What if I were to tell you that at as you read this newsletter, a hostile nation’s intelligence service is beaming secret messages to one of their agents here in the United States. What’s more, you can listen to it.

In another quick-read lesson on real-world intelligence activities I’ve come across in my research on WWII spies and saboteurs, I introduce you to “number stations.”

Number stations are shortwave radio transmitters that broadcast coded messages to a nation’s spies in foreign countries. Because they are transmitted via shortwave, anyone with a shortwave receiver can listen to them. But only those agents they are meant for can read them.

Warring nations began using number stations during the First World War. By the Second World War nearly every house in America had a shortwave receiver so families anywhere could tune in to popular entertainment broadcasts originating from distant coastal studios. The ubiquity of these receivers made them the perfect means of secretly communicating with a deep cover agent.

That communications work like this: Once or twice a week at an appointed day and hour, an agent tunes into a particular shortwave frequency. A prearranged tune tells the agent the message that follows is for him or her. A voice reads the coded message which usually comprises several groups of numbers, with four or five digits in each group. Each group is repeated at least twice.

Once received, the agent decodes the message. This can be done with a “one-time pad” on which each page contains a new code. Both the sender and receiver have identical packets. Once a code is used, it is removed from the packet and destroyed. Since each code is only used once, this form of enciphering is considered unbreakable. (I used one-time pads myself in the military.)

The agent may also use a “book code.” In this case, both sender and receiver have identical copies of the same book, be it a novel, the Bible, or a text book. The first numbers in the message indicate the page of the book. Subsequent numbers indicate a line on that page, and which word on that line is part of the message.

The most famous book code used in WWII is the “Rebecca Code.” German spy Johannes Eppler, working undercover in British-controlled Cario, Egypt, used Daphne du Maurier’s novel Rebecca to encode messages about British battle plans that he sent to Field Marshall Erwin Rommel.

The major drawback to number stations is that they provide only one-way communications. The agent needs to rely on letter drops, dead drops, or couriers to send information to his or her handler. 

Yet, despite today’s satellite communications and the internet, number stations are still widely used. Unlike shortwave broadcasts, digital communications can be intercepted and traced to their intended recipient. As long as whatever code is used remains unbroken, the number stations system is secure.

In my so-to-be-released WWII spy thriller The Last Saboteur, a German sleeper agent in the U.S. codenamed Introvert receives his instructions from Berlin via a number station.

If you’d like to hear what a number station sounds like, you can listen to both recordings and live broadcasts at Priyom.org.