The early days of espionage were devoid of the high tech glamour that we're accustomed to seeing in spy thrillers. For decades, secret messages have been broadcast across the airwaves for anyone to listen to -- and they have.
Enthusiasts and conspiracy theorists have been following these broadcasts for almost as long as they have existed. With very little information to go on, they have dubbed these broadcasts 'numbers stations'.
But what are they, exactly? Who is sending these mysterious -- and often creepy -- messages, and why can't anyone seem to crack them?
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What Are They?
It's difficult to pinpoint exactly when numbers stations first came into use, but some believe them to have started during World War I.
Utilising shortwave radio channels suited for long distance communication, the theory is that they can be easily broadcasted globally, which eliminates a great deal of location-based risk. Both the sender and the receiver could be anywhere. Shortwave is also free of commercial traffic, so the message is unlikely to be interrupted.
There's no exact rule of thumb when it comes to the broadcasts, but there are some commonalities between many of them. Some of the most well-known begun with a piece of music or looping noise that are then punctuated by a string of numbers.
Here's a recording of one of the most well-known stations -- The Lincolnshire Poacher. Named for its use of the English folk song of the same name, it broadcast out of Cyprus from the mid-1970s to 2008.
You'll notice that the voice doesn't begin until almost a full minute into the recording.
How Do They Work?
Government and official agencies have rarely spoken on the record about numbers stations. However, it's generally accepted that someone is instructed to tune into a particular frequency at an appointed time so they can receive the message.
For normal listeners, all they'll hear is a random strings of numbers. But it's assumed that the intended receipient would use a one-time cryptographic pad to decrypt the message.
Are They Still Used?
Technology may have come a long way since the original inception of numbers stations, but this hasn't resulted in their irrelevance. In fact, it's their simplicity that has contributed to their longevity.
This is mostly because of the aforementioned one-time pad. Also known as the Perfect Cipher, it is a cryptoalgorithm where plain text is combined with a random key that can be used to crack it.
The beauty of this is that key is both random and used only once, which makes cryptoanalysis -- cracking the code for later use -- impossible. This is why hiding in plain sight doesn't seem to concern whoever utilises this method of message sending; there's no way for an ordinary listener to decipher the contents.
The setup also remains attractive -- you can send a message from thousands of kilometres away from the attended target with little risk of being traced.
This is why they are still in use today.
In fact, South Korea reported in mid-2016 that their North Korean counterparts had started using this method again, under the guise of a distance learning broadcast on mathematics.
"Now we'll begin a mathematics review assignment for members of the 27th expeditionary unit of the distance learning university... Turn to page 459, question 35; 913, question 55; 135, question 86."
Another famous example is Russia's UVB-76 station, which broadcasts on the high-frequency 4625 kHz band. Nicknamed The Buzzer for its distinctive, repetitive noise, it was first discovered in 1982. It has been known to be occasionally permeated by music or a Russian voice.
This station has drawn particular interest due to activity on the channel increasing long after the Cold War was over. In fact, it hit a peak in 2010 and followers of the channel who had triangulated the location noticed that the signal had moved.
The original site has even been discovered by enthusiasts, some of whom found log books.
Since this particular station resided in a bunker at one stage, it is generally accepted to involve the military, though no one knows to what end. One theory is that it acts as a failsafe for Russia's Dead Hand nuclear system -- if it ever stops transmitting a nuke, or the country's entire arsenal of nuclear-tipped weapons, will automatically activate.
Can I Listen To Them?
Absolutely -- anyone who owns a shortwave radio has the ability to tap into these fascinating broadcasts. There are a plethora of websites that have taken the guesswork out of finding the frequencies, such as Priyom and Global Frequency Database.
Don't have a shortwave? No problem -- it's also possible to listen online in real-time, with a bit of trickery.
You can also find recordings all over the internet, if you're not particularly interested in hearing the broadcasts live.
You can even have a crack at working out what they mean, but you certainly wouldn't be the first. Considering the almost full-proof system they adhere to, the secrecy of numbers station broadcasts are likely to remain as mysterious as ever.