Thanks to KB9BVN for this link via the Flying Pigs QRP Club mailing list…..Dan
While “numbers stations” are perhaps more known to shortwave listeners than amateur radio operators, every now and then one pops up on 30m. Numbers stations are stations that send only sequences of numbers. Ostensibly, intelligence agencies are using these numbers stations to send secret messages to agents in the field. You can find numbers stations sending their sequences in many different languages.
According to the Conet Project,
Shortwave Numbers Stations are a perfect method of anonymous, one way communication. Spies located anywhere in the world can be communicated to by their masters via small, locally available, and unmodified Shortwave receivers. The encryption system used by Numbers Stations, known as a â€œone time padâ€ is unbreakable. Combine this with the fact that it is almost impossible to track down the message recipients once they are inserted into the enemy country, it becomes clear just how powerful the Numbers Station system is.
The Conet Project sells a CD with a number of recordings for $55, but you can find a bunch of mp3 files here for free.