Category: Encryption

Numbers Stations

You can be forgiven if you have dismissed radio as something we can consign to the dustbin of history in this modern digital age of interconnectedness. And yet I wonder if everyone doesn’t perhaps have some fond memory of a time when they played with the tuning dial of a radio to try and find the strange broadcasts that they knew were out there. At least those of you of a certain age will likely have a memory like this. Or in any event I do.

More fascinating still though is the shortwave radio. I remember my grandfather had a portable(ish) radio that featured a display that hinted at broadcasts that might be originating not from the hill on the edge of town (you know, the one with all the radio towers on it) but from distant countries. It was a radio that could pick up, in addition to the usual AM and FM signals, shortwave broadcasts.

Shortwave is a whole different beast from our standard radio broadcasts. Radio signals (and I’m going to get a bit technical now) cover a wide spectrum — just as a piano’s keyboard covers many octaves. And yet what we pick up on AM or FM are but just a few notes on that piano. Television frequencies might be another note, the frequencies used cellular phones another; the frequencies used by radio-controlled models another. You get the idea.

So shortwave represents another set of notes off in another area of the keyboard (sticking with the piano metaphor).

And yet radio frequencies are not all alike. Certain frequencies, it was discovered over a hundred years ago, actually bounce off our ionosphere, many miles above the surface of the earth and therefore can travel very long distances (the more familiar AM and FM frequencies can not perform this feat). It is the unfortunately named “short wave” frequencies that seem so good at traveling so far.

And so it is that certain nationalities have taken advantage of shortwave frequencies to send, apparently, coded messages to their spies abroad in the world. Since anyone with a shortwave radio can receive these signals, it would obviously be foolish for the secret organizations to broadcast clear messages that spell out the orders for their spies.

Instead, people who have made a hobby of listening to shortwave have discovered broadcasts they refer to as “numbers stations”.
These number stations don’t exactly broadcast orders, they broadcast, as the name suggests, just numbers.

What do these numbers mean? Do they tell the spy a date or time? Or a frequency? Do they refer to a dead drop location? Perhaps they refer to which page to use of a one-time pad?

Adding to the intrigue are the sometimes eerie way the numbers stations sign on or present their numbers. Here is an example.

Yankee Hotel Foxtrot:

Many other examples can be found here: The Conet Project

If you have a shortwave radio (or know someone who does) spend a little time exploring with it. Due to the distances that the radio waves must travel and considering that they travel nearly to the edges of space, shortwave broadcasts are temperamental. Consider that activity on the surface of our sun will affect what you will be able to hear. And night time reception is better than day time reception due to the heating versus cooling of the air in the atmosphere.


“Decoder rings” and children’s “code wheels” for sending and decoding secret messages generally used a simple method of encrypting (encoding) the alphabet by simply swapping one letter for another. Such a cipher is known as a Caesar cipher— named for Julius Caesar who is known to have used a similar type of code in his correspondences.

These types of ciphers are famously easy to “crack”. Even children are able to figure out quickly that a letter by itself is almost certainly the letter “I” or “A”. Repeated letter sequences that are three letters long might be the word “the” — especially if the first two letters (“th”) show up at the start of other words (“this”, “that”).

More clever children (who perhaps have played the game Hangman) might apply statistics — knowing that the vowels (especially the letter “e”) are the more common letters in English.

One simple such cipher involves replacing each letter in the alphabet with the letter thirteen places away (if you go off the end of the alphabet you wrap around back to the other end). This cipher is often called a Rot-13 cipher since you are “rot”-ating the alphabet thirteen places.
Rot13_vertical_tableSince there are twenty-six letters in the alphabet you don’t have to worry about remembering whether you are to count the letters to the left or to the right since the code works just as well in either direction. In fact, for the Rot-13 table to the right, I needed to only show the first thirteen letters on the left and the remaining thirteen on the right. See if you can figure out how to use it.

Here is a simple encrypted message you can try to decipher:

frperg zrffntrf

Even a short message like the above though can be tedious to decipher. Here is a text field you can enter text into and encode (or decode) using Rot-13. You can type either the original, unencrypted message in, or the Rot-13 encoded message. Clicking the Rot-13 button once will transform the text, clicking again will transform it again (in effect reverting it).

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