He tells NPR's Guy Raz that the stations are unlicensed, which makes it hard to figure out where they're broadcasting from. And the mystery only deepens: No government has ever officially admitted to using numbers stations. No one's really sure when the stations began broadcasting, though they're most likely a Cold War-era invention.
And, Stout says, no matter how advanced modern computer cryptography is, good old shortwave is often the best option for getting messages to spies in the field.
"Because [a message] can be broadcast over such an enormous area, you can be transmitting to an agent who may be thousands of miles away," he says. And, he adds, computer communications almost always leave traces.
"It's really hard to erase data out of your hard drive or off a memory stick," he says. "But all you need here is a shortwave radio and pencil and paper."
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