GUY RAZ, host:
If you have a shortwave radio, try scanning the dial some night and listen for something like this.
(Soundbite of recording)
Unidentified Woman #1: Three, nine, seven, one, five. Three, nine, seven, one, five.
RAZ: This is a recording of an actual transmission from a so-called number station. You can find these transmissions all over the shortwave spectrum. And usually it's a voice reading out an endless string of numbers - sometimes in English, sometimes Spanish, German, Czech, even Morse code.
And it's long been thought that these stations play an important role in international espionage. In fact, the Russian spies recently arrested may have been getting their orders from Moscow through those transmissions.
Mark Stout is a former CIA man. He's now the official historian for the International Spy Museum here in Washington. And he joins me in the studio. Welcome.
Mr. MARK STOUT (Historian, International Spy Museum): Thank you very much. Pleasure to be here.
RAZ: So, Mark, explain how these number stations work.
Mr. STOUT: Well, as you could probably hear from the clip we just played, they're not actually human beings. Typically these days, they're automated. They're voice-simulators.
Beyond that, it's all a little mysterious. These stations are unlicensed. So it's often not really clear where the physical transmitters are. And short waves bounce back and forth quite a number of times between the ionosphere and the Earth, which is what makes them easy to receive the signal at a long distance, but it also makes them hard to geolocate. So...
RAZ: But that was my question. I mean, shortwave, we think of it as such an antiquated technology, so why bother with shortwave?
Mr. STOUT: Well, there's actually a lot to be said from an espionage point of view for shortwave radio. First off, because it can be broadcast over such an enormous area, you can be transmitting to an agent who may be thousands of miles away.
Secondly, most of the modern communications techniques involving computers and that sort of thing leave behind traces. It's really hard to erase data off of your hard drive or out of a memory stick. But all you need here, however, is a shortwave radio and pencil and paper. And paper is easy to destroy and the shortwave radio in and of itself is not incriminating. Millions and millions of people around the world have them.
(Soundbite of recording)
Unidentified Woman #2: (Spanish spoken)
RAZ: And now we know there was a case of some spies here in the U.S. caught in the past few years spying for Cuba. They were apparently listening to a transmission in Spanish. Presumably, spies are told when to listen to these stations?
Mr. STOUT: That's absolutely right. They're given a schedule on which to tune in to a particular frequency. And the broadcast would usually include some sort of header, basically meaning the message is now starting, and then they'll read out the numbers. They'll write them down.
One of two things happens then, either they decrypt these messages literally using pencil and paper, which is far more secure. Interestingly enough, however, the Cubans have used a different technique of decrypting these using computer diskettes, which meant that the data went onto the computers of these individuals which turned out to be a security problem for them later when the FBI started entering their homes and searching their computers.
From a cryptographic point of view, it doesn't make any difference. But from a security point of view, you don't want to put this stuff on your computer.
(Soundbite of recording)
Unidentified Woman #3: (Foreign language spoken)
RAZ: How long has this been going on?
Mr. STOUT: Well, that's an interesting question. No one's really quite sure, at least since the early Cold War. There are some suggestions that this may have been tried during World War II. But those stories I've been unable to pin down. And frankly, I'm skeptical.
The underlying cryptologic technique of what's called a one-time pad is actually a technology that was invented in 1917 by a U.S. Army officer who, actually in his postwar life, went on to invent Muzak. But the number stations themselves appear to be a Cold War invention.
RAZ: Anybody can go to, as I mentioned earlier, to their shortwave radio if they have one and hear these. So what's to stop me or you from listening and then intercepting these messages?
Mr. STOUT: Nothing's going to stop you from intercepting them. You know, thousands of enthusiasts around the world do that on a daily basis. But just getting the transmission doesn't mean that you can read it.
And in fact, this one-time pad technology is literally the world's only unbreakable cipher system. You really truly cryptanalytically have no traction getting into a one-time pad system. None at all.
RAZ: So, if someone listening has a shortwave radio and they want to see if they can find one of these stations, where should they look?
Mr. STOUT: Well, you can just scan the dial and you're likely to find it. A better way, frankly, is to go to the internet and there are a number of websites of people who monitor these stations and you can go there and literally look up stations we expect are on the air right now and they'll give you frequencies to tune into. So it's actually really quite easy.
RAZ: That's Mark Stout. He's a former CIA officer. He's now the official historian at the International Spy Museum.
Mark Stout, thanks so much for coming in.
Mr. STOUT: My pleasure.
(Soundbite of recording)
Unidentified Woman #4: (German spoken)
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