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Numbers stations
One-way voice link · OWVL

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, transmitting seemingly random sequences of (spoken) numbers or morse coded tokens. In the most common form, it features a female voice, reading long strings of numbers, generally in groups of five, often preceeded by a preamble or a simple musical tune. In most cases, such stations carry OTP encrypted messages.
In the 1970s and 80s, it was often speculated that numbers stations were used by drugs cartels for smuggling and organising clandestine droppings. This seemed highly unlikely however, as a clandestine high-power transmitter could easily be located and would certainly have been taken down by the authorities. It has since become known, that Numbers Stations are operated by governments as a simple and secure method for sending secret messages to agents anywhere in the world, using the SW radio bands, whilst the recipient of the message remains untraceable.

In 2014, the Czech Government confirmed the existence of at least two numbers stations in Czechoslovakia during the Cold War. In reply to a request by Priyom.org, they released several official documents [5]. In 2015, this was confirmed by the Swedisch Intelligence Agency SÄPO [6].

OWVL equipment on this website
Baudot-to-speech converter 'Schnatterinchen' (DDR)
Speech/morse generator, Device 32620
Spy radio sets used during the Cold War
Short-wave receivers used for the reception of Numbers Stations
Introduction
During the Cold War, numbers stations were often operated by Eastern Bloc countries, such as Czechslovakia, East-Germany and the Soviet Union, broadcasting messages to agents operating under cover in West-European countries like Belgium, Germany, France and The Netherlands.

Radio Amateurs will certainly remember the artificial female voice on the short wave (SW) bands, that read endless sequences of seemingly random numbers, with a distinct pronunciation.

Many of the broadcasts were in German, as this language was understood in most European countries, but there were also broadcasts in Polish, Russian and even in English. In the late 1950s, the messages were commonly read 'live' by a team of speakers – usually women – and recorded onto tape. The messages were later transmitted by high-power radio SW stations.
  
Drum with 13 discs

Short wave radio (SW) has the advantage that it spans the entire globe without the need for satellites or the internet. The reason that female voices were often used, is that due to the higher pitch, they could be discriminated more easily on the narrow-band poor-quality fading radio channels. In addition, the words were pronounced in such a way that they were unambiguous.

As this method was labour-intensive and prone to mistakes, the intelligence services started looking for ways to automate the entire process. One of the first electronic numbers stations – developed for the East-German Stasi – was Device 2028 – Codenamed: Schnatterinchen, shown above. It consisted of a rotating drum with 13 discs, each of which held a piece of magnetic tape with a spoken word or number.

The device was driven by a punched paper tape that could be prepared offline on a teleprinter (telex). It made the female speakers redundant.
  
Device 32620 (Stimme) ready for use

In the early 1980s, the disc-based systems were gradually replaced by digital alternatives, such as the East-German Device 32620 shown above. It no longer had moving parts and degrading magnetic tapes, and could easily be converted for other languages. In the following years they were also used in other countries like Poland, Czechoslovakia, Cuba and the Soviet Union.

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 very popular amongst Eastern spies in 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.

Replying to a message was much more difficult. In some cases, the agent posted a hand-written message in a so-called dead letter box, after which it was picked up and handled by someone else. They were then forwarded to their destination (i.e. the homeland) by means of a diplomatic courier or – in encrypted form – by radio, often with the help from an East-European embassy.

Some spies however, had their own transmitter that they could use to reply to a 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 is very dangerous, as the authorities of the guest country were well aware of their presence and are constantly monitoring the waves for illegal transmissions. Once a clandestine transmission is intercepted, the authorities will try to locate the station by means of triangulation, using fixed or mobile Radio Direction Finding (RDF).
  
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 services. One documented example is the capture of a Dutch citizen in 1969, who worked as an agent for the Stasi — the repressive East-German intelligence service. In his house, the BVD (now: AIVD) found a fully operational Russian R-353 spy radio set with burst transmitter (shown above), along with a partly used one-time pad.

Other countries
Numbers stations are not the exclusive domain of the former Soviet Union and the Warsaw Pact countries. America and its Allies had spies operating under (diplomatic) cover behind the Iron Curtain, and used Numbers Stations to pass coded messages to them. Likewise, countries like Cuba and China have operated similar stations, some of which are still active today. In fact, even Russian and European Numbers Stations can still be heard on the SW-bands to this day.

Types
The following types of Numbers Stations are known:

  • 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 short-wave receivers
Sony ICF-7600DS digital general coverage receiver
Ilmenau 210 tabletop radio receiver
VEB Sternradio Sonneberg (DDR) - Sternchen
BND short-wave converter

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 (UK)
Post-war version of the UK Type 36/1 (MCR-1), made by MBLE (Philips Belgium)
Russian PR-56 receiver (USSR)
CIA receiver RR/E-11 (USA)
Wandel & Goltermann BN-58 (FE-8) receiver (Germany)
BN-48 (UHU) backup receiver (Germany)
UHU
PLUTO receiver (1958 - Czechoslovakia)
RR-49 receiver (CIA, USA)
A-610 SEZHA miniature receiver (USSR)
Mk. 328 Receiver (UK)
 Other receivers


Numbers stations today
Although Numbers Stations are typically something from the Cold War, some of them are still active today (2021). Countries are still actively spying on each other, and short-wave (SW) radio is one of the most reliable means of communication, that does not reveal the identity or location of the recipient. Below is an example of a message that was intercepted in 2014.  More examples

G06 · Russia   5422 kHz (German)
      947
      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 Numbers Stations that broadcast coded messages are illegal, they are not officially identified by a name or number. In the past, such 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 amateur interceptors, 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].

Other sources
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.

References
  1. Wikipedia, Numbers station
    Retrieved March 2015.

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

  3. Karsten Hansky, Sound samples of current Numbers 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.

  7. Numbers Stations research and information center
    Retrieved August 2019.
Further information
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Crypto Museum. Created: Sunday 15 March 2015. Last changed: Friday, 03 June 2022 - 05:20 CET.
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