How Numbers Stations Work

Shortwave radio is old technology. In the late 1920s it became a popular way to communicate over long distances, thanks in large part due to its propagation traits.

Shortwave radio energy is determined by the power of the transmitter. The bigger and more powerful the transmitter, the farther the signals travel. The physics of shortwave energy help, too, because it bounces off of the Earth's ionosphere.

The ionosphere is a region of space about 50 to 370 miles (80 to 595 kilometers) above the Earth's surface. It's an area of the upper atmosphere (or thermosphere) in which solar ionization takes place, causing an electrical charge. This charge reflects shortwave radio signals and bounces them back down to the terra firma, and in doing so, greatly increases the distance that the signal travels. This is called skip, or more fancily, skywave.

With a quality handheld radio, a person can tune into a strong shortwave signal from the other side of the planet. That's a useful characteristic for all sorts of communications purposes, and as it turns out, it's also great for spying.

If you're the head of an intelligence agency in the United States and you need to send a secret message all the way to an embedded spy in an area of conflict like Iraq, shortwave messages might be the best method. Power up an extra-powerful transmitter, broadcast your coded message at a predetermined time and frequency, and your agent abroad will immediately know whether to keep pretending to be a businessman or to assassinate a political foe.

And because shortwave radios are so common, the technology doesn't draw suspicion. An agent who is apprehended with a radio has plausible deniability on his side, whereas carrying specialized computer equipment and cryptography software is a red flag that could result in imprisonment ... or worse.

Shortwave is also immune to oppression. In times of strife, authorities can literally switch off a country's Internet access or kill the power to communications satellite. They can't stop every shortwave transmission.

What's more, computerized messages, even those created with supposedly crack-proof encryption, can eventually be traced and broken. In stark contrast, no one can trace who is receiving shortwave radio messages. Often, it's hard work even finding the powerful transmitters.

Because shortwave broadcasts are so easy to tune into, anyone with a cheap radio can eavesdrop by finding the right frequencies. That includes you. Many frequencies are listed at various web sites like SpyNumbers.

What you'll hear varies by station. Some stations broadcast continually. Others spring to life at scheduled times. Most keep strict schedules so that agents know when to tune in to the right frequency. On the next page you'll see more about what, exactly, numbers stations are sending through the airwaves.

The broadcasts you'll hear on numbers stations vary. They generally begin with an alert signal of sorts at the top or bottom of the hour. This alert may be a simple tone, or it can be fragments of song, such as with the famous Lincolnshire Poacher station, which begins broadcasts by playing several bars from the well-known tune of the same name. This indicates to listeners that a message is about to begin and also helps the user tune his or her radio for the best reception.

What follows the opening refrain is usually a set of spoken numbers or letters, often four or five at a time, which are repeated again and again, before moving on to a new set. Depending on the station's origin, the code may be spoken in English, Korean, Czech, Spanish or other languages.

There are a lot of variations on the same structure. Many stations have received colorful nicknames that reflect some defining characteristic of the station. For example, one is called Swedish Rhapsody because it begins with a fragment of that song. Other well-known stations include The Buzzer, Cherry Ripe, The Spanish Lady, Atencion and Yosemite Sam.

Sometimes broadcasts come in loud and clear. Sometimes atmospheric conditions degrade the signals. And sometimes, intentional jamming on the part of opposition countries creates enough interference on the same frequency that it renders the transmission difficult or impossible to understand.

The coded structure of the messages almost certainly indicates that these messages are secret and intended for spies avoiding detection. The question, then, is how do spies understand the latent messages when no one else can?

The manner in which these bizarre broadcasts are encoded is the linchpin of an unbreakable encryption system. Keep reading to see how these codes work.

Enigmatic and mysterious numbers stations broadcasts are weird to the point of being spine-tingling. What makes them perhaps even more compelling is that they're still in use, decades after the world wars that sparked their rise. They've survived the communications revolutions because unlike so much digitized code, these transmissions are unbreakable.

You may scoff at the idea. If codes generated by supercomputers can be hacked, what's so hard about deciphering a few numbers? It is hard. Actually, it's nearly impossible. Done properly, no one will understand the message but the intended recipient.

It works because of the one-time pad concept. In this system, both the sender and receiver have a single copy of paper (or other media) filled with random digits or numbers. Using a key that corresponds to the pad, the recipient can figure out what letters the incoming characters or digits stand for.

Crucially, each key is used only once, thus the name one-time pad. Both the sender and receiver immediately destroy the key at the end of the broadcast, meaning that even if someone else intercepts the series of numbers, they'll have no way to decode the hidden message.

This also means that there's no relationship between past or future messages, so there's no pattern for cryptographers to use in breaking the code. Each new key and message are randomly generated, and done properly, the system is the only way to perfectly encrypt a transmission.

To fool the ingenuity of the best cryptographers, however, the key and message must be truly random. And as it turns out, creating really random sequence is challenging, even for computers. Sans complete randomization, there may be a pattern for experts to detect, and they can crack at least part of the message. Sadly for codebreakers, because of the one-time pad concept, their hard work won't help them decipher future messages.

