Number stations are shortwave transmissions from foreign intelligence agencies to spies in the field of foreign countries. Appearing at just about any time of day or night on the shortwave bands, number stations have been calling out with their automated voices since at least the first World War. Since the fact of operating a number station is technically a state secret, there have been very few confirmations from government organizations - those that have were the Czech Office for Foreign Relations and Information (we also obtained a Top Secret document), Polish Institute of National Remembrance and the Swedish Security Service SÄPO. Usage of number stations has additionally been confirmed by former Cold War spies, and the cases of Erwin van Haarlem (1988), The Cuban Five (1998) and Andreas and Heindrun Anschlag (2011).
Number stations transmit encrypted messages in forms of groups of numbers, or less commonly letters, using either automated voice (many languages), Morse code or digital signals. While the encryption method used for most number stations is unknown, some have used and others are widely believed to use one-time pad - mathematical addition of a set of random numbers (the key) to the plaintext, which can be used only once, and must be destroyed after usage. Some of the stations are believed to transmit pre-defined codebook instructions.
Number stations offer a powerful advantage in our modern world: practically complete anonymity. The recipient of the message can be almost anywhere in the world, and receive instructions without fear of being traced through a phone call or internet connection. All the recipient needs is a shortwave radio and to be in the right place at the right time.
The first account of a number station, as reported in ENIGMA Newsletter Issue #12, was from the Austrian Kurzwelle Panorama magazine dating from World War I. The BBC were noted for sending messages to people overseas which where coded messages to SOE agents during WW2. From then on encrypted messages broadcasted with creepy automated voices have been being sent with stations appearing and disappearing as political events changed over the last 60 years. The amount of active stations has significantly decreased since the late 1990s, though number stations are still actively used by countries such as Russia, Poland and Cuba.
ENIGMA naming system
The most popular number station naming system was devised by the European Numbers Information Gathering and Monitoring Assocation (ENIGMA), a number station research group active in 1993-2000, and later maintained by ENIGMA 2000. It was created to solve ambiguities in number station reporting, and classifies stations by language or type of signal. Each ENIGMA designator consists of an alphabetic prefix followed by an ordinary number.
- E - English language voice broadcasts
- G - German language voice broadcasts
- S - Slavic language voice broadcasts
- V - Voice broadcasts in all other languages
- M - Morse code
- F - Frequency-shift keying digital modes
- P - Phase-shift keying digital modes
- XP - Russian stations using MFSK digital modes
- HM - Hybrids of analog and digital modes
These legacy prefixes saw only one designator assigned each in the past, and can be considered as abandoned: