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Numbers stations   OWVL
One-way voice link

A numbers station, also known as a one-way voice link (OWVL), is a special type of unusual radio broadcast, generally on the Short Wave (SW) radio bands, reading out incomprehensible lists of (spoken) numbers or morse coded messages. The most common type features a female voice, reading long strings of numbers, generally in groups of five, often preceeded by a preamble and/or a series of musical notes. In most cases, such stations carry OTP encrypted messages.
During the Cold War number stations were often operated from Eastern Block countries, such as Czechslovakia, East-Germany (DDR) and Russia (USSR), with their broadcasts aimed at spies and secret agents operating undercover in West-European countries like Belgium, Germany, France and The Netherlands. For this reason, many of the broadcasts were in German, as this language was understood in most countries. Radio Amateurs will certainly remember the artificial female voice that listed endless sequences of seemingly random numbers on the short wave (SW) bands.
The agents were instructed to listen to specific SW frequencies at fixed days and times, often using a commercially available SW-receiver, from brands like Sony, Panasonic and Grundig. The image on the right shows a Sony ICF-2001D, a commonly available receiver that became really popular amongst the spies during the 1980s.

In most cases, the messages hidden behind the numbers, were encrypted by means of the truely unbreakable One-Time Pad (OTP) cipher and the spies were trained to decode such messages and destroy the keys immediately after reception.
Sony ICF-2001D, a commercialy available short-wave receiver that was popular with international spies and agents.

Returning messages was much more difficult. In some cases, the agent delivered a hand-written message is a so-called dead letter box, after which it was handled by someone else. In such cases the messages were sent by courier or radio, often with help from an East-European embassy.
There were also situations in which a spy used his own transmitter to send the message. Such transmitters are commonly known as 'spy radio sets', many of which are covered on this site.

Sending a message from a spy radio station was extremely dangerous as the authorities of the guest country were well aware of them and were constantly monitoring the waves for clandestine transmissions. Once a suspected signal was intercepted, they would immediately try to locate the station by means of fixed or mobile Radio Direction Finding (RDF) and triangulation.
Complete R-353 with burst encoder and OTP cipher booklet

During the Cold War, a number of clandestine radio stations, operated by foreign secret agents, were intercepted and captured by Western intelligence agencies. One documented example is the capture of a Dutch citizen, who acted as an East-German agent in The Netherlands, in 1969. When he was exposed, the intelligence agency BVD (now: AIVD) found a completely operational Russian R-353 spy radio set (shown above) in his home, along with a partly used one-time pad.
Other countries
Number stations are not exclusive to the former Warsaw Pact countries. Like the Russians, the Americans and their Allies also had spies operating undercover behind the Iron Curtain, and number stations were used for passing coded messages to them. In the same vein, countries like Cuba and China have operated such number stations, some of which are still active today. In fact, even Russian and European Number Stations can still be found on the SW-bands today.
Over the years there has been much speculation as to the real purpose of number stations. It has been suggested that the stations were used for drugs smuggling, but this seems highly unlikely. Illegal radio stations operating at high output power are easily located by means of radio direction finding (RDF), and would have been dismantled by the authorities after some time.

It is commonly accepted today, that number stations operate at high output power and are operated by governments as a simple and fool-proof method to send messages to spies and agents working undercover in a foreign country. The reason for using the SW bands for this, is that, under the right conditions, SW signals can span the entire earth without using satellites or common civil communications networks such as telephone lines and the internet. Unlike modern communications methods like the internet, the recipient of a SW broadcast can not be traced.

In 2014, the Czech Government officially confirmed the existence of at least two number stations in Czechoslovakia during the Cold War. In reply to a request by Priyom.org, they released some official documents [5]. In 2015, this was confirmed by the Swedisch Intelligence Agency SÄPO [6].
We currently recognise the following types of Number Stations:
  • Voice stations (reading numbers)
  • Morse code stations (CW)
  • Multi-tone stations (RTTY, FSK, MFSK)
  • Digital stations
  • A combination of the above
Popular receivers
Although in some cases dedicated receivers were supplied to an agent, standard off-the-shelf commercial receivers were often used for the reception of the Number Stations. The reason for this is that such receivers could be bought in nearly every Western country without attracting any attention of the authorities. Furthermore, it gave Eastern Block countries access to the latest technology that was available commercially in the West. One of the first domestic receivers used for this purpose was the Zenith Royal 1000; the first all-transistor SW-receiver from the USA. It was eventually surpassed by the Grundig Satellit 2000 and finally the digital Sony ICF-2001D.
Zenith Royal 1000 Trans-Oceanic receiver Sony ICF-2001D receiver