To make things more confounding, stations may send mostly dummy broadcasts that mean nothing at all. This forces adversaries to expend far more resources attempting to figure out which messages are real and which are fake.

Once in a while the broadcasts are less than professional. The person reading the code may make mistakes, or there could be laughing in the background. Cuban numbers stations in particular are known for their hilarious missteps -- sometimes they have such poor transmission that they're impossible to hear, or they accidentally pipe music from Radio Havana onto the airwaves.

In the one-time pad system, the key is of utmost importance. If the wrong people get their grimy hands on a key, they could potentially figure out which broadcast to monitor and then use the key to decipher a message that was meant to be used against them.

This also means that the key presents a great risk to spies. If they're caught with a sheet of paper covered in random numbers that look like a secret code, they are likely to have a very, very bad time in the hands of the authorities. Thus, keys may be printed on tiny sheets of paper that are easily hidden in obscure everyday items. In the event of discovery, they're also easily eaten, set on fire or discarded.

Those kinds of extremes are probably best for Hollywood, though. These days, it's more likely that the key would be sent via some sort of digital service or device without arousing much suspicion.

Furthermore, it's likely that most spies would use more modern means of communication, if only for their expedience. But judging from the fact that governments still pay to maintain numbers stations, there are plenty of uses for their strange, monotonic messages.

A few stations may have illicit uses. One theory guesses that drug smugglers and other criminals use this system to skirt international authorities. This is especially true in the U.S., where many intercepted signals are broadcast in Spanish. The consistent sloppiness of these particular transmissions is a hint that they are lacking government oversight and execution.

But the best bet is the most obvious – numbers stations are still mostly used for the purposes of espionage. There are some very public and recent instances as evidence.

In 2001, Ana Montes, who worked for the U.S. Defense Intelligence Agency, was arrested and convicted of spying for Cuba. Investigators searched her home and found a shortwave radio, along with a code sheet that was used to encrypt radio transmissions.

In 2011, German authorities arrested Andreas and Heidrun Anschlag for being Russian spies. The married couple moved to Germany in 1988, just before the fall of the Berlin Wall, and remained undercover there until their arrests. When authorities burst into their home, Heidrun was in the middle of receiving a shortwave transmission.

Some experts were bemused by the fact that these spies were relying on such antiquated equipment in the days of satellites and email. Their continued existence, though, is proof that they're effective.

Numbers stations have mostly spoken their otherworldly messages in obscurity. Save for a few intermittent news headlines and the 2013 John Cusack movie, "The Numbers Station," popular culture hasn't really attached many conspiracy theories to these strange transmissions.

In 1997, Irdial Discs released "The Conet Project," which was a four-CD compilation of numbers stations recordings, along with an extensive booklet that speculated about the purpose and origin of the broadcasts. At first the project found little media attention, but "Conet" now has a strong cult following amongst artists, conspiracy theorists, musicians and shortwave radio lovers.

"The Conet Project" arrived in the pre-Web days, long before search engines enabled casual surfers to research obscure topics with just a few keyboard strokes. Enough people were intrigued (or perhaps disturbed) by the things they heard on the CDs that at least one government agency was prompted to offer a statement, possibly to tamp down on further inquiries.

That statement came from a spokesperson from the United Kingdom, and it was appropriately vague. This statement said that the numbers stations are exactly what people think they are. "People shouldn't be mystified by them. They're not, shall we say, for public consumption" [source: Segal].

In the history of numbers stations, this is the nearest a government official has come to unveiling the exact purpose of the broadcasts. Considering that the stations are public knowledge and that hundreds or thousands of people must have worked in or around the transmissions, the stations continue to be obscured by an unusually ironclad secrecy.

The quantity of numbers stations and transmissions dropped dramatically after the end of the Cold War. Lessened political tensions and improvements in other technologies left fewer people relying on shortwave communication.

Still, the numbers stations broadcasts continue. Are they feeding invaluable information to spies who carry out vital missions of the cloak-and-dagger variety? Or are they black budget holdovers from a bygone era, impossible to do away with and simply transmitting useless noise into the void?

From the headlines about cases such as Ana Montes and the Anschlags, it's pretty clear that shortwave radios are still used for spying. But it's impossible to know just how many countries continue to actively use these systems. It could be that they simply broadcast mostly gibberish, as part of a strategy to lull enemies or distract them with decoys.

It could also be that governments mostly maintain the stations for readiness purposes, just in case they might be necessary in emergencies. This continuity not only provides some fallback technology for military or espionage needs, but it also keeps numbers stations workers employed ... and less likely to break their silence and sell secrets to a media outlet.

In the broadest sense, though, the days of numbers stations are probably, well, numbered. The steep decline in the stations following the end of the Cold War was likely the most significant indicator that these stations aren't needed as much as they were during a period when superpowers routinely clashed all over the globe.

As communications technologies continue to evolve and digital encryption gets harder to break and harder to trace, these strange shortwave broadcasts may eventually find themselves relegated to the dustbin of the spy trade.

Or perhaps the opposite will occur. Maybe some catastrophic digital apocalypse will push numbers stations operators back onto the frontlines of a shadowy war, one where a mechanical voice bleats bizarre codes into the airwaves around the globe, spurring action that will forever change human history.