This is one of the better known mysteries out there, but surely someone hasn’t run into it yet, or would like to know more about how it works.
Shortwave radio is not very popular in the United States*, having been supplanted by AM and FM. But there’s still plenty of places in the world that use it, and that makes it really useful for one thing: espionage.
Let’s say it’s 1979, you’re an East German spy, and you’re stationed in Bonn, West Germany. Your bosses back in Moscow East Berlin need to send you instructions. They can’t call you on the phone, even from elsewhere in Bonn; that could be tapped. Your mail could be read. But you have a shortwave radio, like every other person in your neighborhood, so that’s not suspicious in any way.
Of course, a radio only picks up what’s broadcast across the air for everyone to tune into, so they do it in code. Probably, they use a one-time pad. Let me give you a really short example of this.
Say I want to send the message “JOHANN” to my spies in Bonn, because in the field I have Johann, Ilse, and Werner, and only Johann needs to be undertaking a mission this week. Before everyone leaves Berlin, I give them a little pad of tiny sheets of paper, one for each day of the next three months, while they’re scheduled to be on location Bonn.
The piece of paper for this particular day starts with the following numbers: 213247690354.
I want to send the message “JOHANN”. So I take the place of J in the alphabet — it’s the 10th letter — and I add it to the first two digits of my pad. 21 + 10 = 31. O is the 15th letter; next two digits are 32, plus 15 = 47. H is the 8th letter, next two digits are 47, plus 8 = 55. A is the first letter; next two digits are 69, plus 1 = 70. N is the 14th letter; next two digits are 03, plus 14 = 17, and the last two are 54, plus 14 = 68.
So over the radio, on a predetermined frequency at a specific time, I broadcast someone speaking the words:
“Thirty-one. Forty-seven. Fifty-five. Seventy. Seventeen. Sixty-eight.”
All my spies are tuned in. They all write down the numbers they hear on the radio; they all subtract the pairs of numbers from their pads; they all get: 10, 15, 8, 1, 14, 14: J, O, H, A, N, N. Now all three spies know who’s on this week’s assignment. Obviously, I could make the message longer too, as long as I gave them enough numbers on the pad, and it could then contain instructions. “JOHANN, PLEASE BUY EGGS.” And of course, those instructions themselves might be code; “please buy eggs” might mean “kidnap the ambassador”.
Notice too that the decoding uses both the pad (which they got before they went into the field) and the broadcast message (which was not fixed until that moment). So as long as I use that day’s pad to encrypt my message, I don’t need to know in advance what the message for that day would be. I don’t need to know Johann will buy eggs on the 28th when they leave Berlin on the 1st with their pads; I can use that day’s sheet to encrypt any message.
You get the idea. Furthermore, I could simply start each message with a number indicating who’s on assignment that week, and then my other spies would stop listening as soon as they heard someone else’s number. That’s smart, too, because it means Ilse and Werner won’t spend 10 minutes writing down a message — perhaps being caught while doing so — only to find out it wasn’t for them.
But everyone in the entire region — possibly a thousand kilometers away or more — can pick this up if they tune in to the right frequency at the right time, and surely someone can decipher the message? No. Not without the pad they can’t. If used correctly, a one-time pad is proven unbreakable. If you put a computer on the task of finding possible meanings of our six-letter message, you could get “JOHANN” or “WERNER” (or any other string of six letters) depending on which sequence the computer tried, and if you don’t have the pad to know which one is right, you don’t know if Johann is supposed to buy eggs (whatever that means) or if Werner is supposed to kiss Bob. (BUY EGGS and KISS BOB both have the same number of letters.) And for that matter, since no part of a properly-randomized one-time pad can be predicted from any other part … maybe it’s Johann who is to kiss Bob, or Werner who is to buy eggs. Sorry!
Okay. So that is what numbers stations are for and how they work, in a nutshell. They get more elaborate, of course. Many of them became known for their “interval signal” — a short piece of music or other noises played at the beginning of the transmission, to give the listener a chance to get the radio tuned correctly. One of the best known of these was the Lincolnshire Poacher station, which used the melody of the old English folk song of that name as its interval signal.
Numbers stations are still in use. They were reported as early as World War I, so they’ve been around for 100 years. They come and go (some, like Lincolnshire Poacher, were attested for decades). At this very moment, somewhere in the world a numbers station is probably transmitting, and a spy is listening to the radio very surreptitiously, hoping someone doesn’t kick in the door and ask why they’re listening to a very boring show.
There’s a set of CDs called The Conet Project by Irdial Discs which contains a massive collection of spy broadcasts, and it’s available now, legitimately, for free at archive.org. Yay! Some people find them soothing to listen to; others find them creepy.
There’s other weird radio anomalies out there which are undoubtedly connected with espionage or military intelligence in one way or another; the most notorious of these is probably UVB-76, a.k.a. The Buzzer, a station which broadcasts from somewhere in Russia, playing a monotonous harsh “buzz” sound every couple seconds … but then sometimes a voice breaks in saying cryptic things.
The Cold War never ended. Sleep well!
* — There are some shortwave stations in the US, but I’ll say it bluntly: most of them belong to right-wing evangelical religious groups and “Patriot” movement types. You are vastly more likely to hear about the “evils” of all people who aren’t Protestant Christians, or to hear the rantings of Alex Jones, on American shortwave radio than you are to hear anything else.