Above, you will find links to articles on so-called "number stations", grouped by the language or code they transmit in, as well as station network operators.
Number stations are shortwave transmissions from foreign intelligence agencies to spies in the field of foreign countries. They carry encrypted messages in form of groups of numbers or letters, using either automated voice, Morse code, or a digital mode. While the encryption methods used by most number stations are 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 cleartext, which can be used only once, and must be destroyed after usage.
As number stations are part of classified intelligence operations, very few government organizations have released information about them; those that have include the Polish Institute of National Remembrance and other Polish archives, the Swedish Security Service, and the National Archives of Latvia. Additionally, number stations have been involved in the publicly prosecuted espionage cases of Kim Hyon-hui (1987), Václav Jelínek (1988), the Cuban Five (1998), Ana Montes (2001), the Illegals Program (2010), and Andreas and Heidrun Anschlag (2011).
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 the ENIGMA newsletter #12, was from an issue of the Austrian magazine Kurzwelle Panorama dating from World War I. The BBC were noted for sending coded messages to SOE agents during WW2. From then on, encrypted messages broadcast with creepy automated voices have appeared and disappeared as political events have changed over the last 60 years. Many number stations have ceased operation between the late 1990s and the early 2010s, coinciding with the rise of the Internet, mobile phones, and cheaper satellite communications. Conversely, since the mid 2010s, the activity of several remaining stations has significantly increased.
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 7 digital modes
- HM - Hybrids of analog and digital modes
While powerful and convenient, the ENIGMA naming system isn't the be-all and end-all of numbers stations classification. There have been other approaches, such as the predating practice of naming stations after their musical introduction (Lincolnshire Poacher, Swedish Rhapsody...) or just by voice (English Man, German Lady); each with their own limitations. The ENIGMA system was very useful in its pre-internet era of analog radio and distant community, but blind application in a modern context with widespread digital tools might just result in cargo cult. Instead, transversal analysis and classification of intelligence transmissions based on the study and determination that they originate from a same operator, brings a clearer picture than considering them as separate individual "stations" just by virtue of using different ENIGMA designators.