"Numbers stations" are shortwave radio stations that governments use to communicate with their spies in the open, meaning anyone can listen in. But there's plenty of careful encryption to keep the traffic secret anyway.
While spies typically try to hide as much of their communication as possible, there is one method of intelligence communication that is literally broadcasted so that everyone for thousands of miles around can listen in to the messages, but no one else can understand the message.
These were known as "numbers stations," an apt name since they exist solely to broadcast number sequences to spies operating in the area. Governments dispatch their spies with books of codes, and then the numbers broadcasted are used with these books to assemble messages years after the spy was dispatched.
These are typically done with "one-time pad" encryption where the message cannot be cracked without the book of numbers. The list of numbers is compared to a single line of numbers in the book, and comparing the numbers will give the spy the message intended for them. But, importantly, each line in the book is used a single time.
So, someone listening in cannot piece together messages through careful listening or tracking, only through stealing the book, if they can find it. So, governments can broadcast their numbers in the clear, usually from a radio station bordering the country they are spying in, without worry.
America has suffered spies that listened to these stations, like Ana B. Montes, one of the highest ranked spies in U.S. history. But we've also used the method ourselves especially during the Cold War. Our allies in Britain had done so, running a station in Cyprus for years.
Some spies during the Cold War, including some from the U.S. and Britain, were captured with their code books intact. America had its own numbers coup in the 1980s when it turned a source in the Soviet Government that fed them the codes used to instruct communists in the U.S. at the time.
To listen in yourself, you need to live in range of a broadcasting station and to have a "shortwave" radio, a receiver that listens to high-frequency signals. Few places still track the broadcasts.
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