In addition, fully equipped agents could also use the built-in receiver of their spy radio set, such as in the case of the Russian R-353, or the separate receiver that was supplied with modular sets like the Czechoslovakian PLUTO and the West-German SP-15 with its small UHU. Some examples:
Mk. 301 Receiver Rion spy radio set Wandel & Goltermann BN-58 (FE-8) receiver BN-48 (UHU) backup receiver
PLUTO receiver (1958) Mk. 328 Receiver

Number stations today
It may surprise you, but Number Stations are still active today (2015) as countries are still spying on each other and the short-wave band is one of the safest ways of sending messages without leaving traces. Below is an example of a recently picked up message [3]. More examples are here. If you want to know more about the current Number Stations, please check out the links below.
G06 · Russia   5422 kHz (German)
      532 20
      06132 75514 79681 94217 21443 31441 81797 17512 62689 33103
      48930 93432 25709 93628 48683 18809 85052 49870 63962 04884
      532 20 00000

  1. Sender ID is in bold (947).
  2. Recipient is in red (532) followed by the number of groups in the message (20).
  3. Recipient and number of groups are repeated at the end of the message (532 20).
  4. All number groups are repeated (except for the terminator).
  5. 00000 is the message terminator.
  6. Sample kindly supplied by Karsten Hansky [3].

 More examples

Station identification
As the Number Stations that broadcast clandestine messages are illegal, they are not officially identified by a name or number. In the past, Number Stations were often given nicknames, such as Lincoln Poacher and Cherry Ripe, often based on certain characteristics of the transmission, such as the opening tune. For this reason, a group of enthusiastic listeners, known as ENIGMA 1 , has assigned unique ID number to each station, along with prefixes, suffixes and family IDs.

G06a IA
In the example above 06 is the station ID. The prefix G indicates that the (voice) transmission is in German. The suffix 'a' indicates that it is a variant of the regular station G06. In some cases, if a stations belongs to a family of stations, the Family ID is given in Roman numerals. In this case the family ID is IA, which means it is related to family I (KGB, GRU, FSB). For a full list of Station IDs, prefixes, suffixes and familiy IDs, please refer to the current Enigma Control List [4].

S17e Bulgarian Betty
In some cases, the (nick)name of the station is written behind the station ID, such as in the example of the S17e above. In this case, the station was operated by the Czechoslovakian StB, just like S05 OLX (with the OLX callsign officially being assigned to a Czech news agency) [5].
  1. In this context, ENIGMA does not refer to the German Enigma Cipher machine, but is the abbreviation of European Numbers Information Gathering and Monitoring Association. Although the original ENIGMA group still exists, the list of station IDs, known as the Enigma Control List, is now maintained by ENIGMA 2000 [4].

More about Number Stations
If you want to know more about Number Stations, their locations, frequencies and the time at which they are expected to broadcast, here are a couple of useful links to websites with the latest information. Enigma 2000 are the current maintainers of the Enigma Control List, whilst website Priyom.org offers a real-time schedule of expected transmissions. In addition, The Conet Project has compiled an impressive collection of sound samples, past and present, onto a stack of CDs.

  1. Wikipedia, Number station
    Retrieved March 2015.

  2. Simon Mason, The Euronumbers Mystery
    1991. ISBN 0-936653-28-0.

  3. Karsten Hansky, Sound samples of current Number Stations
    Received August 2015. Many thanks!

  4. ENIGMA 2000, Enigma Control List
    Website. Retrieved August 2015.

  5. Priyom.org, Czech Intel confirms it used to run a numbers station
    4 March 2014. Retrieved August 2015.

  6. Priyom.org, Swedish Security Service Tells us about their Numbers Stations Experience
    25 January 2015. Retrieved August 2015.

Further information

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Crypto Museum. Created: Sunday 15 March 2015. Last changed: Sunday, 15 January 2017 - 09:29 CET.
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