well he knows advanced cryptography also isn't clear, but he's receiving assistance from Wikileaks, which is skilled in it. Wikileaks has also used its own form of "insurance files" in the past.

Snowden remains in diplomatic limbo in the transit area of Sheremetyevo airport in Moscow. As to what his insurance file contains, beyond what has already been revealed by the Guardian, Washington Post, and South China Morning Post, we have yet to find out; Greenwald told the Daily Beast that just the non-encrypted material Snowden gave him consists of "thousands of documents."

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Zachary M. Seward is a senior editor at Quartz. He previously worked at The Wall Street Journal and Harvard's Nieman Journalism Lab. He teaches digital journalism at NYU.

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