A numbers station is a shortwave radio station characterised by broadcasts of formatted numbers, which are believed to be addressed to intelligence officers operating in foreign countries. Most identified stations use speech synthesis to vocalise numbers, although digital modes such as phase-shift keying and frequency-shift keying, as well as Morse code transmissions, are not uncommon.

Most stations have set time schedules, or schedule patterns; however, other stations appear to broadcast at random times. Stations may or may not have set frequencies 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 organisations.

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 SEO 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 numbers community is now most commonly using a naming system devised by the European Numbers Information Gathering and Monitoring Assocation (ENIGMA), which classifies number stations by language or type of the signal. Prefixes used:

  • 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
  • XP – Russian stations using MFSK modes
  • SK – Other digital mode stations that do not use MFSK [Only SK01]
  • HM – Hybrids of both voice/Morse and digital modes
  • DP – Hybrids of (M)FSK and other types of digital modes [Only DP01]

The digit “9” after the letter indicates temporary assignment for stations under investigation (since early 2014).

The system is currently maintained by Enigma 2000.